Potassium in Crop Production

ADVANCES IN AGRONOMY. VOL. 33

POTASSIUM IN CROP PRODUCTION Konrad Mengel and Ernest A. Kirkby Institute of Plant Nutrition, Justus Liebig University, Giessen, Federal Republic of Germany and Department of Plant Sciences, The University, Leeds, England

. . . . . . . . . . . . . . . . . . . . . . . . . .. . . , . . .. . . . . . . . . A. Soil Potassium Fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B . Factors and Processes of Potassium Availability . . . . . . . . . . . . . . . . . . . . . . . , . . , . C. Assessment of K Availability in Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11. Potassium Availability in the Soil

D. Plant Root Soil Interactions 111. Potassium in Physiology

. . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . .. . . . . . . ... . . . .. . . .

59

60 60 64 70 71 74 74 81 83 85

A. Potassium Transport across Biological Membranes and Cation Competition B. Cell Turgor and Water Economy of Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Energy Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Long-Distance Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Enzyme Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 IV. Potassium Application and Crop Growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 A. Crop Response and Potassium Application . . . . . . . . . . . . . . . . . . . . , . . , . . . . , . . . 91 B. Effect of Potassium on Yield Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 C. Secondary Effects of Potassium on Crop Yield.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 103 V. Conclusions 103 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

I. INTRODUCTION Potassium was first recognized as an essential element for plant growth following the work of the Englishman Home in 1762 from experiments in which he grew barley in pots of soil and used plant analysis as a means of investigating uptake. Later researchers such as Th. de Saussure and Carl Sprengel recognized that potash was present in plant ash obtained from a large number of different plant species. In reviewing the analytical data of the period, Liebig (1 841) proposed that K was in some way involved in plant metabolism. The experience of farmers around Giessen, the German university town in which Liebig worked, had indicated the beneficial influence of manuring crops with plant ash. Liebig recognized that potash was the essential growth factor in the ash. Furthermore, Liebig was aware +

59 Copyright @ 1980 by Academic Rcss. Inc. All rights of Rpaoducrion in any form reserved. ISBN 0-12600733-9

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KONRAD MENGEL AND ERNEST A. KIRKBY

that the clay fraction of the soil provided a source of K+ for plant growth. In his book, “Die organische Chemie in ihrer Anwendung auf Agrikultur und Physiologie” he wrote, “There must be a component in clay which has an influence on plant life and which directly participates in plant development. This component is the ever-present potash or sodium. The paramount importance of clay minerals in binding or releasing K+ recognized by Liebig has been confirmed by much subsequent research work. The same is also true of Liebig’s suggestion that K+ was involved in plant metabolism. Potassium is now known to be required by plants in large quantities, and potassium fertilizer application has had a considerable impact on crop production, particularly under conditions where there has been a shift from extensive to intensive agricultural practice (Amon, 1969). In this article, three main aspects of K+ in crop production are reviewed, namely, K availability in the soil, the function of K+ in the plant, and potash fertilizer application. The soil is considered as a source of K+ to plant roots. Pedological and mineralogical problems relating to soil K+ have been reviewed elsewhere by Rich (1968, 1972) and by Schroeder (1976). These aspects are considered here only insofar as they are of direct importance to the availability of K+ in the soil medium and hence to crop growth. The use of K+ in practical crop production is also emphasized in the discussions on the physiological role of K+ in the plant and in fertilizer application. ”

II. POTASSIUM AVAILABILITY IN THE SOIL A. SOILPOTASSIUM FRACTIONS

The potassium status of a soil may be assessed on its content of K+-bearing minerals, since the amount of these minerals present in a soil gives some indication of the potential source of K+ to plants. However, in terms of the ability of the soil to supply K+ to plant roots, the quantity of K+-bearing minerals plays only an indirect role. More important in determining the K+ supply to plants are the soil K+ fractions. These fractions, which have been established experimentally using different extraction techniques, are soil solution K+, K+ adsorbed to clay minerals or humus, and K+ present in minerals. The total quantities of K+ in these three fractions differ considerably between soils. However, in mineral soils in which K+ is present in average amounts, the soil solution K+ makes up about 1 to 3% of the exchangeable K+, which in turn represents only a small fraction-at most a few percent-of the total K+ (Scheffer et al., 1960). Potassium in soil solution tends to equilibrate with K+ in the adsorbed fraction so that these two soil K+ fractions are closely interdependent. The equilibrium between solution and adsorbed K+ is controlled to a large

POTASSIUM IN CROP PRODUCTION

61

extent by the degree of K+ selectivity of the adsorption sites in the exchangeable fraction. Adsorption sites of organic matter and of kaolinitic clay minerals are low in K+ selectivity. Potassium adsorbed on these sites is thus in equilibrium with a relatively high concentration of solution K+ (Ehlers et al., 1968). On the other hand, the 2: 1 clay minerals possess adsorption sites that are much higher in K+ selectivity and that bind K+ very strongly. This is especially true for illitic clay minerals (Ehlers et al., 1967). As shown in Fig. 1 , three types of adsorption sites may be distinguished: planar sites (p-position) with a low K+ selectivity, edge sites (e-position) with a medium K+ selectivity, and inner sites (i-position) with a high K+ selectivity. These highly selective inner sites are of particular interest, since the adsorbed K+ can be considered an integral part of the clay mineral. This is true of the micas, in which the “interlayer K+ ” bridges the adjacent layers by electrostatic bonds. Interlayer K+ is not easily displaced by other cation species and particularly not so by larger cation species such as Ca2+ and Mg2+. For this reason, this particular K+ fraction is termed “nonexchangeable K+.” All three K+ fractions, solution K+, exchangeable K+, and nonexchangeable K+, are interrelated and all play a part in supplying plants with K+. The interrelationshipsbetween the K fractions may be illustrated by considering what happens when a K+ salt such as KCl is added to the soil. At first the salt dissolves and the K+ concentration of the soil solution increases rapidly. Potassium is then removed from the solution by the adsorption sites, the rate at which this occurs depending on the particular equilibrium conditions of the system. This removal of K+ from soil solution is accompanied by an equivalent increase in the soil solution concentration of other cations. The application of K+ to a soil may saturate all three fractions with K+. However, the time required for K+ equilibrium to be reached under field conditions may be as much as several weeks. Saturation of i-positions by K+ is a particularly long-term process since the diffusion of K+ in the interlayer zone is p - position i position

I

I

-

K not exchangeable to IargeCations

*+

I +

‘e-

position

Hydroxy-Al(or Fe) Islands

FIG. 1. Model of an expandable 2: 1 clay mineral with interlayer K+,wedge zones, and p-. e-, and i-positions.

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KONRAD MENGEL AND ERNEST A. KIRKBY

slow (diffusion coefficient lo-’’ to cm-2 sec-l). In examining this fixation process on a K+-fixing soil (44% clay), Karbachsch (1978) found that even after a very high fertilizer application rate of 1000 kg Wha, the K+ concentration of the soil solution was only on the order of 1 mM K+. As the experiment progressed, this concentration steadily declined until after a period of 90 days it fell to a value of 0.3 mM, which was only about 50% higher than the K concentration of the soil solution prior to K+ fertilizer application. Similar results have been reported by Amberger et al. (1974) who found that the process of K+ adsorption to the i-position of clay minerals extended over a period of about 6 months. From these results it is clear that soils in which the K fixation capacity is high may more or less immobilize fertilizer K+. The agricultural implications of this are considered in the final section of this article. All i-positions, even those occupied by cations other than K+, are involved in the K fixation process. Potassium fixation takes place by means of K+ adsorption to these K+-specific binding sites of the interlayer zone, and in this process cation exchange occurs. As shown in Fig. 1, the replacement of interlayer K+ by larger cation species (Ca2+,MgZ+)expands the lattice and wedge zones are formed. The reverse process occurs when these larger cation species are replaced from interlayer sites by K+ or NH:. The contraction in the mineral is accompanied by a decrease in cation exchange capacity. This is the process by which K+-depleted 2:l clay minerals fix K+. The K+ fixation capacity of soils differs widely and depends much on the type of soil clay minerals present in the soil and on their degree of K depletion. According to investigations of Arifin and Tan (1973) the proportion of wedge zones decreased for the various minerals in the following sequence: micas > illite > vermiculite > montmorillonite. These authors found K+ fixation capacities ranging from 0.3 to 0.6 me K/g clay. The data refer to K+ fixation under dry conditions. The so-called “wet K+ fixation capacity” is a lower value, due to the fact that under wet conditions only micas, illites, and vermiculites fix K+, whereas under dry conditions smectites are also able to fix K+, because of a shrinkage of the mineral. The behavior of clay minerals in relation to K fixation or release on drying, however, is by no means clear-cut (Ahmad and Davies, 1971). Potassium depletion of K+-fixing soil minerals may be of anthropogenic origin or may have occurred during soil development. Potassium fixation is often found on alluvial soils associated with high amounts of organic matter, as is the case for numerous K+-fixing sites in Bavaria in the Federal Republic of Germany. The role of organic matter in K fixation in these soils is not clear, but it is believed that the mineral constituents may have lost substantial amounts of K+ during the period of soil material transportation by water (Niederbudde, 1967), thereby making the soils prone to K+ fixation. Continuous cropping without K+ application may also induce the buildup of a

POTASSIUM IN CROP PRODUCTION

63

K+ fixation potential in the soil. An example of this has been reported by Nielsen (1970). In a long-term field experiment he observed the highest K fixation capacity on a soil which had received no K+ fertilizer for 70 years. Potassium fixation is also dependent on soil acidity, being generally low or absent on more acid soils. Under such conditions more soluble forms of A1 such as AI(OH)?j, Al(OH)2+are available which compete with K+ and are selectively bound to the i-position of the clay minerals (Nemeth and Grimme, 1972). In the long term, low pH conditions also favor the formation of Al-chlorites, a type of mineral which does not fix K+ (Laves, 1978). Under certain circumstances interlayer K+ may contribute to a considerable extent in supplying K+ for plant uptake (Niederbudde et al., 1969; Tabatabai and Hanway, 1969). Mengel and Wiechens (1979) found that under optimum conditions the nonexchangeable K+ fraction of a soil rich in K-bearing minerals (illite, vermiculite) could completely meet the K+ demands of ryegrass. This pot experiment also showed that the proportion of K+ absorbed from the nonexchangeable K+ fraction increased the more the exchangeable K+ fraction was depleted. Below a level of 300 ppm exchangeable K+, most of the K+ absorbed by the ryegrass originated from the nonexchangeable K+ source. Potassium quantities present in this nonexchangeable fraction may be considerable. The availability of this K+, however, decreases as K+ is released. Drews (1978) found that only a very small fraction-at most a few percent-of the interlayer K+ was available to Lolium perenne. In this permanent cropping experiment the rate of K + released from the nonexchangeable soil K + fraction finally became so small that the plants suffered severely from K+ deficiency. These observations of Drews (1978) as well as the experiments of Wiechens ( 1 975) and pot and field experiments of v. Boguslawski and Lach ( 1 97 1) clearly demonstrate that the huge pool of interlayer K can be tapped by plants to only a very limited extent. One may suppose that only interlayer K+ located in the marginal zones of clay minerals is available to plant roots in adequate amounts. The above view is supported by the work of Newman (1969). In studies on the release of K+ by micas he has reported that the K+ concentration in equilibrium with the interlayer K+ decreased as the interlayer K+ is depleted. The process of interlayer K+ release is not yet completely understood. According to v. Reichenbach (1972) it is an exchange process associated with diffusion in which K+ adsorbed to i-positions of the interlayer zone is replaced by other cation species. If the replacing species is a large one (Na+, Mg2+, Ca2+), then K+ exchange results in an expansion of the clay mineral and the formation of wedge zones (see Fig. 1). The resulting widening gap between the two layers of the mineral favors the diffusion of the replaced K+ out of the mineral. A demonstration of this type of behavior has been reported recently by Jackson and During (1979) for New Zealand topsoils of widely different clay mineralogy. Pretreatment of the soils with Ca2+ (as acetate) resulted in an expansion of the +

64

KONRAD MENGEL AND ERNEST A. KIRKBY

clay mineral and an increase in K+ desorption. Potassium desorption is also often associated with the oxidation of F2+ to Fe3+ (Farmer and Wilson, 1970; v. Reichenbach, 1972). Low pH and high moisture conditions are also beneficial to the release of interlayer K'. Net K+ release occurs if the rate of K+ release is higher than the rate of K+ fixation. Since the fixation rate depends directly on the K+ concentration in solution in contact with the clay mineral, a high net rate of release is only likely to occur if the K+ concentration in the solution is extremely low. Diluting the soil solution with distilled water should thus promote the release of interlayer K+. This has been demonstrated in a comparative leaching experiment by Drews (1978). The continuous leaching by water of a soil column containing 300 g soil over a period of 180 days resulted in a loss of 9.8 mg K/100 g soil. However, in the comparative parallel treatment, in which the water was replaced by 15-60 x M KCl no loss of K+ from the nonexchangeable fraction was observed. This experiment shows that net K+ release from clay minerals occurs only if the K+ concentration in the soil solution is extremely low. The question of whether plant roots have a direct influence on the release of interlayer K+ is discussed in the next section. B. FACTORS A N D PROCESSES OF POTASSIUM AVAILABILITY

The contact exchange process as postulated by Jenny and Overstreet (1938) was long held to be the most important means by which plant roots obtain and mobilize K+ adsorbed to clay minerals. This concept was criticized by Lagerwerff (1961), who concluded from his experimental data that the bulk of cations absorbed is taken from the solution and that the exchangeable cation fraction is only indirectly available by means of exchange with cations in solution. Further evidence against the predominant role of contact exchange in K+ uptake came from the findings of Barber et al. (1963). These workers evaluated the amount of K+ accessible to plant roots by interception, or in other words, the K+ in direct contact with the roots as they push their way through the soil. It was concluded that the amount of intercepted K+ was far too small to satisfy the needs of the plant. This conclusion is also consistent with the calculations of Mengel and Kirkby (1978) which show that even for a soil high in exchangeable K+ the amount of K+ in direct contact with plant roots can only satisfy a small fraction of the plant's K requirements. Experiments of Drew and Nye (1969) with Lolium perenne also revealed that only 6% of the total K+ demand was supplied by the soil volume of the root hair cylinder. Ninety-four percent of the K+ taken up therefore originated from beyond the limit of the root hair cylinder. It can thus be concluded that the bulk of K+ required by plants must be transported toward the roots.

POTASSIUM IN CROP PRODUCTION

65

The transport of K+ in the soil medium toward plant roots may take place by mass flow or diffusion. Differentiation between both processes is difficult, and only a rough calculation can be made. According to Barber et al. (1963) only about 10% of the total K+ requirement of crops is transported by mass flow, although the contribution can be somewhat greater when the amount of water transpired by the crop is increased. Generally, however, it is accepted that diffusion is the main process by which K+ is transported to plant roots. Both processes, K+ diffusion and K+ mass flow, have been incorporated into an equation that describes the K+ flux toward plant roots (Barber, 1962).

+ C ~ V+ u

J = Dl(d~l/dr) + Dp(d~2/dr)

(1)

where flux rate (total quantity of ions reaching the root per unit time per unit area of root surface) c, = K+ concentration in the soil solution cp = K+ concentration moving at the soil surfaces (adsorbed K+) c3 = K+ concentration in mass flow water v = velocity of water flowing through the soil toward the roots a = replenishment factor D, = diffusion coefficient of the ions in the soil solution D, = diffusion coefficient for the movement of ions at soil surfaces (exchange diffusion) J

=

This equation shows that quite a number of factors have an influence on the K+ flux rate in the soil medium. If one assumes that the mass flow component ( c 3 v ) is of minor importance and one also neglects the replenishment factor, then the flux rate can be seen to be controlled mainly by factors influencing diffusion. Two kinds of diffusion may be considered, diffusion in the soil solution and diffusion in the zone of adsorbed cations (exchange diffusion). Data for exchange diffusion are very rare in the literature. However, De Lopez-Gonzales and Jenny (1959) reported an exchange diffusion coefficient for Sr2+ of 1.5 x lo-* cm-2 sec-’. This is lower than the diffusion coefficient of S r p + in solution by a factor of lo3. It would therefore seem reasonable to assume that the “exchange diffusion” coefficient for K+ is also by some orders of magnitude lower than the K diffusion coefficient in solution, and that one may neglect the exchange diffusion component of the equation (Nye and Tinker, 1977). If the term for the exchange diffusion in Eq. (1) is dropped, then the equation for the K+ flux may be reduced as follows: This is Fick’s diffusion law. Nye and Tinker (1977) argue that despite the heterogeneous nature of soil, it is legitimate to treat the soil as a quasi-

66

KONRAD MENGEL AND ERNEST A. KIRKBY

homogeneous body to which this law may be applied and that the diffusivity of such a system is described by the diffusion coefficient. They hold the view that this is valid so long as a representative sample of gas- and liquid-filled pores and adjacent adsorbed phases are included. In a more recent paper Nye (1979) has substituted the diffusion coefficient by a dispersion coefficient (see below). This alteration, however, does not affect the principle of the following deductions. According to Nye and Tinker (1977) the diffusivity may be described by the equation! (2)

D = D18fldC1ldC where D1 = diffusion coefficient of K+ in free solution 8 = the fraction of the soil volume occupied by solution f , = impedance factor C, = concentration of K+ in soil solution C = concentration of K+ in the whole soil system

From this equation it follows that the diffusivity of K+ in the soil media increases with 8, which in turn is closely related to soil moisture. The impedance represents the factor also increases as soil moisture increases. This term tortuous pathway along which K+ has to pass on its way through the soil medium to plant roots. It can readily be visualized that the tortuosity increases as the soil becomes drier. C1 is the K+ concentration in the soil solution, and C is the total K+ directly or indirectly involved in K+ transport. Generally the exchangeable K+ is used when measuring the term C. The ratio dC,ldC is of particular importance since it is the reciprocal of the buffer capacity.

vl)

b = dCIdC, = buffer capacity Substituting b for dCldC, the following equation is obtained: D

= D19fl/b

(3)

From this equation it is clear that the diffusivity of K+ decreases as the K buffer capacity increases. This close relationship between the K+ buffer capacity and the K+ diffusion coefficient has been shown experimentally by Vaidyanathan et al. (1968). The importance of soil moisture for the diffusion of K+ or related cation species has been demonstrated by several authors. Graham-Bryce (1963) found a diffusion coefficient for Rb+ of 1 x lo-' cm+ sec-I at a soil water content of 23%. The coefficient decreased to 5 x lo-* ern+ sec-' when the soil moisture was reduced to 10%.Patrick and Reddy ( 1977) measured a diffusion coefficient of 2.5 x 10-6cm-2sec-l for NH: in paddy soils. In comparison with these values,

POTASSIUM IN CROP PRODUCTION

67

the diffusion coefficient for K+ in pure water is about 1.5 X cm-' sec-'. As it seems likely that the diffusion coefficients for NH:, Rb+ , and K+ should not differ greatly, the coefficients cited above for NH: and Rb+ should also be more or less in the same order of magnitude as those of K+ . It can thus be seen that soil moisture is of crucial importance in K+ availability. From the equations cited above the K+ flux in the soil medium (J) can be described by the following equation: J =

-(*)

(2)

(4)

In this equation exchange diffusion and mass flow are neglected. The rate of K+ diffusion is controlled by the K concentration gradient ( d C , / d r )as well as by diffusivity . If the assumption is made that the rate of K+ absorption by the root is the same as that diffusing to the root surface, then the K+ concentration at the root surface remains constant. This, however, is an exceptional case, and generally the rate of K+ absorption by roots is higher than the rate of K+ transported toward the root surface. For this reason K+ depletion zones develop around the root, and the K+ concentration at the root surface thus declines. The degree of K+ depletion at the root surface can be expressed by the C,/Ci ratio, where Ci represents the initial K+ concentration before uptake begins and C, the K+ concentration at the root surface. Experimental evidence of Rb+ depletion around plant roots has been reported by Barber (1962) and Farr et al. (1969) using autographs. The K+ concentration at the root surface is of crucial importance in relation to K+ uptake, according to the following equation (Drew et al., 1969):

F = 21raaC,

(5)

where F = Flux rate across the root surface (mole cmP sec-') a = radius of the root a = root absorbing power C, = concentration of K+ in solution at the root surface

In this case a single root or single root segment is considered as a cylinder, so that the term 27ra represents the root surface of I-cm root length. The term a is the root absorbing power. A high power means that a high proportion of K+ impinging on the root surface is absorbed and vice versa. The root absorbing power depends much on root metabolism and is thus not a constant term but changes depending on metabolic conditions and plant species involved. From Eq. ( 5 ) it can be derived that the K+ flux across the root surface is related to the K+ concentration at the root surface (C,) in a linear way. This is not

68

KONRAD MENGEL AND ERNEST A . KIRKBY

completely correct, as the K+ uptake rate in relation to C , is rather described by a Michaelis-Menten type of curve, as shown by Barber (1979). However, in cases in which diffusion is the limiting process in transporting K+ to the root surface, the K+ concentration at the root surface is low ( d o p M ) , and in this low concentration range the relationship between K+ uptake and K+ concentration is more or less linear. If the time factor is integrated, K+ uptake can be described by the following equation:

M, = 21ratuC,t

c,

(6)

The term represents the average K+ concentration at the root surface during the uptake period t . The decrease of C , during this time is related to the replenishment of solution K+. The higher the K+ replenishment, the higher is the mean K+ concentration at the root surface. This K+ replenishment is controlled by the K+ buffer capacity (b) of the soil, which can be expressed as the ratio of K+ quantity (exchangeable K+) over K+ intensity (K+ concentration of the soil solution) (Mengel, 1974). Figure 2 from the data of Grimme et al. (197 1) shows the K+ buffer curve of a sandy soil and a loamy soil. From these curves it can be derived that if the same amount of K+ is absorbed by plants (quantity factor) from both soils, the K+ concentration in the soil solution of the sandy soil is depressed to a much higher extent than the K+ in the soil solution of the loamy soil. The steepness of the curve is a direct measure of the K+ buffer capacity. This K+ buffer capacity provides also some information about the K+-supplying power of a soil according

(c,)

me K+ in solution ( intensity )

FIG.2. K+ quantityhntensity relationship of a sandy and a clay soil. The steepness of the curves represents the K+ buffer capacity (after data of Grimme er al., 1971).

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POTASSIUM IN CROP PRODUCTION

to the following equation (Nye, 1979): (6a)

U, = bdC,/dt

where U, = K uptake r a t e h i t soil volume. It is evident from Eq. (6a) that the K + concentration at the root surface and the K+ buffer capacity are most important factors controlling K+ uptake of plants. The K+ concentration at the root surface is difficult to measure. It is related to the average K+ soil solution concentration according to an equation established by Baldwin et al. (1973): +

a a av X c, = C,/(l + InD*Bfl 1.65a -

)

(7)

where v x

D*

= flux of water through the root surface = radius of the depletion zone =

dispersion coefficient of the solute in the soil solution (cm' sec-')

This equation takes into account the mass flow (v).The term D* stands for the diffusion coefficient. According to Nye (1979) the dispersion coefficient is more appropriate than the molecular diffusion coefficient. Under normal plant water consumption D* is unlikely to exceed the molecular diffusion coefficient by more than a factor of 2. Major factors controlling C , are the K+ concentration in the bulk soil solution ( C , )and the root absorbing power (a).the former increasing and the latter decreasing the K+ concentration at the root surface. Also the extension of the depletion zone has an influence on C,; the larger the extension, the higher is the K+ concentration at the root surface. The theory of K+ flux toward plant roots as outlined above has been tested with young maize plants by Claassen and Barber (1976), using the following equation for the calculation of plant K+ uptake:

where I, I,,, K, C, E

= = = =

=

K+ uptake rate (influx) influx rate at infinite K+ concentration Michaelis-Menten constant

K+ concentration at the root surface K+ efflux

C , was computed from an equation established by Nye and Maniott (1969). From the plot CJC, versus the radial distance from the root, the K+ concentration

70

KONRAD MENGEL AND ERNEST A. KIRKBY

at the root surface (C,) was derived. Thus it was possible to calculate the K+ uptake according to soil and root parameters and to test this calculation by actual K+ plant uptake. Although four different soils with varying K+ levels were included in this experiment a fairly good correlation (Rz= 0.87) was obtained between the predicted and experimental K+ uptake. Calculated K+ uptake was overestimated by about 50% possibly because competition occurred between roots for soil K+. The good agreement between the predicted Kf uptake and the actual K+ uptake of plants proves that the theory of K+ flux and diffusion in soils is based on sound assumptions. The mathematical model used by Claassen and Barber (1976) takes into account the following factors: effective average diffusion coefficient, initial K+ concentration in the soil solution, and the buffer capacity. Hence these factors are the most important parameters controlling K+ availability in soils. C. ASSESSMENT OF K AVAILABILITY I N SOIL

Potassium concentration in soil solution, K+ buffer capacity, and the soil diffusivity should be considered in estimating K+ availability for practical purposes. Under practical farming conditions the soil diffusivity is difficult to assess in advance since it depends much on soil moisture. Potassium concentration in the soil solution and K+ buffer capacity until now have rarely been used in estimating soil K+ availability. In most cases exchangeable K+ is still regarded as a satisfactory measure of the K+ availability status of soils. This fraction, however; comprises both solution K+ and Kf adsorbed by varying strengths to adsorption sites (p-, e-, and even i-positions). Soils with the same values for exchangeable K+ may thus differ considerably in K+ concentrations in soil solution (Nemeth et al., 1970), because more selectively bound K+ is equilibrated with a relatively low K+ concentration and vice versa. If this specifically bound K+ is taken into account, exchangeable K+ may also be a good indicator of the K+ availability status. Rezk and Amer (1969) thus found a significant correlation between K+ uptake by plants and the “corrected” exchangeable K+ of the soil. This “correction” was obtained by dividing the exchangeable K+ through the Gapon coefficient. By this procedure numerical values are obtained that are closely related to the K+ concentration of the soil solution. Poor correlations between plant response and exchangeable K+ have been obtained especially in investigations where soils of different clay contents and degrees of K+ saturation have been used. Jankovic and Nemeth (1974) even found a negative correlation between the exchangeable soil K+ and sunflower seed yields harvested from five different sites. The same yields, however, were positively correlated with the K+ concentration of the soil solution. A close relationship between K+ concentration of the soil solution and the grain yield of

POTASSIUM IN CROP PRODUCTION

71

wheat, grown under field conditions, has also been reported by Nemeth and Harrach (1974). Similarly in studying the K+ availability of 21 different soils in a pot experiment with oats, v. Braunschweig and Mengel (1971) found a highly significant correlation between the K+ concentration of the soil solution and grain yield. More recently During and Duganzich (1979) have also reported that K+ uptake by white clover was best reflected by the K+ concentration of the soil solution. Exchangeable K+ alone correlated very poorly with uptake except in soils of very low K status. Recent experiments of Wanasuria et al. (1980) have shown that the K+ of paddy soils extracted by electroultrafiltration (EUF) was positively correlated with the grain yield, whereas no significant correlation with the exchangeable K+ was obtained. EUF-extractable K+ does reflect the K+ concentration of the soil solution (Nemeth, 1979). Although the K+ buffer capacity is of paramount importance for K+ availability, little quantitative data are available concerning its influence on K+ supply to crops. Nemeth (1975) in investigating three soils with different K+ buffer capacities found a close negative relationship between the K+ buffer capacity and the decrease in grass yield of four consecutive cuts harvested during the experimental period. Barrow (1966) reported that the correlation between the K+ uptake of clover and the content of exchangeable K+ was improved if in addition to the exchangeable K+, the K+ buffer capacity was also taken into account. Recent experimental results of Busch (1980) obtained in pot experiments with a number of soils differing widely in texture have shown that 50-80% of the variability in K+ uptake could be explained by the K+ buffer capacity and the K+ concentration of the soil solution. Only under extreme K+-deficiency conditions, was K+ uptake much controlled by other, still unknown factors. D. PLANTROOTSOILINTERACTIONS

The quantity of K+ absorbed by crops is also related to root growth, extension, and metabolism. Although root interception contributes only to a minor extent to the total K+ requirement of a crop, root extension and root density in the soil are of importance for the quantity of K+ accessible to plant roots. The extension of the K+ exploitation zone around a plant root represents the soil volume that can be "mined" for K+. Since K+ is mainly transported by diffusion toward plant roots, the bulk of K+ absorbed by plants originates from these zones around roots. It is easy to deduce from this relationship that a dense rooting system can exploit a larger soil volume for K+ than can a poor one. The rooting density ( L , ) can be defined as total root length per unit soil volume. The rooting density has an impact on the extension of the K+ exploitation zone around the root (Nye, 1979) according to the following relationship: x = 1 l(7rL.")~

72

KONRAD MENGEL AND ERNEST A. KIRKBY

where x = radius of exploitation. Thus in dense root systems, the K+ depletion zones around roots are less extended and often an overlapping of the exploitation (depletion) zone occurs. A further factor of importance is the soil volume accessible to plant roots. Pot experiments of Newman and Andrews (1973) have shown, for example, that if only a small soil volume is available, the amount of K+ absorbed by plants is also reduced. When the soil volume was restricted, dense rooting systems were observed and K+ uptake was inadequate. It seems likely that this resulted from root competition for K+ between overlapping depletion zones. Root extension and root mass are both of particular importance if available K+ in the soil is low (Chloupek, 1972). On the other hand, as has been shown by Maertens (1971) using young maize plants, only a small portion of a root system may suffice to ensure ample K+ uptake, provided the K+ availability is high. Thus, in general, the K+ absorption potential of a root segment by far exceeds the rate at which K+ is actually absorbed. In order to assess K+ uptake rates, K+ uptake should be calculated per unit root segment (e.g., cm root length). Such experiments and calculations have been canied out by Mengel and Barber (1974). These workers observed K+ uptake rates per unit length of plant root in the early weeks of plant development. Similar results have also been reported by Adepetu and Akapa (1977), who studied nutrient uptake of five cowpea cultivars. The K+ uptake rate per m root length of 5-day-old plants was four times higher than the K+ uptake rate of 30-day-old plants. These results clearly indicate that especially at an early growth stage, high K + availability is required and that the susceptibility to K deficiency is particularly marked during this period. Barber (1979) reported that maximum K+ uptake rates may differ considerably. Thus for young maize roots maximum K+ uptake rates of 2 pmole and for young soybean roots of 0.4 pmole K + cm-I sec were found (cm refers to root length). This author stresses the fact that the K+ content of the tops rather than the K+ content of the roots has a decisive influence on the maximum K+ uptake rate. Thus in 17-day-old maize plants maximum K+ uptake rates ranged between 1.3 and 4.0 pmole cm-' sec - I according to the K + content of the tops; rates being low in tops with high K+ contents and vice versa. The K+ absorption rate of the root is highly dependent on the root metabolism and particularly on respiration and thus on the carbohydrate content of the root (Mengel, 1967). Generally, younger plants have higher root carbohydrate contents than older plants, and even young root tips of older plants are less capable of absorbing K+ than root tips of younger plants (Vincent et al., 1979). Plant species and even cultivars of the same species may differ in their capability of exploiting soil K+. These differences can be explained in terms of rooting pattern and root metabolism, although the whole complex of K+ uptake by field crops growing in soil is still only poorly understood. Halevy (1977) compared +

POTASSIUM IN CROP PRODUCTION

73

two cotton cultivars differing in their capability of exploiting soil K+. The cultivar with the higher uptake potential for soil K+ was found to maintain vigorous root growth to a later growth stage than the cultivar with the poor K+ exploitation capability. This result suggests that root growth at a later stage in plant development in the higher K+ uptake potential cultivar was the cause of the difference in K+ uptake. A spectacular difference in the K+-exploiting capability exists between grasses and legumes. The legumes are inferior to grasses, and when grown together grasses successfully compete with legumes for soil K+. If abundant K+ is not available, the legumes suffer from K+ deficiency, whereas the grasses still grow vigorously and compete strongly for K+ (Blaser and Brady, 1950). This effect may be explained in part by differences in the rooting patterns of the two plant groups; although this explanation is not completely satisfactory. In this context an experiment of Malquori et al. (1975) is of particular interest, in which wheat and lucerne were grown in nutrient solutions. In one treatment where the K+ source of the nutrient solution was biotite, wheat was able to “extract” K+, whereas lucerne was not. This shows that of the two plant species, interlayer K+ was only accessible to wheat. Similar results have been obtained by Steffens and Mengel (1979), who grew ryegrass and red clover on a soil low in exchangeable K+. It was found that ryegrass was more capable of feeding from the nonexchangeable soil K+ than was clover. One may speculate as to the mechanism by which grasses are more able to utilize this interlayer K+ . In this context the results of Baligar and Barber ( 1978a) are of particular interest. These workers observed that after the addition of Rb+ to a number of different soils the K/Rb ratio of corn plants grown on the soils was more similar to the K/Rb ratio of the exchangeable soil fraction than to the K/Rb ratio of the soil solution. In an analogous experiment with onions the K/Rb ratio in the plants was found to be between the K/Rb ratio of the exchangeable fraction and the K/Rb ratio of the soil solution. Baligar and Barber (1978b) discuss their results in terms of exchange diffusion as proposed by Jenny (1966). Tinker (1978) in commenting on Baligar and Barber’s results suggests that the effect might be related to H+ excretion by roots displacing K+ and Rb+ from adsorbing sites around the root. This question needs to be investigated further. Exchange diffusion from the interlayer K+ of clay minerals to the surface of plant roots does not seem a likely mechanism for K+ release. In neither the experiment of Malquori et al. (1975) mentioned earlier nor the work of Ristori (1973, in which clay mineral was added to a nutrient solution, was evidence provided that close contact between clay mineral and root is essential for exploiting interlayer K+ . It is possible that an extremely low K+ concentration in the soil solution achieved by a high rate of K+ uptake is associated with a net release of interlayer K+ . Research in this direction merits further attention. The results of Drews (1978) are encouraging in this line of approach for he

74

KONRAD MENGEL AND ERNEST A . KIRKBY

found that plants grown under energy stress were less capable of exploiting interlayer K+ than control plants. The control plants also depressed the K+ concentration of the soil solution to a greater extent, and this may have resulted in a higher K+ release by clay minerals. According to Barber (1979) plant roots can deplete the K+ concentration of the nutrient solution to as low a level as 2 pM K+. Clay minerals are also capable of absorbing K+ from plants. In the experiment already mentioned by Malquori et al. (1975) in which biotite was added to a nutrient solution supplying wheat, these authors observed an expansion of biotite to a 14-h; peak indicative of K+ release. The maximum of the peak was obtained at the flowering stage. However, in the period following flowering the 14-h; peak declined, and the authors suggest that K+ released by plant roots after flowering was again fixed by interlayer adsorption sites. This observation is consistent with experiments of Kurdi and Babcock (1970), who found that at low K+ concentrapA4 K+)K was released by the roots and fixed by a tion in the root medium (4 bentonite suspension. The question of whether the excretion of H+ by plant roots can mobilize soil K+ is not yet clear. Newman (1969) found that at a low pH (3.5 to 4.0) the release rate of Kt from biotite was about twice as high as from more neutral pH. If a plant root depresses the pH of the rhizosphere due to H+ excretion, the release of interlayer K+ may therefore be promoted. Net Ht excretion of roots occurs when plants are fed with NHi-N (DeJaegere and Neirinckx, 1978) or in the case of legumes, when they are exclusively dependent on N fixation as an N source (Israel and Jackson, 1978). As yet, however, it is still uncertain whether this process of H+ release can mobilize interlayer K+ in quantities of importance in plant nutrition.

Ill. POTASSIUM IN PHYSIOLOGY A . POTASSIUM TRANSPORT ACROSS BIOLOGICAL MEMBRANES AND

CATION

COMPETITION

Physiology may be considered as a sequence of biochemical and biophysical reactions in living systems. For some of these reactions there is a direct or indirect association with K+, and it is for this reason that the entry of K+ into living systems merits attention. Of all cation species K+ is known to traverse biological membranes most rapidly. It has been shown by numerous authors that K uptake by plant cells is closely associated with metabolism, and especially with root respiration. Whether K+ is actively absorbed as defined by transport against an electrochemical potential is not completely clear. The application of the Nernst equation to studying electrochemical equilibria of ions in roots in bathing solutions has established this for other nutrient ions.

POTASSIUM IN CROP PRODUCTION

75

All anions are transported and accumulated against an electrochemical gradient (Bowling et al., 1966; Higinbotham et d., 1967), whereas the cations Ca 2+ and Mg2+invariably move into roots down an electrochemical gradient. The use of the Nernst equation for K+ transport studies provides evidence of both active accumulation (Pitman and Saddler, 1967; Bowling and Ansari, 1971) and passive equilibrium (Higinbotham et al., 1967; Pallaghy and Scott, 1969). The results of Etherton ( 1963) also indicate that the K+ concentration in the external solution can determine the form of uptake. At low external K+, the internal content was higher than that predicted by the electrochemical gradient indicating active uptake, whereas at high external concentrations the internal K+ was less than predicted and an outpump was suggested. To some extent the apparent discrepancy in the above results reflects the small differences obtained between observed internal concentrations of K+ and those predicted by the Nernst equations. Bowling (1976) has drawn attention to the possibility that since K+ is so mobile in plant tissues, passive diffusion may mask the activity of a K+ pump. The use of the Nernst equation may therefore be an inappropriate test for active K+ transport in higher plants. In reviewing the literature Higinbotham (1973) suggests that although there is good evidence for active K+ influx by algae a clear-cut case for higher plants has still to be made. Recent experiments of Cheeseman and Hanson (1979) with corn roots have shown that K+ can be taken up against an electrochemical gradient. The authors assume that this active K+ uptake is brought about by an ATPase which is inhibited by higher K+ concentrations and thus works only at concentrations <5 mM K+. From the above comments it is not surprising that the mechanism of K+ uptake is still much a matter of speculation. Certain points, however, have been established. The uptake process is known to depend on metabolic energy. ATP generated by respiration or photosynthesis is believed to be closely linked with the transport of K+ across biological membranes. This view is consistent with the observation that K+ uptake and K+ retention by roots responds very sensitively to 0,supply (Hopkins, 1956; Mengel and Pfluger, 1972). Potassium uptake by green leaves is also light dependent as has been shown by Jeschke (1970) with leaves of Elodea densa, by Nobel (1970) with pea leaves, and by Jacoby et a f . (1973) with bean leaves. Energy may be used directly in active uptake to transport K+ against an electrochemical gradient or used indirectly as, for example, by inducing an electrical potential gradient across the plasmalemma down which K+ may move into the cell. It is also well established that K+ uptake is selective in relation to other cations. It is therefore generally accepted that K+ uptake involves the combination of a hypothetical carrier with a K+ ion. This is believed to take place in such a way that the intermediate ion-carrier complex formed at the outer part of the membrane is transported inward. At the internal side of the membrane the ioncarrier complex breaks down, and K+ is released inside the cell.

76

KONRAD MENGEL AND ERNEST A . KIRKBY

A carrier has yet to be isolated from cell membranes. However, within the last 15 years organic compounds have been discovered that are capable of transport-

ing monovalent cations across membranes, and some bind very selectively with

K+. These substances, which are antibiotics, are collectively called ionophores because of their property of acting as ion-carrying agents (Hinkle and McCarty, 1978). Two main groups have been categorized by Pressman (1968). The first group includes valinomycin, gramicidin, and the macrotetralide actins. These are all neutral and form complexes by acquiring the charge of the complexing cation. The second group includes nigericin, which contains a carboxyl group thus giving the molecule a negative charge, which is neutralized by the cation. The structures of all these compounds are extremely complex (Kilbourn et al., 1967; Dobler et al., 1969). All, however, have lipophylic properties, so they are soluble in the lipid matrix of membranes. By selectively binding with K+ they thus facilitate the transport of K+ across biological membranes. The binding is similar to that in the interlayer of micas. In both, the nonactin-K complex and the valinomycin-K complex, K+ is held at the center of a ball and bound by eight 0 atoms. These 0 atoms of the organic complex replace H,O molecules from the K+ ion hydration shell during the process of binding (Kilbourn et al., 1967). The degree of tightness by which the water molecules are held controls the capability of the organic ligand to form a complex cation. Water is held more strongly by the hydrated Na+ ion than by the hydrated K+ ion. The Na+ ion is therefore less readily able to form an Na complex, and it may be supposed that it is this difference by which living organisms are enabled to distinguish very sensitively between K+ and Na+. The considerably higher rates of uptake of K+ than Na+ by most plant species probably also depend on this difference in complex formation. The selective properties of valinomycin in transporting alkali ions across a synthetic membrane were investigated by Mueller and Rudin (1967). In agreement with the above concepts, valinomycin increased membrane permeability to K+ so that the rate of K+ transport was about 300 times higher than that of Na+ . Nigericin also selectively transports K+ but only in exchange for protons (Hinkle and McCarty , 1978). Such or similar compounds to those discussed above may be responsible for K+ selectivity in uptake by higher plants. If the process of K+ complex formation and K+ release were linked directly to metabolism to provide the energy for uphill transport against an electrochemical gradient, the system in effect would be that of active K+ carrier transport. On the other hand, passive K+ transport may also be brought about by these selectively K+-binding compounds by enabling K+ to move down an electrical gradient from the outer solution into the cell. An ion uptake model in which cation influx takes place down an electrochemical gradient has been proposed by Hodges (1973). As shown in Fig. 3, cation uptake is regulated by an ATPase in the plasma membrane. For each ATP +

POTASSIUM IN CROP PRODUCTION

77

KEar$ ar K'

K'

Carrier trcjnsport (Valinomycin)

ATP+ HO , ADP+ Pi

carrier NOj

FIG.3. Scheme of membrane-located ATPase-driving Kf carrier transport, facilitated K+ diffusion, and carrier-mediated NO, uptake.

molecule split by the ATPase, one H+ is produced which is extruded from the cytoplasm into the outer medium, and one OH- is generated in the cytoplasm. This process induces an electropotential gradient across the membrane, the cytoplasm being more negative compared with the outer medium. The cytoplasm thus attracts cations from the outer medium. Most cation species are probably absorbed by this mechanism, which per se is nonselective. However, as already mentioned, the presence of such substances as nonactin or valinomycin in the membrane could induce selective uptake by preferential binding with K+ in the downhill transport. This passive selective uptake has been called facilitated diffusion. The findings of Ratner and Jacoby (1 976) indicate that the high rate of K+ uptake by plant cells can be accounted for in terms of an ATPase-driven facilitated diffusion of K+ . There is considerable experimental support for the uptake mechanism of K+ as described above. As required in the Hodges scheme, all living plant cells are negatively charged with respect to the outer medium (Dainty, 1962). There is good evidence too that the plasmalemma contains ATPase (Hodges et al., 1972). Fisher et a / . (1970) observed a highly positive correlation between ATPase activity and ion uptake. A striking similarity between the kinetics of ATPase action and the kinetics of K+ absorption by oat roots has also been reported by Leonard and Hodges (1973). Recently an auxin involvement in K+ uptake has been suggested. Erdei et al. ( 1 979) found that auxins stimulated K+ uptake as

78

KONRAD MENGEL AND ERNEST A. KIRKBY

well as ATPase activity. These authors concluded from their in vivo and in vitro experiments with young rice roots that ATPase is directly involved in the K+ uptake process. Travis and Booz (1979) came to the same conclusion in experiments with meristematic and mature root tissue of soya. Thus K+ uptake is directly or indirectly associated with the electrogenic H+ pump (ATPase). Whether ATPase itself functions as a K+ carrier is still a matter of speculation (Poole, 1978). In addition to the carrier type of K+ transport across biological membranes already discussed, another mechanism of ion diffusion involving membrane pores has also been suggested. Such pores can be formed by macrocyclic antibiotics including gramicidin A (Mueller and Rudin, 1967) and may allow “tunnel transport” of cations. In the case of gramicidin A, the pores are each made up of two helical molecules and are permeable to ions with one positive charge (Stryer, 1975). There is now a considerable body of experimental information which strongly suggests that membranes contain selective K+ carriers as well as pores which enable a less selective K+ uptake by tunnel transport. The dual pattern of K+ uptake which Epstein (1966) has described as mechanism I and mechanism I1 may be explained in these terms. Mechanism I probably describes the selective carrier transport, and mechanism I1 the less selective tunnel transport. Cation efflux may also occur through membrane pores, although the process does not appear to be selective. Mengel and Pfluger (1 972) observed similar rates of release of K+ and Na+ from corn roots, whereas in the same tissues the K+ uptake rate was about 10 times higher than that of Na+. In the Hodges’ model anion uptake takes place by exchange for the OHgenerated in the cytoplasm as a result of the ATPase-driven H+ extrusion (see Fig. 3). An additional source of cytoplasmic OH- also arises after the uptake and assimilation of NO; and to a lesser extent also of S q - assimilation. NO; SO:-

+ 8H+ + 8e+ 8H+ + Re-

NH, + 2H20 + OHSH2 2 b O 20H-

+

+

This continuous OH- production resulting from NO-, assimilation may directly provide a means for sustained NO, uptake without accompanying cation uptake by NO;/OH- exchange between the root and the nutrient medium. On the other hand, the OH- produced may stimulate the accumulation of an organic acid anion in the plant via the PEP carboxylase mechanism (Hiatt, 1966; Davies, 1973) with accompanying cation uptake (Kirkby, 1968; Kirkby and Knight, 1977) to balance the organic anion charge. It is also possible that organic acid anions formed in this way may be oxidatively decarboxylated to again give rise to an internal source of OH- to drive anion uptake independent of cation uptake as already described. This mechanism has been considered in relation to longdistance K+ transport and is discussed later. The influence of the accompanying anion such as NO; or C1- on the uptake of

POTASSIUM IN CROP PRODUCTION

79

K+ by young barley plants was studied by Blevins et al. (1974). The results obtained in this experiment are consistent with the concepts discussed above. The uptake and accumulation of K was greater in the NO ;-treated plants particularly toward the end of the experiment (4-36 hours). The authors suggest that one of the main reasons for this finding was a more rapid sustained uptake of NO; which provided a mobile counter-anion for K+ transport. In addition, the synthesis and accumulation of organic acid anions by NO; reduction increased the capacity for K+ accumulation by providing a nondiffusible organic anion source. Similar enhancing effects of NO, on the uptake of K+ and other cations by tomato plants have been observed by Kirkby and Knight (1977). The influence of NO, assimilation in providing a major source of OH- to induce further NO, uptake without accompanying cation uptake, by NO,/OH- exchange, has been reported recently by Kirkby and Armstrong (1980) in the castor oil plant (Ricinus cornmunis). This process also appears to predominate in grasses and cereals where the uptake of anions is about twice that of cations (Kirkby, 1974). The above findings are in agreement with the observation of Johansen and Loneragan ( 1975) who suggest anion-dependent and anion-independent components in the K+ uptake process. The proposed model for cation uptake as considered above is also of relevance to the well-known phenomenon of cation antagonism, which is more precisely described by the term cation competition. Numerous experiments on growing plants in complete nutrient solutions have shown that increasing the concentration of K+ in the outer solution depresses the uptake of other cation species. The reverse effect of other cation species on K+ uptake has also been observed. Since there is little evidence that the uptake of Ca2+, M$+, and Na+ is carrier mediated, it is unlikely that the depression of uptake of these cation species results from a competition for a common carrier site. A more likely explanation for cation competition is that all cation species are attracted by the negative electropotential of the cell, which is continuously being regenerated by H+extrusion and by NO, uptake and assimilation. Cation species that can traverse the plasma membrane more easily, for example, by facilitated diffusion, have a greater chance of saturating the continuously generated negative potential of the cell than cation species for which the plasma membrane represents a banier to uptake. A cation species that enters the cell rapidly thus depresses the uptake of other cation species. Such a mechanism should result in nonspecific cation competition (Mengel, 1973). Competition is nonspecific, because all cation species are involved and the extent of uptake depression depends mainly on the concentration of the cation species in the outer solution and the permeability of the membrane for the individual cations. Most plant species absorb K+ at a rather high rate as compared with the uptake rate of other cation species. From the model described above, therefore, K+ +

80

KONRAD MENGEL AND ERNEST A. KIRKBY

would be expected to be a strong competitor in uptake with the other cation species. This has been demonstrated in numerous experiments (Smith and Robinson, 1971; Mengel and Nemeth, 1971). Nonspecific cation competition becomes especially evident when plants are grown with a suboptimal supply of K+. Such an experiment was conducted by Forster and Mengel (1969) in which the K supply to young barley plants (tillering stage) growing in solution culture was interrupted for a period of one week. This interruption resulted in a dramatic increase in the uptake of the other cation species. The main result of this experiment is given in Table I. Although the cation sum was hardly changed by the K+ interruption, the content of various cation species was much altered. This example demonstrates that in the absence of K+, the uptake of other cation species is considerably enhanced. Similar results have been obtained by Terry and Ulrich (1973a) with leaves of sugar beets and by Dijkshoorn ef al. (1974) in experiments with rice plants. Most plant species absorb K+ at a higher rate than Na+; but there are some, such as Beta species and spinach, that also take up considerable amounts of Na+ (Marschner, 1971). In these species Na+ may partially substitute in the biochemical or biophysical functions of K+. The Na+ uptake mechanisms of these natrophilic species are not yet understood. One may suggest, however, that where Na+ is taken up in such large amounts, this may be achieved by facilitated diffusion through membrane channels formed by gramicidin A or similar compounds. In the carrier-type uptake, the carrier, for example, valinomycin, enniatin A, or related compounds, may act as a shuttle between one side of the membrane and the other. Potassium uptake and K+ release of the carrier should thus be controlled by the electrochemical potential gradient across the membrane. Net K+ uptake should be dependent on the K+ concentration in the cytoplasm, because the net K+ release rate of the carrier at the inner side of the membrane Table I Effect of Interrupted K Supply on the Cation Content (meq/100 g dm) of Roots and Shoots of Young Spring Barley Plants" Roots

Shoots

Control

Intemption

Control

Intemption

K Ca Mg Na

157 9 36 3

28 12 74 78

170 24 54

Traces

52 66 121 12

Sum

205

I92

248

25 1

Forster and Mengel (1969).

POTASSIUM IN CROP PRODUCTION

81

should be depressed if the K+ concentration in the cytoplasm is high and vice versa. Such a relationship between the K+ concentration in the plant and the net rate of K+ uptake has been reported by several authors (Hoffmann, 1966; Johansen et al., 1970; Barber, 1979). The observation that the K+ efflux rate of plant roots is higher if K+ is present in the outer solution (Mengel and Pfliiger, 1972) also supports the concept of a specific K+ carrier working as a shuttle in which K+ loading and unloading is dependent on the electrochemical potential gradient across the membrane. According to Zimmermann (1978), K + uptake is also regulated by the osmotic pressure of the cell, higher turgor pressure decreasing the uptake rate and vice versa. Since a high cell K+ content is generally associated with a high turgor, K+ uptake may also be subjected to osmoregulation. From kinetic studies on uptake it appears that competition does occur for the carrier cation binding sites for chemically closely related cation species such as Ca2+ and Sr2+,Rbf and K + . Organic cations may also react with the carrier binding site. Lepe and Avila (1975) thus reported that alkylguanidines considerably depressed K+ uptake by excised barley roots. The depression in uptake was greater, the longer the alkyl chain. The authors therefore suggest that the positive site of the guanidine complex reacts with the carrier binding site, whereas the lipophilic chain is bound to the membrane. Whether the bulk of NH :-N absorbed by plants is transported through biological membranes by a carrier-type mechanism as outlined above, is in question. For such a mechanism to be operative one might expect a marked competition for binding sites with K +. The uptake of NH: should therefore be depressed by K + . Mengel et al. (1976), however, were not able to detect any such depressing effect of K when studying NH:-N uptake by young rice plants. +

B . CELLTURGOR A N D WATERECONOMY OF PLANTS

A high rate of K+ uptake by root cells depresses the osmotic potential in the cells, and this induces water uptake. The uptake of water by roots and the ability of the plant to exploit soil water therefore depend on the K+ nutritional status of the plant. Water transport into the xylem vessels is also mainly an osmotic process in which K+ in its function as an osmoticum is very important. Electron probe analysis of Lauchli et al. (1971) has shown that parenchyma cells may accumulate K+ to a high extent and that K+ is secreted into the xylem. During this process K+ has to traverse the membrane that separates the symplasm of the living stelar cells from the free space of the conducting vessels. Whether K+ leaks out passively from the symplasm into the xylem vessel or whether it is actively transported across this membrane is not yet clear. It is feasible that this K+ transport mainly occurs by facilitated diffusion dependent on ATPase activity

82

KONRAD MENGEL AND ERNEST A. KIRKBY

and active anion transport. Lauchli (1972) holds the view that the K+ transport is active. This is supported by results that have shown that the xylem parenchyma cells contain numerous plasmodesmata and pits forming a plasmatic pathway across the stele up to the vessels (Lauchli et al., 1974). Whatever the mechanism of K+ release into xylem vessels, K+ is the important osmoticum which drives the water flux from the surrounding cells into the xylem vessels (Baker and Weatherley, 1969). Only in plant species that absorb Na+ in considerable amounts can K+ be substituted in this osmotic function by Na+. The root pressure, which can be of importance for the upward movement of organic and inorganic solutes, is much controlled by the K+ nutritional status of plants. If K+ is low or absent in the root medium, both the quantity of water moved upward by root pressure and the concentration of a number of solutes such as nitrate and amino acids in the xylem sap are considerably depressed. Such observations have been made by Mengel and Simic (1973) in experiments with decapitated sunflower plants. Potassium plays a spectacular role in stomatal opening and closure (Fischer and Hsiao, 1968). Convincing evidence for this essential K+ effect has been provided by electron probe analysis studies of Humble and Raschke (1971) which showed that the increase in turgor in the guard cells associated with stomatal opening resulted from an increase in K+ concentration in the cells. Under light conditions, photophosphorylation seems to provide the ATP required for pumping K+ into the guard cells (Humble and Hsiao, 1969; Turner, 1972). The K+ accumulation is associated with an accumulation of malate which appears to be the major anion charge balancing the accumulated K+. Thus under dark conditions Allaway (1973) found very little malate in the guard cells of Vicia faba, whereas on exposure to light there was a rapid increase in the malate concentration. Similar results have been found by Raschke and Schnabl(l978) who showed that when KCl was applied a proportion of the guard cell K+ was balanced by Cland the rest by malate. Abscisic acid prevents stomatal opening and simultaneously inhibits K+ uptake by guard cells (Horton and Moran, 1972). Fusicoccin, on the other hand, which is a phytotoxin, promotes the uptake of K+ by guard cells and thus induces stomatal opening (Turner, 1972). Since fusicoccin stimulates ATPase selectively (Giaquinta, 1979), it is likely that the K+ uptake is closely related to ATPase activity. The role of K+ in stomatal opening is very specific, and other univalent cations are generally unable to replace K+ in this function, except in a few plant species, for example, Kalanchoe marmorata where Na+ drives the stomatal opening mechanism. The subject of ion transport in stomatal guard cells from the viewpoint of the chemiosmotic theory has recently been considered by Zeigler et al. (1978). Potassium is also a major osmotically active component in other plant cells contributing to cell turgor and enhancing the capacity of plant cells to retain

POTASSIUM IN CROP PRODUCTION

83

water. In this function K+ seems to be of particular importance in young tissues. Thus Arneke (1980) found that the turgor of older leaves of Phaseolus vulgaris was hardly influenced by K+ nutrition, whereas in the younger leaves, the turgor was dependent on the K+ supply to the plants. In the low K+ treatment the turgor of young leaves showed an average value of 5.05 bar which differed significantly from the average turgor (7.17 bar) obtained in the young leaves of the high K+ treatment. This experiment also showed that a first sign of inadequate K+ nutrition was the higher dry weight content of the plant tissues. Turgor appears to be the most sensitive parameter indicating K+ nutritional status. Other K+-related processes such as COz assimilation, phosphorylation, and protein synthesis are less sensitive in registering inadequate K+ supply. Arneke ( 1980) found that the turgor in young leaves had a direct effect on the size of cells and on the growth rate of the entire plant. This work indicates that a high turgor in meristematic cells is a prerequisite for cell expansion, a process which precedes cell division. According to Arneke’s results, K+ seems to be an indispensable osmoticum in young leaves of P haseolus vulgaris. This finding is well supported by experimental findings of Green and Muir (1978) who observed that the expansion of cucumber cotyledons was dependent on the K+ supply. It is feasible that in the natrophilic plant species this osmotic effect of K+ may also be brought about by Na+. Marschner and Possingham ( 1975) thus found that K+ as well as Na+ promoted cell expansion and the production of chloroplasts in the leaves of sugar beet and spinach. The overall effect of K+ on the water economy of plants results from the process cited above and probably also from processes that are not yet known or well understood. This beneficial effect of K+ is of particular importance in practical crop production, since K+ reduces water losses by transpiration (Brag, 1972), so that more organic matter can be produced per unit water consumed by a crop well supplied with K+ (Blanchet et al., 1962; Linser and Herwig, 1968). C. ENERGYMETABOLISM

The basic process of energy metabolism-the conversion of radiation energy into chemical energy-is much controlled by the K+ status of the plant. The beneficial influence of K+ on phosphorylation has thus been reported by various researchers using different plant species (Hartt, 1972; Watanabe and Yoshida, 1970; Pfliiger and Mengel, 1972). Photoreduction (production of NADPH) is also promoted by K+ (Pfliiger and Mengel, 1972). Indirect evidence for this effect has also been provided by experimental findings of Weller and Hofner (1974) and Overnell (1975). Both publications report that K+ increases the photosynthetic release of Oz. The specific function of K + in the energy conversion process is not yet

84

KONRAD MENGEL AND ERNEST A. KIRKBY

completely understood. However, it is recognized that K+ is involved in metabolic reactions including those of energy (ATP), synthesis, and energy transfer. Metabolic energy is generated by the chloroplasts in the process of photophosphorylation. Light-driven electron transport releases protons from the stroma into the inner space of the thylakoid compartment of the chloroplasts (Trebst, 1974). Cations and especially K+ are moved out into the stroma of the chloroplast in exchange for this inward movement of protons. According to Lauchli and Pfluger (1978), K+ in a concentration range of about 100 mM seems to be necessary for high efficiency in energy transfer. Of the other cations Mgz+ is particularly important (Barber, 1977). Portis and Heldt (1 976) have proposed that the light-driven uptake of Mg2+ from the inner space of the thylakoids to the stroma is enough to “switch on” RuBP carboxylase by increasing its affinity for C02. Potassium also promotes CO, fixation but this occurs according to Peoples and Koch (1979) not by direct activation of RuBP carboxylase but rather by favoring the synthesis of this enzyme. The fluxes of protons, electrons, and cations across the thylakoid membrane as described above constitute the major features of the Mitchell (1966) chemiosmosis hypothesis of ATP synthesis. The same is true for charge transfer across the inner mitochondrial membrane. However, in the case of oxidative phosphorylation which occurs during respiration in the mitochondria, the direction of charge transfer is reversed in the synthesis of ATP. Again a proton concentration gradient and an electrical potential difference across the membrane drives ATP synthesis, but in this case the protons are moved from the inside to the outside of the mitochondria. Potassium and calcium thus move inward in exchange for protons. It is generally accepted that K+ plays an analogous role in mitochondrial as in chloroplast energy turnover. However, convincing evidence for the role of K+ in mitochondrial energy transfer is lacking. In the older literature there is some evidence of a beneficial effect of K+ on oxidative phosphorylation (Latzko and Claus, 1958; Latzko, 1961). Jackson and Volk (1968) have also presented data which show that under conditions of suboptimal K + supply a greater proportion of ATP is produced by respiration to compensate for low production under conditions of poor photophosphorylation. This finding is consistent with recent results of Peoples and Koch (1979), who observed an inverse relationship between the rate of dark respiration and the K content of alfalfa leaves. The promoting effect of K+ on photosynthetic ATP synthesis and NADPH production has a general impact on various energy-requiring processes in plant metabolism. The effect of K+ on CO, assimilation has been observed by numerous authors (Ilyashouk and Okanenko, 1970; Estes et al., 1973; Terry and Ulrich, 1973b). Other beneficial influences include those on protein synthesis (Jeanniot et al . , 1970; Hsiao et al., 1970; Koch and Mengel, 1972), on N2 fixation by Rhizobium bacteria (Mengel et al., 1974; Feigenbaum and Mengel, 1979), on amino acid synthesis (SeGer, 1978), on synthesis of vitamin C (Werner, 1957), and on long-distance transport. All these effects can be explained

POTASSIUM IN CROP PRODUCTION

85

in terms of K+ enhancing the production of ATP or NADPH, or both of these compounds. It is well known that K+ activates numerous enzymes, a subject that will be considered in more detail below. The beneficial effect of K+ on protein synthesis may thus originate from K+ enzyme activation, and frequently such effects have been interpreted in these terms. Studies with entire plants, however, often do not allow the distinction of the cause of this beneficial effect between direct enzyme activation and an increase in energy supply associated with higher levels of K nutrition. In this context the findings of Seqer (1978) are of particular interest. In an experiment with spring wheat growing at two levels of K nutrition, she studied the amino acid turnover of the grains during the grain filling period. In the higher K treatment the content of soluble amino acids in the grains during the first weeks of grain filling was about twice as high as that of the low K treatment. In addition the incorporation of amino acids into the grain proteins and especially the turnover of glutamate occurred at a much higher rate in the high K plants. In this treatment too the K+ content was higher in both the roots and the culms. However, no major difference between treatments was observed in the K+ contents of the grain. For this reason it seems unlikely that the higher protein synthesis in the high K treatment was a consequence of K+-stimulated protein synthesis. It seems more feasible that the high level of K nutrition enhanced amino acid translocation from the vegetative plant parts to the grains and that this higher import of amino acids into the grains promoted protein synthesis. This example demonstrates that the beneficial effect of K+ on long-distance transport of assimilates may be of greater importance for protein synthesis than the effect of K+ on enzyme activation. Insufficient K+ in the plant may often result in an increase in protein content of vegetative plant material. This has been observed by Hsiao er al. (1970) in corn seedlings and by Mengel and Koch (1971) in young sunflower plants. The effect cannot be ascribed to improved protein synthesis under suboptimal K+ conditions, for under such conditions the rate of protein synthesis is restricted, as was shown in experiments of Koch and Mengel (1972, 1974) using labeled N. The increased protein content is a secondary effect of suboptimal K+ supply. Hsiao et al. (1970) demonstrated that the growth process was more closely related to the K supply than was protein synthesis. This explanation is consistent with the observations that cell turgor and thus cell expansion are the most K+-sensitive processes. If the growth rate is depressed more than protein synthesis, an accumulation of protein occurs. D. LONG-DISTANCE TRANSPORT

Potassium is known to be very mobile in an upward and downward direction in the entire plant. A high rate of translocation occurs in the xylem because of the

86

KONRAD MENGEL AND ERNEST A. KIRKBY

rapid rate at which K+ is selectively secreted into the root xylem vessels. Of all (.is also by far the most abundant in the phloem sap, where the cation species, I it may reach concentrations of 100 mM and more (Hall and Baker, 1972). This finding indicates that K+ is selectively absorbed by the sieve tubes, and it also explains why K+ can so easily be translocated from the upper plant parts to the basal plant organs and roots. Ben Zioni et al. (197 1) have suggested that K+ circulation in the entire plant is of significance for the upward translocation of nitrate. From their investigations with Nicoriana rustica they concluded that K+ acts as the main counter-ion for the upward translocation of nitrate. These workers propose that on reduction of nitrate in the upper plant parts an equivalent of malate is formed, and some of the K originally accompanying the NO; is then transferred together with malate via the phloem to the roots. Here the malate is oxidized and decarboxylated, and the HCO 3 produced is released into the nutrient medium in exchange for uptake of an NO 3 ion. The K remaining in the root together with this NO ;is then transported upward and the cycle repeated. For the Ben Zioni et al. (1971) K-recirculation scheme to operate, two criteria must be met. In the first place NO; reduction must take place largely in the tops. Second, the OH- resulting from this NO; reduction must be transferred to the nutrient medium in exchange for a net excess anion over cation uptake. From this it follows that in plant species such as tobacco and tomato in which the uptake of anions is not greatly in excess of that of cations, when supplied with NO;-N, that the scheme is probably of little importance. In the case of tomato, for example, about 90% of the OH- charge arising from NO; reduction in the tops is retained in situ as organic acid anions in association with inorganic cations (Kirkby and Knight, 1977). On the other hand, a number of plant species including grasses and cereals do take up a considerable excess of anions over cations (Dijkshoorn, 1962; Kirkby, 1974). Evidence supporting this hypothesis has thus been provided by Blevins er al. (1978), who compared barley seedlings well supplied and poorly supplied with K+. The shoots of the low K+ seedlings had much lower NO; concentrations and lower NO;-reductase activities than the roots, suggesting that K+ plays a major role in NO; translocation. Further support for this scheme has been demonstrated by Kirkby and Armstrong (1980) in the castor oil plant (Ricinus communis). In this species the uptake of anions is again approximately twice that of cations when the plants are fed with NO;-N. Evidence of K recirculation was concluded from an experiment in which a low and high level of NO; nutrition were compared, keeping the cation concentrations in the nutrient solution constant by compensating for the difference in NO 3 concentration by SO:-. In both NO; regimes, the reduction of NO; took place largely in the tops. In agreement with the Ben Zioni et al. (197 1) model the dominant ions in the xylem sap were NO,, K+, and Ca2+,whereas +

+

POTASSIUM IN CROP PRODUCTION

87

those in the phloem were K and organic anions. At the higher level of NO 3 nutrition, N uptake was increased threefold but there was no influence on K uptake. It was therefore concluded that the increase in upward NO, transport was facilitated by K recirculation. Whether K+ is transported in the phloem with malate is still an open question. Mengel and Haeder (1977) reported much lower concentrations of malate (approximately 1 mM) than K+ in the phloem sap of Ricinus communis, a finding consistent with the data of Smith (1978). Since the pH of phloem sap is high (pH 8-9), it would be reasonable to assume that amino acids may provide negative charge to balance the K+ charge. The question of which amino compounds act as counter-ions in K+ transport in xylem and phloem still needs to be investigated. The use of a legume plant would be particularly useful in this respect because it would allow the comparison to be made between NO,, NH:, and molecular N2 fed plants. The significance of an antiport (OH- exchange) as compared to a symport (associated cation uptake) mechanism for NO, uptake has been suggested by Cram (1 976) to relate to the osmotic pressure generation in the plant. Cram argues that if all the N were taken up as KNO, and one K + and one organic acid produced per N reduced to organic form, as in tobacco or tomato, the osmotic pressure of the cells of the Grumineae might be twice the values observed. The switch to NO, uptake without accompanying cation uptake appears to prevent the osmotic pressure from rising and may hence regulate internal osmotic pressure. It is of interest that those plant species which do take up nitrate by an antiport mechanism also take up a relatively high proportion of their cations as K+. For plants taking up ions by the symport mechanism, the cation uptake is higher but so too are the proportions of Ca2+and Mg2+,two cation species that may to a large extent form insoluble salts and thus not contribute to internal cellular osmotic pressure. This difference in behavior lends further support to the Cram ( 1976) internal osmotic regulation hypothesis. The beneficial effect of K+ on the long-distance transport of photosynthates has been shown by several authors for various plant species: by Amir and Reinhold (197 1) for beans, by Hartt (1970) for sugarcane, by Ashley and Goodson (1972) for cotton, by Ilyashouk and Okanenko (1970) for sugar beet, by Haeder et ul. (1973) for potatoes, and by Mengel and Viro (1974) for tomatoes. Barankiewicz ( 1978) reported that under the conditions of suboptimum K+ nutrition, the turnover of C from malate and aspartate to sugar phosphates in corn leaves was affected. From these results it is concluded that K+ promotes the transport of malate and aspartate from the mesophyll tissue toward the bundle sheath cells of C-4 plants. The main organic constituents of the phloem sap consist of sugars, mainly sucrose, and amino acids. For this reason the beneficial effect of K+ on phloem transport also has a direct impact on the long-distance transport of N. This can be +

88

KONRAD MENGEL AND ERNEST A. KIRKBY

of importance for the utilization of N in crop production. Koch and Mengel (1977) thus showed that N stored in vegetative plant parts of wheat could be used for the grain production of wheat to a greater extent when the plants were well supplied with K+ than when they were not. Magnesium transport in the phloem also appears to be promoted by K+. The Mg contents of flax seeds (Linser and Herwig, 1968), of potato tubers (Addiscott, 1974a), and of tomato fruits (Viro, 1973) were thus increased when the level of K nutrition was raised. This finding is of particular interest since generally the Mg content of plant organs is adversely influenced by K+ nutrition as a result of cation competition. The data of Viro (1973) show that although cation uptake competition occurred in tomato plants, retranslocation of Mg2+ from roots, stems, and leaves to the fruits was promoted by K+. The higher K+ treatment thus resulted in a greater MgZ+ concentration in the tomato fruits. The beneficial effect of K+ on phloem transport has also been found where CO, assimilation was not significantly influenced by K+ . This shows that the K+ status of plants is registered more sensitively by phloem transport than by CO, assimilation. The mechanism by which K+ influences phloem transport is not completely understood. Addiscott (1 974b) has discussed several concepts that could explain the positive influence of K+. He concluded that K+ probably favors the process of phloem loading. Mengel and Haeder (1977) in studying the composition of phloem sap in Ricinus communis found that the level of K nutrition did not greatly influence the concentration of phloem sap solutes. In particular the concentrations of the major solutes, sucrose and amino acids, were not significantly altered by different K treatments. The most important finding of this experiment was that K+ raised the flux rate of phloem sap considerably. The authors suggest that K+ stimulates ATP synthesis by its beneficial effect on photophosphorylation. This in turn, they argue, enhances the phloem loading process so that the higher rate of phloem loading also results in a higher rate of phloem flux. This explanation agrees well with the phloem loading concept of Giaquinta (1977) who has suggested that the process of sugar transport across biological membranes is driven by an ATPase. Recent research data of Travis and Booz (1979) provide evidence that the plasma membrane-bound ATPase is activated by K+ . The beneficial effect of K+ on phloem loading could thus also be related to K+ activation of the plasma membrane-bound ATPase. Malek and Baker (1977) have proposed a direct relationship between K+ and the sugar uptake mechanism of the phloem. In this scheme H+ efflux is associated with the uptake of K+, an exchange process similar to that across the thylakoid membranes. The uptake of K+ gives rise to an equivalent release of protons into the apoplast. This raises the H+ concentration at the outer side of the plasma membrane, which according to Giaquinta ( 1977) promotes phloem loading with sugars. This effect of increasing the H+ concentration in the apoplast in

POTASSIUM IN CROP PRODUCTION

89

promoting phloem loading with sugars has been shown experimentally in Ricinus cotyledons by Hutchings (1978). Since phloem loading with amino acids is also enhanced by K+ (Koch and Mengel, 1977), it will be of interest to see whether the transport of amino acids through the plasma membrane of sieve tubes is also brought about by a similar mechanism. Very recent research data of Doman and Geiger ( 1 979) show that the release of photosynthates from the mesophyll cells into the apoplast is dependent on the K+ status of the leaf tissue. In their experiments they were not able to find a direct beneficial effect of K+ on phloem loading. These authors claim that it is the release of photosynthates from the mesophyll cells into the apoplast rather than the phloem loading process itself that is promoted by K+. As most photosynthates have to pass through the apoplast before being collected in the sieve tube companion cell complex, enhanced export of photosynthates out of the mesophyll cells also results in a higher rate of phloem loading. In recent years it has become increasingly clear that hormonal effects may control the movement of nutrients in plants. Such an effect may well account for the findings of Pitman (1972) from an experiment with barley plants in which the growth rates of the plants were varied considerably by altering the photoperiod. It was observed that the net uptake of K by the roots and the transport in the shoots closely reflected changes in shoot growth. Pitman (1972) suggests that the uptake of K+ was regulated by a “feedback” mechanism between roots and shoots, in which the translocation of growth substances may be involved. Direct evidence of growth regulators on the movement of nutrients in intact plants is difficult to obtain because of the problem of interpretation, since the effects of hormones on growth and metabolism may obscure effects on transport. A detailed investigation on the fluxes of K+ in the root of an intact corn plant has been carried out by Richter and Marschner (1973). Influx rates were found to be similar along the length of the primary root. Owing to the higher demand of the growing tip, however, an additional K+ flux occurred toward this site from the basal parts of the primary roots. Newly developed lateral buds were also found to act as sinks for K+. The control of K+ transport within the root by growth substances has been demonstrated by the results of Luttge et al. (1968) for gibberellic acid. Cram and Pitman (1972) have also shown that abscisic acid influences long-distance K+ transport. The results of Mansfield and Jones (1971) indicate that in guard cells ABA increases the permeability of the cell plasmalemma thereby increasing K+ uptake. Collins and Kemgan (1974) also observed that very dilute solutions of ABA (10-8-10-9 M) added to a bathing solution containing detached maize roots increased both the flux of exudate and of K+ into the xylem. Kinetin had the reverse effect. The mechanism by which such hormonal effects operate in the long-distance transport of K+ has yet to be established. +

90

KONRAD MENGEL AND ERNEST A. KIRKBY

E. ENZYMEACTIVATION

More than 60 different enzymes are known that require univalent cations for activation. In most cases K+ is the most efficient cation species in this activation process. This question of enzyme activation has been thoroughly treated by Evans and Sorger (1966), Wilson and Evans (1968), and Evans and Wildes (1971). For this reason the biochemical aspect of enzyme activation by K + is considered here only briefly. A number of K+ specific enzymes are equally activated by NH: and Rb+ under in vitro conditions. This finding supports the concept that the ion radius or the energy of dehydration or both are of direct importance for the activation mechanism. In vivo, however, NH: and Rb+ cannot substitute for K+ in activating enzymes, since these ions are toxic at the concentrations required (El-Sheikh and Ulrich, 1970; Morard, 1973). Potassium activates different groups of enzymes. For crop production the synthates such as starch synthase, phosphorylase, and ADP glucose pyrophosphorylase (Marschner and Doring, 1977; Hawker et a f . , 1979), as well as the enzymes involved in protein synthesis are of particular interest. The general finding that inadequate K+ nutrition results in the accumulation of low molecular weight sugars and amino acids (Okamoto, 1967; Ratner and Yeliseova, 1968; Nowakowski, 1971) in plant tissues may be explained in terms of depressed enzyme activity. In this context, however, it should be remembered that plants poorly supplied with K+ can suffer from a lack of ATP, which in turn can also result in impaired synthesis of polymers such as protein, starch, cellulose, and even nucleic acids. In vitro experiments have shown that maximum K+ activation is obtained within a concentration range of between about 40-80 mM K. Besford and Maw (1976), for example, report optimal activity of pyruvate kinase in vitro at about 45 mM K in fresh tomato leaf tissue. Their findings show that this value is in excess of that required for optimal succinyl CoA synthetases in the same tissue. In many plant tissues it appears that K+ may be present in relatively high concentrations in relation to enzyme activation requirements. Experiments of Pierce and Higinbotham (1970), for example, indicate that the K+ concentration in the cytoplasm of Avena coleoptile cells is in the range of 140-215 mM. Even in K+-deficient tissues the K+ concentration may appear quite high. In an experiment with young K+-deficient bean leaves, Arneke (1980) reported values of 50-70 mM K+ in the press sap, and it seems highly probable that the K+ concentration in the cytoplasm of these leaves was higher. From this evidence it would appear that even under conditions of a suboptimal K+ supply, the K+ concentration for optimum enzyme activation may still be adequate. This implies that for registering inadequate K nutrition, enzyme activation by K+ is a relatively insensitive parameter as compared with K+-stimulated ATP synthesis or cell expansion. This hypothesis needs further investigation, but

POTASSIUM IN CROP PRODUCTION

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it is well supported by available research data. Its practical consequence is that impairment of enzyme activation by a lack of K+ is unlikely to limit the crop production process. A very good practical example illustrating this point has already been considered in discussing the work of Seser (1978) (see Section 111,

C).

IV. POTASSIUM APPLICATION AND CROP GROWTH A. CROPRESPONSEA N D POTASSIUM APPLICATION

In agricultural practice it is important to know for any given location whether K fertilizer application can significantly increase crop yield. Numerous field experiments have therefore been carried out to investigate crop responses on particular soils. However, it is difficult to extrapolate these results in order to predict fertilizer recommendations because of the many and varying factors influencing the growth of a crop at a particular site. Clearly, though, such generalizations must be made since it is quite impracticable to carry out trials on all the numerous combinations of crops and soils. The main factors influencing crop response to K fertilization are available soil K + , soil moisture, other growth factors, and the particular crop species under consideration. The common practice of expressing K availability in terms of exchangeable K+ is one immediate problem that must be recognized. As already outlined in Section I1 “exchangeable K + ” is not a good indicator of K availability. The equilibrium K+ concentration in the soil solution and the K+ buffer capacity are more reliable parameters for assessing the rate of K+ supply to plant roots in the soil medium. On sandy soils small applications of K+ increase the K+ concentration in the soil solution appreciably and may thus result in substantial yield increases (Mengel and Aksoy, 1971). On more highly textured soils, however, K+ fertilizer applications may scarcely influence K+ concentration in the soil solution, so yield responses are often not obtained. In such soils it must be established whether a lack of response is due to an already high enough level of available K+ in the soil or to too low a K fertilizer application. Generally soils rich in 2:1 clay minerals contain fairly high amounts of exchangeable K+ associated with relatively low K+ concentration in the soil solution (Nemeth et ul., 1970).Solution K+ may thus become a limiting factor in supplying soil K+ to plants (Jankovic and Nemeth, 1974). On such soils therefore the application of K fertilizers must be high enough to increase the K+ concentration of the soil solution if yield increases are to be obtained (Mengel, 1974). v. Braunschweig (1979a) has established from numerous field experiments carried out in West Germany that approximately optimum K+ concentrations in the soil solution are

92

KONRAD MENGEL AND ERNEST A. KIRKBY

obtained when the relationship between lactate-soluble K+ and soil clay is as shown in the following equation: Lactate soluble K+ (ppm)

= %

clay x x

where x is a value between 12 and 20. The value for x increases as the % clay content falls. “Lactate soluble K+ differs from “exchangeable K+,” but the exchangeable K+ value can be used in an analogous equation to that shown above. The equation is valid only in soils in which the clay fraction is made up predominantly by 2: 1 clay minerals. If soils do not differ too greatly in clay content, the value for exchangeable Kt may provide a reliable indicator of the K+ availability and therefore allow comparisons to be made regarding K status. Thus in more than 300 field experiments with wheat, carried out in France, it has been shown that K fertilizer applications resulted in high grain yield increases on soils with values for exchangeable K+ <80 ppm. Medium yield increases were obtained on soils with exchangeable K+ levels between 80 and 160 ppm, and only small yield increases (0.2 ton graidha) were found when the exchangeable Kt was > 160 ppm (LOUC, 1979). Soils capable of fixing large amounts of K+ frequently require particularly high K+ fertilizer dressings to show yield responses. Typical results of K fertilizer applications to a K+-fixing site are presented in Table 11. The data show that the K+ response obtained on a given site can differ considerably and depends on the crop species and on the weather conditions of the particular year. Generally, dry soil conditions during spring provide better K+ responses than moist conditions. The data of Table I1 also indicate that in some cases even rates of 300 kg K,O/ha did not result in spectacular yield increases, and as much as 900 kg K,O/ha were needed to obtain maximum grain yields. The results shown in Table I1 were obtained by Burkart (1975), who camed out field experiments on nine K+-fixing sites located in Southern Bavaria in the ”

Table I1 Effect of K Fertilizer Application on the Grain Yield (ton/ha) on Two Different Sites“ +

Domwag

Weng

K rate (kg K,O/ha)

Spring wheat, 1972

Maize, 1973

Maize, 1972

Spring wheat, 1973

0 300 600 900

3.27 3.96 6.16 4.48

2.48 3.88 5.04 5.48

5.34 5.63 8.66 9.37

4.83 4.62 5.07 5.21

“ Burkart (1 975).

POTASSIUM IN CROP PRODUCTION

93

Federal Republic of Germany. Highest yield responses were found with corn, whereas winter wheat and oats responded only by small to medium yield increases. The extent of the yield response obtained on these sites was not related to the K+ fixation capacity, which ranged from 349 to 783 ppm K as measured as “wet fixation. Dramatic yield increases by high applications of fertilizer K+ have also been reported by Doll and Lucas (1973) on K+-fixing soils in Michigan. Since applied K+ does not immediately equilibrate with soil K+ , but remains for weeks and even months in a readily accessible state, it is opportune to apply potash fertilizer to K+-fixing soils just before sowing. Potassium top dressing is also recommended. During the process of weathering of K-bearing minerals, K+ is released. Whether the rate of release is adequate to meet crop demands depends much on the kind of soil and the intensity of cropping. Young, unweathered soils are mainly rich in K-bearing minerals and may release high quantities of K+. This is borne out by field experiments by Singh and Brar (1977) on young alluvial soils in the Punjab (India). Even though exchangeable K+ values in the soils were low, yields of corn and wheat were high, and responses to fertilizer K+ were not obtained. In the long term, however, it is probable that intensive cropping even on such young soils would result in a major depletion of available soil K+; so fertilizer K+ would be required for optimum grain yields. This is especially the case when the cropping system shifts to a more intensive form and higher amounts of K+ are exported from the field (Kemmler, 1972). Thus Prasad (1977) has reported that numerous field trials carried out in India were highly responsive to N, P, and K fertilizer applications. Potassium responses were particularly high under the conditions of rainfed crops. Stephens (1969) also found with various crops grown in East Africa that during the first two-year cycle of cropping, the response to K+ fertilizer supply was poor or even negative, whereas in the second two-year cycle marked yield increases were obtained. Long-term field experiments with coastal Bermuda grass conducted by Woodhouse (1968) also showed no major K responses in the first five years. After this period, however, spectacular yield increases were obtained in the K-treated plots, and over the eleven years of the experiment K fertilizer application raised yield considerably. Similar results have been reported by Heathcote (1972) in Nigeria and by Anderson (1973) in East Africa. Anderson (1973) found striking K + responses most commonly on very sandy or moderately to strongly acid soils, but also on soils highly saturated with calcium. Crops that especially responded to K fertilizer applications were tea, tobacco, bananas, potatoes, sweet potatoes, coconuts, and grassland cut for silage or hay. DeDatta and Gomez (1975) reported that intensive paddy cropping on vertisols led to a decrease in yield level if K was not applied. The effect of N fertilizer application was poor on these K-deficient soils and could be substantially increased by K+ dressings. These positive effects of K+ were obtained on two sites ”

+

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KONRAD MENGEL AND ERNEST A. KIRKBY

with a value for exchangeable K+ <200 ppm, whereas on the third site with an exchangeable K+ value of about 350 ppm K, fertilizer application did not result in yield increases. The differences in response between the three sites were attributed to imgation water that contained 5 ppm in the third nonresponsive site, but only 1 ppm in the two responsive sites. The authors noted that K+ responses were higher for rice grown in the dry season than for rice grown in the rainy season. That differences in response should occur between seasons is not completely understood. Generally higher rice grain yields are obtained in the dry season because of better light conditions (Tanaka, 1973). It is feasible that the higher response to K+ under such circumstances relates to the increase in potential yield attributable to the higher light intensity. Recent field experiments on 23 sites in Java in most cases showed considerable rice yield increase as a consequence of K+ fertilizer application. Best results were obtained when K+ was applied during the tillering stage or given in split applications. The most striking result of these field experiments, depicted in Fig. 4, was that yields in the rainy and dry seasons converged with increasing K+ application rates. Each point in Fig. 4 represents the yield average of the 23 sites (Ismunadji et al., 1977). From this pattern of yield trend, it would appear that higher K+ application rates can compensate for the yield-limiting effect of the rainy season. This particular K+ effect during the rainy season is not always observed as has been shown by very recent experimental results of Ismunadji and Partohardjono (1979). In field experiments carried out on a broad range of different soil types in Indonesia, K+ application resulted in remarkable grain yield increases. In con-

Grain yield

( kglha

30 K

60

x lo-')

90 120 150 180

Treatment ( k g K&/ ha)

FIG.4. Effect of K fertilizer application on rice yield during the rainy season and the dry season

(Ismunadji er al., 1977).

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trast to the curves shown in Fig. 4, however, the K+ response curves diverged rather than converged. Response to K+ fertilizer application is especially likely to occur when high-yielding rice varieties are grown and when N- and P-containing fertilizer is applied. According to Tanaka (1973) crops grown on sandy latosols and in calcareous habitats are particularly susceptible to K+ deficiency. On these soils, responses to K+ application are likely to occur. Whether a crop responds to potassium is also dependent on other growth factors, and particularly on nitrogen and water supply. Highest K+ responses are likely if these two factors are not growth limiting. This relationship has been clearly demonstrated by Gartner (1969) in an experiment with tropical grasses. In the low N treatment (1 12 kg N/ha), K application had hardly any effect on dry matter yield, whereas in the highest rate treatment (448 kg N/ha) a more than 40% yield increase was attributable to K+. Similar relationships between N and K supply have been reported for grain yields of barley by Macleod (1969), for grain yield of oats by Mengel and Helal ( 1 968), and for grain yield of wheat by Helal and Mengel (1968). In a recent paper, George et al. (1979) reported that K+ application only resulted in a significant yield increase of smooth grass (Bromus inermis) if N was abundantly supplied. The results of this investigation also indicate that the highest K response was obtained in a year with almost no deficit in precipitation during the growth period. The effect of soil moisture on K+ response is often difficult to interpret. When water supply directly limits growth, high rates of K+ application are without effect. On the other hand, when low soil moisture conditions are limiting K+ availability, the application of K+ may result in a yield response, particularly if it encourages the root system to take up the water in deeper soil layers. As already discussed in Section 11, K+ diffusion and the release of nonexchangeable K+ depend greatly on soil moisture. Optimum soil moisture conditions favor K + diffusion in soil medium and are thus beneficial to the supply of K+ to plant roots (Mengel and v. Braunschweig, 1972). Under such conditions indigenous soil K+ may suffice for optimum plant growth, and so fertilizer K+ dressings may be without response. However, under drier conditions on the same soil with the same crop, K+ fertilizer responses may be obtained. This is the main reason why K+ fertilizer responses may differ considerably from one year to the next for the same crop growing on the same soil. The importance of soil moisture conditions in regulating K+ fertilizer response of potatoes and wheat has been demonstrated in long-term field trials (19351949) by van der Paauw (1958). Yields were found to be highly dependent on the total rainfall during the period from May to July. Potassium responses were higher the lower the rainfall during this period. The relationship between rainfall and the effect of a K+ application on grassland is also documented in long-term field experiments of Kuntze and Bartels (1975). These workers found that in dry years highest yields were obtained for a soil with a K+ level of 21 mg lactate-

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soluble K+/100 ml soil. In rainy years the corresponding value was only 13 mg lactate-soluble K+/100ml soil. High K+ responses are especially likely to occur if the top soil layer is rather dry during the early development of a crop. At this stage of growth the K+ demand per unit root length is particularly high (Mengel and Barber, 1974) and thus K+ diffusion toward plant roots may well become a growth-limitingprocess. For most crops K+ nutrition in the early growth stage is decisive in determining the yield level harvested later, so K+ availability conditions during this period of plant growth merit special attention. The release of interlayer K+ much depends on soil moisture. High soil moisture conditions favor a net release, and this may play an important part in supplying plants with K+. If the soil medium is relatively dry, however, K+ release is much restricted. Under such dry conditions, fertilizer K+ may give rise to significant responses that would very infrequently be obtained under more moist conditions. This relationship between K+ release of interlayer K+, soil moisture conditions, and K+ response has been considered by Mengel and Wiechens (1979) in pot experiments with ryegrass. Whether a K+ response is obtained or not also depends on the crop species and even sometimes on different cultivars of the same species. Why such interspecies differences exist is not yet completely understood. Probably several factors are involved. One important factor especially during the vegetative period is the growth rate. If the growth rate is high, so too is K+ demand. For a given soil with given environmental conditions a response to fertilizer K+ is more likely to occur with a fast- than a slow-growing crop. This is probably the reason that corn generally responds better than small grain cereals to K+ fertilization (Burkart, 1975). Another factor about which not much is known is rooting pattern and root metabolism in relation to K nutrition. In this respect, differences between grasses and dicotyledons are of particular interest. Blaser and Brady (1950) found that K+ application preferentially favored the growth of leguminous species in a grass-legume sward and that potassium depletion of the soil resulted in a considerable decrease of the proportion of legumes in the sward. This observation that K+ fertilization especially favors the growth of legumes in grassland was noted over 40 years ago by Konig (1935) and has since been confirmed by various authors (Kemmler et al., 1977; Schmitt and Brauer, 1979). Steffens and Mengel (1979) reported that when ryegrass and clover were grown together, the K+ content of the ryegrass was higher and that of the red clover lower than the corresponding values when the two species were grown separately. These authors suggest that the better response of red clover to ryegrass to K application reflects the successful competition of ryegrass over clover for soil K+. Field experiments also support the results discussed above. Thus van der Paauw (1958) reported that potatoes responded much better to K+ fertilizer dressings than wheat did.

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Similarly Schon et al. (1976) in a 20-year field experiment carried out on a loess soil also confirmed these interspecies differences in response to K fertilizer. Cereal responses were rather low, but with broad beans and potatoes spectacular yield increases were obtained. The mechanism of these differences in response between species is not yet understood. The large variation in response of crops to K fertilization has been used by Greenwood et al. (1974) to formulate fertilizer recommendations for vegetable crops. These workers argue that the large numbers of vegetable crops grown on widely different soils make it impossible to carry out trials to cover the possible combinations of crops and soil. They have therefore proposed a shortcut developed from the concept that a crop which is more responsive than another to fertilizer on one particular site should be more responsive on other sites. Using this approach Greenwood et al. (1974) have made measurements of response curves of yield against level of fertilizer for many crops at one site, and the response curves of one of the crops on a range of sites. From this information the response curves for all crops on all sites are predicted and then tested against the results of independent experiments. This approach may also prove useful for agricultural crops. From the above discussion it is clear that it is not possible to provide general K fertilizer recommendations that are applicable to different soils, climates, and crops. The basis for recommendations should be made using K soil analysis and determining critical levels by K fertilizer field experiments for representative soils and crops. Of the analytical methods used at the present time to formulate K+ fertilizer requirements much stress is placed on exchangeable K+. This is not always satisfactory. Future soil analysis should include two main factors that control K availability, namely the K buffer capacity of the soil and the K concentration of the soil solution, both measured under standard conditions. The optimum soil K level can differ for various crops; for example, it will be higher for potatoes or sugar beets than for cereals. For this reason the crop species in a rotation with the highest K requirement needs particular attention in K fertilizer practice. If the level of available soil K+ is much below the optimum, higher amounts of K fertilizer must be applied than those removed by the crop. Under K+-fixing soil conditions these K application rates may be extremely high, as has been outlined in Section IV, A. If the soil K+ level is higher than optimum, lower K fertilizer rates should be applied. In some cases even the omission of K fertilization may be opportune. Soils with an optimum K+ level should receive amounts of K+ that maintain this K+ status. These quantities can be calculated from the removal of K+ by the harvested produce, provided that losses due to K+ leaching are negligible. Potassium fertilizer policy also depends much on soil texture and the types of clay minerals present in the soil. Medium to highly textured soils have a medium to high K buffer capacity, and are not prone to K leaching. For such soils K+ can

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be applied in autumn, winter, or before the rainy season without danger of K losses by leaching. Two or even three crops of a rotation can be supplied by only one K treatment. The amount of K+ given in the application, however, should meet the total K requirement of the crops in question. This technique of K fertilizer application has been tested by Ansorge (1967) in numerous field experiments on medium-textured soils in Germany. No yield depressions were observed when K+ was applied on a two-year cycle as compared with K treatments applied each year. On poorly buffered soils, especially on sandy soils under humid conditions, a K fertilizer policy must take into account the possibility of loss through leaching. These soils should be treated with fertilizer K+ just before the crop is sown or planted. In this context it is also worth pointing out that the K+ in crop residues (straw, leaves, and roots) can also be leached by winter rainfall or under monsoon conditions. B. EFFECTOF POTASSIUM ON YIELDCOMFQNENTS

It is well known that K+ is mainly taken up during the vegetative period of plant growth. Pitman (1972) has demonstrated that in barley plants the rate of uptake is directly related to the growth rate. As already indicated, there is evidence that the K+ uptake is dependent on the hormonal status of plants (Cram and Pitman, 1972). Abscisic acid is known to inhibit K+ uptake, whereas indole acetic acid promotes the uptake of K+ (Erdei et al., 1979). Inadequate K+ supply retards the vegetative development of the plant, which may not only affect the production of vegetative plant material but also the development of reproductive organs and the filling of storage tissues with photosynthates. Mengel and Forster (1968) in studying the effect of interrupting the K+ supply on the development of spring barley found that grain yields were more depressed the earlier the K+ interruption took place. Thus a 16-day interruption between the tillering and the stem elongation stages resulted in a grain yield depression of about 40%. This yield depression was brought about mainly by a reduction in the number of ears per plant and a lower single grain weight. The interruption in K+ supply during the stage of ear emergence also depressed grain yield. In this case, however, depression resulted largely from a decrease in single grain weight. In a further treatment of this solution culture experiment in which plants were grown without K+ from pollination until maturity, no significant grain yield reduction was obtained. Analogous results have been reported by Forster (1973b) for spring wheat and oats and by Chapman and Keay (1971) for wheat. These results obtained in solution culture experiments are consistent with observations in the field. Thus Ralph (1976) found in field experiments on clay soils in England that the grain yield of winter wheat was increased as a consequence of K+ application by improving the single grain weight. In some cases

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this treatment also increased the number of grains per ear. Ralph (1976) observed that K+ especially promoted the development of the proximal, central, and distal spikelets. Baier and Smetankova (1974) who carried out a large number of field experiments also found that K+ mainly increased the grain yield of cereals by improving the single grain weight. Single grain weight depends much on the K+ status of the plant at the flowering stage. Late K+ application has little effect on the grain development (Mengel and Forster, 1971), and the rate of K+ uptake by cereals after flowering is probably very low. However, the K+ status of leaves and culms at the grain filling period has a substantial impact on photosynthesis and on the translocation of photosynthates from these organs toward the ears (Koch and Mengel, 1977). Ralph (1976) and Forster (1976) made the observation that under optimum K nutritional conditions the senescence of the flag leaf is delayed. This results in a prolonged “leaf area duration” which according to Evans et al. (1975) is important for grain development. Forster’s (1976) main results, shown in Table 111, reveal that K+ had a beneficial effect on increasing leaf area, chlorophyll content, and succulence of the flag leaf. This significance of succulence is not yet understood. It has been suggested that it may have a beneficial effect on phloem loading and probably also on the mobilization of photosynthates deposited in the leaves prior to the grain filling period. This view is supported by experiments of Seqer (1978) who observed that shortly after pollination a substantial amount of nitrogen was still stored in the culms and leaves of wheat in the form of protein. These proteins were mobilized at the stage of highest grain growth and used for the synthesis of grain proteins. Plants with a high K+ status were found to be more efficient in mobilizing the stored leaf proteins and in translocating the resulting amino acids toward the grains. From Seqer’s results it is also clear that the beneficial effect of K+ on grain filling was not related to the K+ content of the grains but resulted excluTable I11

Effect of K + Supply on Grain Yield, Grain Yield Components, Chlorophyll Content, Flag Leaf Area, and Succulence of the Flag Leaves”

Grain yield, g/plant Single grain weight, mg/grain Number of graindear Number of eardplant Flag leaf area, cm2/leaf Chlorophyll content, mg/cm2 leaf area Succulence, mg HZO/lOOcm’ leaf area

1.86 25.1 24.6 3.05 30.4 1.4 349

2.85 32.1 28.1 3.17 41.6 8.3 380

“The figures represent an average of five cultivars of spring wheat (Forster, 1976).

3.48 34.1 31.8 3.22 50.2 34.2 442

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sively from the influence of K+ on the translocation of assimilates from the vegetative plant parts toward the ears. Grain growth is not only dependent on the provision of a source of assimilates to the ears but also on the ability of the ears to provide a “sink” for these assimilates. Shading, for example, has a depressive effect on grain development, which cannot be alleviated by increasing the level of K nutrition (Mengel and Haeder, 1976). On the other hand, the depressing effect of shading on vegetative growth can be reduced to some extent by a high K+ supply (Haeder and Mengel, 1975). This demonstrates that K nutrition can beneficially influence the “source” but not the sink. If the sink metabolism is the limiting factor in grain development, then increasing K supply is without effect. This has been shown by Mengel and Haeder ( 1976) with spring wheat and by Beringer and Koch (1977) in experiments with the high-lysine barley variety “Riso 1508.” It is very likely that the production of seeds, tubers, and roots is influenced by K+ in the same way as has been shown for cereals. The beneficial effect of K+ is to promote phloem loading and phloem transport and thus provide the “physiological sink” with assimilate. Thus K nutrition enhances tuber growth of potatoes (Haeder et al., 1973). Obigbesan (1977) has drawn attention to the observation that K+ application to cassava grown at two different locations in Nigeria particularly increased the storage roothop ratio. An analogous observation for potatoes has been reported by Haeder et al. (1973). These authors also found that the number of potato tubers per plant were highest in the low K+ treatment, but the tubers remained very small. The effect of K+ particularly was to increase tuber size. From experiments of Haeder (1975) it appears that the negative effect of chloride on tuber filling results from impaired translocation of photosynthates from the leaves toward the tubers. This is the main reason that the application of potassium sulfate produces better potato starch yields than treatment with potassium chloride. It is generally accepted that K+ increases the starch content of tubers and the sugar content of roots of sugar beet. This positive effect, however, is not always observed and depends much on the crop yield and the degree of K” deficiency in the soil. In perennial crops a substantial amount of K+ is retranslocated from leaves into stems and twigs before leaf fall begins. This K+ is mobilized again in the following spring when new leaves are formed. For these crops the early development also depends considerably on K+ nutrition. Experiments of Fremond and Ouvrier (1971) on sandy soils in the Ivory Coast have shown that young coconut palm trees which received a K+ application of about 1 kg Wtree grew more vigorously than the control plants without K fertilizer treatment. The K+treated plants fruited earlier and produced much higher yields than the untreated palms. A late application of KS to these untreated palms did result in a yield increase, but the yields obtained were not as high as those obtained when K+ was applied at an early stage. This example demonstrates the importance of K+ in

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establishing high-yield crop stands of plantation crops. According to investigations of Ollagnier and Ochs (1971) and v. Uexkull(l972) oil and coconut palms have a special chloride requirement, and KCI applications may increase yield not only as a consequence of K+ but also because of the beneficial effect of chloride. C . SECONDARY EFFECTS OF POTASSIUM ON CROPYIELD

A number of more indirect effects of K+ occur which are pertinent in discussing crop yields. These secondary effects cannot be considered here in detail, but some major points are presented. Much research work has been done which shows that under conditions of an inadequate K+ nutrition, the susceptibility of crops to plant disease is increased. This topic has been thoroughly treated by Goss (1968), and the 12th Colloquium of the International Potash Institute, Berne, was particularly devoted to this subject. Ismunadji (1976) in discussing rice diseases and physiological disorders draws attention to the observation that K+ improves the resistance of rice to fungal diseases. Analogous observations have been reported for other crops (Kriiger, 1976). Why optimum K+ status especially improves fungal disease resistance is not yet completely understood. Trolldenier and Zehler (1976) have considered the relationship between plant nutrition and rice diseases. These authors suggest that under the conditions of insufficient K+ supply the formation of the cuticle and epidermal cell walls is affected so that fungal hyphae may more easily penetrate these barriers than through the wellestablished cuticles and epidermal cell walls of plants adequately supplied with K+ . The higher contents of sugars and soluble amino acids, frequently found in K+-deficient plant tissues, are also considered to provide a good nutrient medium for fungi and bacteria. The findings of Baule (1969) also indicate that forest trees well supplied with K+ are more resistant to sucking insect infestation than are trees that are deficient in K+. In paddy rice growing, Fe toxicity associated with high Fez+concentrations in the root zone is often associated with K+ deficiency. Trolldenier (1977) has reported that under conditions of suboptimal K+ nutrition the redox potential in the rice rhizosphere decreases and the relative proportion of Fez+ to Fe3+ increases. The resulting Fe toxicity can be alleviated or even completely eradicated by K+ application. It appears that in plants well supplied with K+ that 0, transport is enhanced from the plant tops via the stems to the roots, thus enabling the oxidation of Fez+ to Fe3+ in the rhizosphere. The partial precipitation of Fe3+ at the root surface gives rise to a red color which is indicative of healthy rice roots. The disease “greenback” is a physiological disorder of tomato fruits which is highly dependent on K nutrition as well as on tomato cultivar (Forster, 1973a). The results of Forster and Venter (1975) have shown that the proportion of

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tomato fruits with greenback can be considerably reduced at higher levels of K+ nutrition. On the other hand, if too high a level of K nutrition is supplied, this can bring about the occurrence of “blossom end rot” in tomatoes and “bitter pit” in apples. These Ca-related disorders can be induced by the effects of K-impairing calcium uptake and translocation. Vertregt (1968) reported a close relationship between “black spot” in potato tubers and their K content. Black spot susceptibility disappeared if the K content exceeded 600 me Wkg dry matter. According to Macklon and DeKock (1967) the K content of tubers is positvely correlated with the citric acid content of tubers. Citric acid is known to inhibit the formation of an Fe chlorogenic complex, which induces black spot formation. This relationship may therefore account for K+-black spot association. V.Braunschweig (1979b) also suggests that high turgor in potato tubers prevents the disease. Since K+ is a major osmoticum in potato tubers, the beneficial effect of K+ may also be explained in terms of increased turgor. There are several indications in the literature that a high K content in plant tissues increases frost resistance. This relationship has been demonstrated recently by Eifert and Eifert (1976) in grapes. These workers found an inverse relationship between the K content of vine leaves and frost damage. A beneficial effect in this respect of K fertilizer application to forest trees has also been found by Koskela (1970). The mechanism of this direct effect of K+ on frost resistance is not yet completely understood. It is supposed that K+ not only has an influence as an osmoticum but also improves resistance by affecting other biochemical reactions. Salt tolerance can also be enhanced by K+. In solution culture experiments with barley, Helal et al. (1975) reported that negative effect of NaCl salinzation (60 mM NaCI) on growth could be completely suppressed by the addition of KCI (5 or 10 mM) to the nutrient medium. In further experiments Helal and Mengel (1979) observed that Na salinity impaired N turnover and that this effect could be alleviated by the presence of K+ in the medium. Unpublished data of Held indicate that salt tolerance is related to the energy status of plants and that plants grown under saline conditions consume more energy than plants that are not suffering from salt stress. Helal suggests that as a result of this improvement of K+ on the energy status of the plants, the resistance to salinity is increased. It is well known that Na+ can substitute for K+ to some degree in plant nutrition. In this respect considerable differences occur between crop species. This question has been thoroughly treated by Marschner ( I 97 1) and for this reason will not be considered here in detail. In some species, however, Na+ has an additional influence on growth and crop production. This has been shown for sugar beets by Draycott et al. (1970), El-Sheikh and Ulrich (1970), and Jude1 and Kuhn (1975). The K efficiency of crops has been a subject of recent interest. An “efficiency ratio” has been measured relating the quantity of plant material produced to the

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amount of K+ taken up. Gerloff (1976) reported significant intercultivar differences for this efficiency ratio in Phaseolus vulgaris and tomato. High efficiency was frequently related to a high uptake of Na+, presumably because Na+ was to some extent replacing the nonspecific physiological role of K+ in plant metabolism.

V. CONCLUSIONS Potassium is characterized by unique behavior in soils and living systems and contrasts greatly with related cation species such as Na+ or Ca2+. It is suggested that this unique property of K+ is dependent on the relatively low energy required for the dehydration of K+ and on the high affinity of the dehydration K+ to oxygen atoms. Interlayer K+ in micas or mica-related minerals is embedded at the center of a structure formed by six peripherally arranged oxygen atoms. Analogous structures are found in living systems such as enniatins, nonactin, and valinomycin. It is feasible that these K+ complexes are essential for specific K+ functions in living systems. It appears that typical K+ reactions in the soil as well as those in the plant basically originate from the same property of K+, namely its tendency to substitute the hydration water by other oxygen-containing ligands. This property thus controls K+ fixation and release by clay minerals as well as the K+ buffer capacity of soils, on one hand, and the selective K+ transport through biological membranes and all processes related to this transport, on the other. Both the characteristic behavior of K+ in soils as well as in plants are of agronomic relevance. The understanding of the K relationships and interactions in soils is pertinent of estimating optimum fertilizer application rates. The importance of K fertilizer application will increase the more plant cultivation and production shifts from an extensive to an intensive form. The question of the economics of K fertilization thus requires increasing attention especially in developing countries. The understanding of the physiological functions of K+ in crops is relevant to the production of plant products of high quality. In developing new cultivars or even new crops, knowledge of the physiological role of K+ in plants will also be useful. The fact that K+ is especially involved in the conversion of solar energy into chemical energy could be of some importance in developing crops that are solely grown for energy production. REFERENCES Addiscott, T. M. 1974a. J . Sci. Food Agric. 25, 1173-1 183. Addiscott, T. M. 1974b. I n “Potassium Research and Agricultural Production,” pp. 175-190. Proc. 10th Congr. Int. Potash Inst., Berne.

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Adepetu, J. A., and Akapa, L. K. 1977. Agron. J . 69, 940-943. Ahmad, N., and Davis, C. E. 1971. SoilSci. 112, 100-106. Allaway, W. G. 1973. PIanra 110, 63-70. Amberger, A., Gutser, R., and Teicher, K. 1974. Planr Soil 40, 269-284. Amir, S., and Reinhold, L. 1971. Physiol. Plant. 24, 226-231. Anderson, G. D. 1973. In “Potassium in Tropical Crops and Soils,” pp. 413-437. Proc. 10th Colloq. Int. Potash Inst., Berne. Ansorge, H. 1967. Drsch. Akad. Landwirtschafiswiss. (Berlin) Rep. No. 76. Arifin, H. F., and Tan, K. H. 1973. Soil Sci. 116, 31-35. Arneke, W. 1980. Ph.D. Thesis, FB 19, Justus Liebig University, Giessen. Arnon, I. 1969. In “Transition from Extensive to Intensive Agriculture with Fertilizers,” pp. 13-25. Proc. 7th Colloq. Int. Potash Inst., Beme. Ashley, D. A . , and Goodson, R. D. 1972. Crop Sci. 12, 686-690. Baier, J . , and Smetankova, M. 1974. In “Potassium Research and Agricultural Production,’’ pp. 161-170. Proc. 10th Congr. Int. Potash Inst., Beme. Baker, D. A,, and Weatherley, P. E. 1969. J. Exp. Bor. 20, 485-596. Baldwin, J. P., Nye, P. H., and Tinker, P. B. 1973. PIanr Soil 38, 621-635. Baligar. V. C., and Barber, S. A. 1978a. Soil Sci. Soc. Am. J . 42, 575-579. Baligar, V. C., and Barber, S. A. 1978b. Soil Sci. SOC.Am. J . 42, 618-622. Barankiewicz, T. J. 1978. Z . Pflunzenphysiol. 89, 11-20. Barber, J . 1977. In “Fertilizer Use and Production of Carbohydrates and Lipids,” pp. 83-93. Proc. 13th Colloq. Int. Potash Inst., Berne. Barber, S. A. 1962. SoilSci. 93, 39-49. Barber, S . A. 1979. In “The Soil-Root Interface” (J. L. Harley and R. Scott Russell, eds.), pp. 5-20. Academic Press, New York. Barber, S . A., Walker, J. M., and Vasey, E. H. 1963. Agric. Food Chem. 11, 204-207. Barrow, N. J. 1966. Aust. J . Agric. Res. 17, 849-861. Baule, H. 1969. Landw. Forsch. 23(I), 92-104. Ben Zioni, A,, Vaadia, Y., and Lips, S . H. 1971. Physiol. Planr. 24, 288-290. Beringer, H . , and Koch, K. 1977. Landw. Forsch., Sonderh. 34(II), 36-44. Besford, R. T., and Maw, G. A. 1976. Ann. Bor. 40, 461-471. Blanchet, R., Studer, R., and Chaumont, C. 1962. Ann. Agron. 13, 93-1 10. Blaser, R. E., and Brady, N. C. 1950. Agron. J . 42, 128-135. Blevins, D. G., Hiatt, A. J., and Lowe, R. H. 1974. PIanr Physiol. 54, 82-87. Blevins. D. G., Barnett, N. M., and Frost, W. B. 1978. Plant Physiol. 62, 784-788. Boguslawski, E. v., and Lach, G. 1971. Z. Acker Pflanzenbau 134, 135-164. Bowling, D. J. F. 1976. “Uptake of Ions by Plant Roots.” Chapman & Hall, London. Bowling, D. J. F., and Ansari, A. Q. 1971. PIanra 98, 323-329. Bowling, D. J. F., Macklon, A. E. S., and Spanswick, R. M. 1966. J . Exp. Bor. 17, 410-416. Brad, J. 1971. Biochemistry 14, 127-134. Brag, H. 1972. Physiol. Planr. 26, 250-257. Braunschweig, L. C., v. 1979a. Landw. Forsch. Sonderh. 35, 219-231. Braunschweig, L. C., v. 1979b. Karroffelbau, Hefi Jan. Braunschweig, L. C., v., and Mengel, K. 1971. Landw. Forsch. Sonderh. 26(I), 65-72. Burkart, R. 1975. Ph.D. Thesis, FB Agriculture and Horticulture, Technische Universitat, Miinchen. Busch, R. 1980. Ph.D. Thesis, FB 19, Justus Liebig Universitat, Giessen. Chapman, M. A., and Keay, J. 1971. Ausr. J . Exp. Agric. Anim. Hush 11, 223-228. Cheeseman, J. M., and Hanson, J. B. 1979. PIanr Physiol. 64, 842-845. Chloupek, 0. 1972. 2. Acker Pflanzenbau 136, 164-169. Claassen, N . , and Barber, S . A. 1976. Agron. J . 68, 961-964.

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