Diagnosis of Deficiency and Toxicity of Mineral Nutrients

12 Diagnosis of Deficiency and Toxicity of Mineral Nutrients

12.1

Nutrient Supply and Growth Response

The well-known growth (dry matter production) versus nutrient supply curve (growth response curve) has three clearly defined regions (Fig. 12.1). In thefirst,the growth rate increases with increasing nutrient supply (deficient range). In the second, the growth rate reaches a maximum and remains unaffected by nutrient supply (adequate range). Finally, in the third region, the growth rate falls with increasing nutrient supply (toxic range). In crop production, optimal nutrient supply is usually achieved by the application of fertilizer. Rational fertilizer application requires information on the nutrients that are available in the soil, on the one hand, and the nutritional status of the plants, on the other. The possibilities and limitations of using visual diagnosis and plant analysis as a basis for recommending whether or not to use fertilizer, and if so of what type and quantity, are discussed in this chapter.

Nutrient supply Fig. 12.1 Relationship between nutrient supply and growth.

462

Mineral Nutrition of Higher Plants

12.2

Diagnosis of Nutritional Disorders by Visible Symptoms

As a rule, nutritional disorders that inhibit growth and yield only slightly are not characterized by specific visible symptoms. Symptoms become clearly visible when a deficiency is acute and the growth rate and yield are distinctly depressed. However, there are exceptions. For example, transient visible symptoms of magnesium deficiency in cereals which can sometimes be observed under field conditions during stem extension are without detrimental effect to the final grain yield (Pissarek, 1979). Furthermore, many annual and perennial plant species of the natural vegetation, particularly those adapted to nutrient-poor sites, adjust their growth rate to the most limiting nutrient and, thus, visible deficiency symptoms do not develop (Chapin, 1983, 1988). Diagnosis based on visible symptoms requires a systematic approach as summarized in Table 12.1. Symptoms appear preferentially on either older or younger leaves, depending on whether the mineral nutrient in question is readily retranslocated (Section 3.5). The distribution pattern of symptoms might also be modified by the method employed to induce deficiency, i.e., permanent insufficient supply or sudden interruption of a high supply (Section 3.5). Chlorosis or necrosis and the pattern of both are important criteria for diagnosis. As a rule, visible symptoms of nutrient deficiency are much more specific than those of nutrient toxicity, unless the toxicity of one mineral nutrient induces a deficiency of another. Visible deficiency symptoms of individual nutrients have been described

Table 12.1 Some principles of visual diagnosis of nutritional disorders Plant part

Old and mature leaf blades

Young leaf blades and apex

Old and mature leaf blades

Prevailing symptom

<

y

K

Disorder

Chlorosis

Uniform Interveinal or blotched

Deficiency N(S) Mg (Mn)

Necrosis

Tip and marginal scorch Interveinal

K Mg (Mn)

Chlorosis

Uniform Interveinal or blotched

Fe(S) Zn (Mn)

Necrosis (chlorosis)

Ca, B, Cu

Deformations

Mo (Zn, B)

Necrosis Chlorosis, necrosis

Spots Tip and marginal scorch

Toxicity Mn(B) B, salt (spray injury) Nonspecific toxicity

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

463

briefly in Chapters 8 and 9. For details (including colour pictures) of symptoms of nutrient disorders the reader is referred to Wallace (1961) and Bergmann (1988,1992). Diagnosis may be especially complicated in field-grown plants when more than one mineral nutrient is deficient or when there is a deficiency of one mineral nutrient and simultaneously toxicity of another. Such simultaneously occurring deficiencies and toxicities present difficulties in diagnosis and can be found in practice as for example, in waterlogged acid soils, where both manganese toxicity and magnesium deficiency may occur (complex symptoms). Diagnosis may be further complicated by the presence of diseases, pests, and other symptoms caused, for example, by mechanical injuries including spray damage. In order to differentiate the symptoms of nutritional disorders from these other symptoms, it is important to bear in mind that nutritional disorders always have a typical symmetric pattern: leaves of the same or similar position (physiological age) on a plant show nearly identical patterns of symptoms, and there is a marked gradation in the severity of the symptoms from old to young leaves (Table 12.1). In order to make a more precise visual diagnosis, it is helpful to acquire additional information, including soil pH, results of soil testing for mineral nutrients, soil water status (dry/waterlogged), weather conditions (low temperature or frost) and the application of fertilizers, fungicides, or pesticides. In some instances the type and amount of fertilizer to be used can be recommended on the basis of visual diagnosis immediately. This is true of foliar sprays containing micronutrients (iron, zinc, or manganese) or magnesium. In other instances (e.g., iron deficiency chlorosis), however, visual diagnosis is an inadequate basis for making fertilizer recommendations. Nevertheless, it offers the possibility of focusing further attention on chemical and biochemical analysis of leaves and other plant parts (plant analysis) of selected mineral nutrients. This is of particular importance for annual crops, because the results are required immediately and seasonal fluctuations in the nutrient content of the plants often do not justify the high cost of carrying out a complete mineral nutrient analysis.

12.3 12.3.1

Plant Analysis General

The use of chemical analysis of plant material for diagnostic purposes is based on the assumption that causal relationships exist between growth rate of plants and nutrient content in the shoot dry or fresh matter, or in the nutrient concentration in the tissue press sap. Mineral element composition of plant tissues is usually expressed as content per unit dry or fresh weight (e.g., mg g" 1 dry wt). Although the term concentration is often used synonymously, it refers in the strict sense to a volume (e.g., mg Γ 1 ) . Depending on the mineral nutrient, plant species and age, the most suitable plant part or organ differs for this purpose, as well as whether or not the total content or only a certain fraction of the mineral nutrient (e.g., water extract able) should be determined. In general the nutritional status of a plant is better reflected in the mineral element content of the leaves than in that of other plant organs. Thus leaves are usually used for plant analysis. For some species and for certain mineral nutrients, nutrient contents in

464

Mineral Nutrition of Higher Plants

the dry matter may differ considerably between leaf blades and petioles, and sometimes the petioles are a more suitable indicator of nutritional status (Bouma, 1983). In fruit trees the analysis of the fruits themselves is a better indicator, especially for calcium and boron in relation to fruit quality and storage properties (Bould, 1966). Under certain climatic conditions, drought stress during seed rilling in particular, in legumes seed content of zinc appears to be a more sensitive parameter to zinc fertilizer supply as, for example, foliar analysis (Rashid and Fox, 1992). Samples from field-grown plants are often contaminated by dust or sprays and require washing procedures (Jones, 1991; Moraghan, 1991). The most severe problems in the use of plant analysis for diagnostic purposes are the often short-term fluctuations in nutrient contents (e.g., 'dilution effects' by fast growth). It is particularly difficult to establish mineral nutrient contents which are considered to reflect deficiency, sufficiency or toxicity range in relation to the effects of environmental factors as well as plant genotype and developmental stage of plants and leaves. For example, the percentage of dry matter usually increases with age of plants or organs (Walworth and Sumner, 1988) or at elevated C 0 2 concentrations because of starch accumulation (Kuehny et al. ,1991) with a corresponding decline in the critical deficiency concentrations of mineral nutrients. At elevated C 0 2 concentrations lower critical leaf contents of nitrogen in leaves of C 3 species are also related to less Rubisco, whereas critical leaf contents of phosphorus tend to be higher for reasons which are not clear (Conroy, 1992). Strict standardization of sampling procedure and availability of suitable reference data are therefore of crucial importance for foliar analysis. The use of nutrient ratios instead of contents is another approach to meet this difficulty (Section 12.3.4). For more recent reviews on plant analysis for diagnostic purposes the reader is referred to Reuter and Robinson (1986); Martin-Prevel et al (1987); Bergmann (1988, 1992); Walworth and Sumner (1988); Jones (1991), and Heinze and Fiedler (1992). 12.3.2

Relationship between Growth Rate and Mineral Nutrient Content

A representative example on the relationships between plant growth and mineral nutrient contents in shoots is shown in Fig. 12.2 for manganese. Both under controlled environmental conditions and under field conditions the critical deficiency content

10

20 30 0 5 10 15 Manganese content (/L/g g"1 dry wt.)

Fig. 12.2 Relationship between manganese content in youngest emerged leaf blades and shoot dry weight in barley grown in a growth chamber (A) and under field conditions (B). (From Hannamitffl/., 1987.)

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

465

(defined also as 'level') of manganese in the dry weight of the youngest emerged leaf blade of barley plants is in the range of 12 μ% g _1 dry matter, if 95% of the maximal shoot dry weight (or yield) is used as reference point. It should be kept in mind that the critical deficiency contents (CDC) for example of the youngest emerged leaf blade is not necessarily identical to the CDC at the sites of new growth, the shoot meristem where the CDC might be much higher as has been shown for zinc in rice plants (Section 9.4.10). Usually, afigureof 90% of the maximum dry matter yield is taken as a reference point in order to define the CDC of a mineral nutrient (Bouma, 1983; Ohki, 1984). In a low input system the reference point is sometimes only 80% of the maximum dry matter yield, the CDCs are correspondingly lower in the shoot dry matter, for example for phosphorus in maize and cowpea (Smyth and Cravo, 1990). Differences in reference point are often overlooked in comparisons of literature data on CDC. The method employed for determination of the CDC is also important and contributes to different values, as has been shown for magnesium in subterranean clover, where the values of CDC for plants provided permanently with a low supply have been compared with plants where a high supply was suddenly interrupted (Scott and Robson, 1990b). It is unlikely that the latter procedure reflects the situation under field conditions. Sudden interruption of nutrient supply leads to unusual high CDCs for various nutrients (Burns, 1992), as fast growing plants become suddenly totally dependent on remobilization and retranslocation of mineral nutrients. In view of the various factors discussed above, and also in the following sections, the use of a 'range' of values (e.g., critical deficiency range) would seem more appropriate and realistic than a single value (e.g., 12/*g g" 1 ). At least it should be borne in mind that the use of a single value encompasses a range of contents and that the probability of deficiency, or sufficiency increases, with the extent of deviation from this single value. The general pattern of relationships between plant growth and mineral nutrient contents in plant tissue is shown in Fig. 12.3 in a schematic presentation. There is an ascending portion of the curve where growth either increases sharply without marked increase in nutrient content (I and II) or where increases in growth and mineral nutrient content are closely related (III). This is followed by a more or less level portion where growth is not nutrient limited and nutrient content markedly increases (IV and V) and, finally, by a portion where excessive nutrient content causes toxicity and a corresponding decline in growth (VI). Occasionally, with an extreme deficiency of copper (Reuter et al., 1981) or zinc (Howeler et al., 1982b), for example, a C-shaped response curve is obtained (Fig. 12.3, region I, dashed line) in which a nutrient-induced increase in growth rate is accompanied by a decrease in its content in the dry matter, which is often referred to as the Tiper-Steenbjerg' effect (Bates, 1971). Possible explanations for this type of response are a lack of remobilization from old leaves and stem (Reuter et al., 1981) or necrosis of the apical meristems with a corresponding cessation of growth despite further uptake of small amounts of the mineral nutrient by extremely deficient plants. Concentration and dilution effects of mineral nutrients in plants are common phenomena which must be carefully considered in interpreting nutrient contents in terms of ion antagonism or synergism, or both during uptake. This holds true in particular when the contents of mineral nutrients are in the deficiency or toxicity range

466

Mineral Nutrition of Higher Plants

ible toxicity symptoms

Visible deficient symptoms χ>^~"

ω *>*

I

c 15 Q.

-^W-

Content of mineral nutrient in plant tissue Range

I

H

deficient

n in low

IE

adequate

Element 0.16-0.25 0.26-0.50 P(%) <0.16 1.26-1.70 1.71-2.50 K(%) <1.26 21 -100 15-20 Mn (mg kg"1) < 15 * Soybean, upper leaves (from Jones, 1967)

3ZI

high 0.51-0.80 2.51-2.75 101-250

a:

toxic >0.80 >2.75 >250

Fig. 12.3 Relationship between nutrient content (e.g., mg g"1 dry matter) and growth or yield (upper) and examples of the nutrient content in the dry matter of soybean leaves in the various nutrient supply ranges (lower). (Jarrell and Beverly, 1981). If, for example, the contents of two mineral nutrients are in the deficiency range and only one of them is supplied, growth enhancement causes a 'dilution' of the other mineral nutrient (a decrease in its content in the dry matter) and severe deficiency is induced without any competition occurring in uptake or within the plant. Central to the use of plant analysis for diagnostic purposes are the critical deficiency and toxicity contents (Fig. 12.3). Growth is maximal between the critical deficiency and toxicity contents, but for practical reasons the 90-95% of maximal growth value rather than the maximal value itself is chosen as a reference point. The mineral nutrient contents can be grouped into ranges, as shown in the lower portion of Fig. 12.3 for soybean. If nutrient contents are in the adequate range there is a high statistical probability that these nutrients are not growth-limiting factors. Certainly, contents in the luxury range further decrease the risk that these nutrients will become deficient under conditions unfavorable for root uptake (e.g., dry topsoil) or when the demand is very high (e.g., retranslocation to fruits). However, there is a greater risk of growth reduction by direct toxicity of these nutrients or by their effect in inducing a deficiency of other nutrients, i.e. nutrient imbalance (Section 12.3.5). In defining critical toxicity contents the heterogenous distribution of a nutrient within a plant organ has to be considered, for example, of boron in leaf blades (Section 9.7.9). 12.3.3

Developmental Stage of Plant and Age of Leaves

Next to the mineral nutrient supply, the physiological age of a plant or plant part is the most important factor affecting mineral nutrient content in the plant dry matter. With

467

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

Table 12.2 Critical Deficiency Content (CDC) of Copper (at Maximum Yield) in Subterranean Clover and Plant Organs and Plant Agea Age of plant (days after sowing) Plant part

26

40

55

98

Fb

Whole plant tops Youngest open leaf blade

3.9 3.2

3.0 ~3

2.5 ~3

1.6 ~3

1.0 -3

"CDC expressed as mg g 1 dry weight. Based on Reuter et al. (1981). ''Early flowering. the exception of calcium and sometimes iron (Sanchez-Alonso and Lachica, 1987b) and boron (Section 9.7) there is usually a fairly clear decline in mineral nutrient content in the dry matter as plants and organs age. This decline is caused mainly by a relative increase in the proportion of structural material (cell walls and lignin) and of storage compounds (e.g., starch) in the dry matter. Mineral nutrient contents corresponding to the adequate or critical deficiency range are therefore lower in old than in young plants. For example, in grain sorghum the CDC of phosphorus in the leaf dry matter has been shown to decrease from about 0.4% to 0.2% throughout the growing season (Myers et al.y 1987). In field-grown barley the potassium content in the shoot dry matter decreased from 5-6% in young plants to about 1% towards maturation, although the plants were well supplied with potassium (Leigh et al., 1982). In this instance the decline in content was exclusively a 'dilution effect' as the potassium concentration in the tissue water (mainly representative of vacuolar sap) remained fairly constant at —100 mM throughout the season. Complications arising from changes in the critical deficiency content with age can be lessened by sampling tissues at specific physiological ages. For example, as shown in Table 12.2, the critical deficiency level of copper in the whole plant tops decreases drastically in clover with age but remains fairly constant at ~ 3 ^g in the youngest leaf blades throughout the season. The use of the youngest leaves, however, is suitable only for those mineral nutrients which either are not retranslocated or are retranslocated to only a very limited extent from the mature leaves to areas of new growth, that is, when deficiency occurs first in young leaves and at the shoot apex (Table 12.1). The situation is different for potassium, nitrogen, and magnesium; since the contents of these mineral nutrients are maintained fairly constant in the youngest expanded leaves, the mature leaves are a much better indicator of the nutritional status of a plant, as shown for potassium in Fig. 12.4. Here, the youngest leaf is not a suitable indicator because the potassium levels indicating deficiency and toxicity vary only between 3.0 and 3.5%, respectively, compared with 1.5 and 5.5% in mature leaves. This illustrates the necessity of using mature leaves to assess the nutritional status of mineral nutrients which are readily retranslocated in plants. If young and old leaves of the same plant are analyzed separately, additional information can be obtained on the nutritional status of those mineral nutrients which are readily retranslocated. A much higher content of, say, potassium in the mature

468

Mineral Nutrition of Higher Plants

T

-

c J5

6

Mature leaf ° Youngest leaf

Q. U)

r 4

Critical contents for 9Q% of maximum growth Deficiency Toxicity 3.0 3.5 %K Youngest leaf: 1.5 5.5 %K Mature leaf:

-C

g)

"O

2I

o o

■I—·

-C

CO

1

1

2

3 4 5 K (% dry weight)

6

Fig. 12.4 Relationship between shoot dry weight and potassium content of mature and youngest leaves of tomato plants grown in nutrient solutions with various potassium concentrations. Inset: calculated critical contents. leaves indicates luxury consumption or even toxicity. The reverse gradient, on the other hand, is an indicator of the transition stage between the adequate and deficient ranges; if this gradient is steep, a latent or even acute deficiency may exist. The use of gradients is particularly helpful under conditions in which relevant reference data on critical contents are lacking (e.g. for a species or cultivar) or under certain ecological conditions. If toxicity is suspected, the old leaves are the most suitable organs for plant analysis. When choosing a given plant organ such as the most recently developed, fully expanded leaf for analysis it should be taken into account that the CDC value will decline throughout plant development, even when expressed as a concentration in the plant sap. For potassium, for example, in soybean this value falls between podset and podfilling from 65 mM to 29 ΙΉΜ (Bell et al., 1987). This decline throughout plant development is particularly evident for the nitrate which acts as a storage form of nitrogen in the leaves and as an indicator of the nitrogen nutritional status of the plants (Section 12.3.7). In the petioles of potato leaves the CDC of NO3-N decreases from 2.7-3% in the dry matter at the onset of tuberization to 1.0-1.6% at later stages (Williams and Maier, 1990) and in the midribs of cauliflower from 1.1% at the 4-6 leaf stage to 0.15% at preharvest (Gardner and Roth, 1990). A similar decline holds true for sulfate as the main storage form of sulfur in plants (Huang et al., 1992c). This decline in CDC for a given organ with age can occur for various reasons. For example, as plants become older there is a decrease in demand for nutrients for new growth. Also nutrient supply from the roots may increase although this is less likely in most field conditions in view of declining root activity. The decline in CDC appears rather to be the consequence of increase in total shoot biomass and, thus, storage capacity of mineral nutrients in the shoots, as illustrated in an example for maize in Table 12.3. Between the 4-5-leaf stage and heading both the critical deficiency concentrations of nitrate-N in the press sap and the concentrations considered as adequate (Fig. 12.3) decline during plant development. However, as the total aboveground biomass increase is more than linear, a given concentration of nitrate-N per liter

469

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

Table 12.3 Critical Deficiency and Adequate Concentrations of Nitrate Nitrogen (NO3-N) in Press Sap of Leaf Sheath from Basal Stem at Different Developmental Stages of Maize, and Estimated Amounts of NO3-N Stored in the Above-ground Biomass0 Concentration range (mg NO3-N1- 1 )

Developmental stage

Estimated amounts of stored NO3- N ( k g h a _ 1 )

Critical defic.

Adequate

Critical defic.

Adequate

800 375 250 250

1400 700 550 550

3.6 6.2 7.0 10.6

6.3 11.6 15.3 23.3

4-5 leaf stage Onset of shooting Shooting Heading

"Based on Geyer and Marschner (1990) and Geyer (unpublished).

press sap (or per unit dry matter) represents an increasing amount of stored nitrogen which can act as an internal buffer and maintain similar growth rates for several days when supply from the soil declines. Using a model which takes into account changes in growth rates and biomass as a parameter of internal demand, a single critical leaf sap concentration of 380 mg NO3-N 1 _ 1 could be identified for Brussels sprout at all growth stages and also for various growing seasons (Scaife, 1988). Compared with the changes in the mineral nutrient content of annual species, the fluctuations throughout the growing season of the mineral nutrient content of leaves and needles of trees are relatively small because of the nutrient buffering capacity of twigs and trunk. In evergreen trees the simultaneous analysis of leaves and needles differing in age (Table 12.4) may provide additional information on the nutritional status of the tree. With increasing age of the needles, the content of all macronutrients decreases, except that of calcium (Table 12.4). In Norway spruce, the silicon content also increases with needle age (Wyttenbach et aL, 1991). The decrease in nitrogen,

Table 12.4 Mineral Nutrient Content of Norway Spruce (Picea abies Karst) Needles of Different Agea Age of needles (years) Mineral nutrient Nitrogen Phosphorus Potassium Magnesium Calcium

1

2

3

4

1.79 0.20 0.63 0.04 0.28

1.76 0.17 0.56 0.04 0.40

1.46 0.14 0.47 0.03 0.50

1.22 0.13 0.44 0.03 0.59

°Contents of mineral nutrients expressed as a percentage of the dry weight. Based on Bosch (1983).

470

Mineral Nutrition of Higher Plants

phosphorus, potassium and magnesium with needle age (Table 12.4) might in part indicate retranslocation, but is presumably mainly caused by a dilution effect, resulting from increased lignification of the old needles. Only with calcium (and silicon) dilution is overcompensated for by a continuation of high influx into old needles. With the exception of magnesium, the data of Table 12.4 are indicative of trees well supplied with these macronutrients. 12.3.4

Plant Species

Critical deficiency contents differ between plant species even when comparisons are made for the same organs at the same physiological age. This is also true for the adequate range. These variations are mainly based on differences in the plant metabolism and plant constitution, as for example, differences in the genotypical demand of calcium and boron in cell walls. When grown under the same conditions the CDC of boron in the dry matter of the fully expanded youngest leaf is 3//g g _1 in wheat, 5/*g in rice, but as high as 25 μ% in soybean and 34 μ% in sunflower (Rerkasem et al., 1988). Indigenous plant species from nutrient-rich habitats seem to have higher fcriticäl deficiency concentrations of potassium in the shoots (~100 mM) than species from nutrient-poor habitats (~50 mM; Hommels et al., 1989a). Representative data for adequate nutrient ranges of selected species are given in Table 12.5. More extensive and detailed data, including deficiency and toxicity contents, can be found in Chapman (1966); Jones (1967; 1991), Bergmann and Neubert (1976) and Drechsel and Zech (1991). As shown in Table 12.5 the contents of macronutrients in the adequate range are of similar orders of magnitude for the various plant species; an exception is calcium, the content of which is substantially lower in monocotyledons. In all species the adequate range is relatively narrow for nitrogen, because luxury contents of nitrogen have unfavorable effects on growth and plant composition (Section 8.2.5). In apple leaves, for example, a nitrogen content of more than 2.4% often affects fruit color and storage adversely (Bould, 1966). On the other hand, the adequate range for magnesium is usually broader, due mainly to competing effects of potassium; with high potassium contents, high magnesium contents are also required to ensure an adequate magnesium nutritional status. The contents of micronutrients in the adequate range vary by a factor of 2 or more (Table 12.5). Manganese shows the greatest variation, indicating that for manganese in particular, leaf tissue is bufferingfluctuationsin the root uptake of this nutrient. In plants growing in soil, manganese exhibits more rapid and distinctfluctuationsin uptake than any other mineral nutrient, the rate depending on variations in soil redox potential and thus on the concentrations of Mn2+ (Section 16.4). The data given in Table 12.5 are average values and offer no more than a guide as to whether a mineral nutrient is in the deficient, adequate, or toxic range. This should be borne in mind when only one or a few mineral nutrients have been analyzed and the information on possible nutrient interactions is therefore insufficient. The critical toxicity contents of sodium and chloride are in general closely related to genotypical differences in salt tolerance. The interpretation of these contents is complicated because in saline substrates a decline in growth is often caused in the first

a Based on Bergmann (1988,1992). ^Sodium content below 1.5%.

Spring wheat (whole shoot, booting stage) Ryegrass (whole shoot) Sugar beet (mature leaf) Cotton (mature leaf) Tomato (mature leaf) Alfalfa (upper shoot) Apple (mature leaf) Orange (Citrus ssp.) (mature leaf) Norway spruce (1-2 year-old-needles) Oak; Beech (mature leaves)

Species (organ) 2.9-3.8 2.5-3.5 3.5-6.0* 1.7-3.5 3.0-6.0 2.5-3.8 1.1-1.5 1.2-2.0 0.5-1.2 1.0-1.5

0.35-0.5 0.35-0.6 0.3-0.5 0.4-0.65 0.3-0.6 0.18-0.30 0.15-0.3 0.13-0.25 0.15-0.30

3.0-4.2 4.0-6.0 3.6-4.7 4.0-5.5 3.5-5.0 2.2-2.8 2.4-3.5 1.35-1.7 1.9-3.0

K

0.3-0.5

P

3.0-4.5

N

0.3-0.5

0.35-0.8

3.0-7.0

1.3-2.2

1.0-2.5

3.0-4.0

0.6-1.5

0.7-2.0

0.6-1.2

0.4-1.0

Ca

Contents (% dry wt)

0.15-0.30

0.1-0.25

0.25-0.7

0.20-0.35

0.3-0.8

0.35-0.8

0.35-0.8

0.3-0.7

0.2-0.5

0.15-0.3

Mg

7-15 8-20 6-12 6-15

20-80 25-80 30-80 25-70 20-50

15-60 15-50

35-100 35-100 40-100 30-100 35-100 25-125 50-500 35-100

0.25-1.0 0.6-2.0 0.3-1.0 0.5-2.0 0.1-0.3 0.2-0.5 0.04-0.2 0.05-0.2

40-100 20-80 40-80 35-80 30-50 30-70 15-50 15-40

25-60

6-12 20-50 40-100 0.15-0.5

6-12

6-12

4-10

6-15

5-12

5-10

Cu 20-70

Zn 30-100

Mn

dry wt)

0.1-0.3

Mo

1

5-10

B

Contents (mg kg

Table 12.5 Mineral Nutrient Contents in the Adequate Range of Some Representative Annual and Perennial Species 0

472

Mineral Nutrition of Higher Plants Table 12.6 Effect of Foliage Phosphorus Content on the Critical Deficiency Content (CDC) of Nitrogen and Vice Versa in Araucaria cunninghamiia Foliage phosphorus content (% dry wt)

CDC of nitrogen (% dry wt)

Foliage nitrogen content (% dry wt)

CDC of phosphorus (% dry wt)

0.06 0.09 0.12 0.16 0.21

1.07 1.18 1.24 1.31 1.35

0.60 1.05 1.35 1.65 1.80

0.07 0.08 0.10 0.11 0.12

a

Based on Richards and Bevege (1969).

instance by effects on the water balance of plants and not necessarily by direct toxicity of sodium or chloride or both these ions in the leaf tissue (Section 16.6). 12.3.5

Nutrient Interactions and Ratios

There is a whole range of nonspecific as well as specific interactions between mineral nutrients in plants (Robson and Pitman, 1983) which affect the critical contents. A typical example of a nonspecific interaction is shown in Table 12.6 for nitrogen and phosphorus. The CDC of nitrogen increases as the phosphorus content increases and vice versa. Similarly in maize, at low phosphorus content an increase in nitrogen content of the earleaf from 2.1 to 2.9% had little effect on yield, but at high phosphorus content yield continued to increase as earleaf nitrogen content rose well above 3% (Sumner and Farina, 1986). Interactions between two mineral nutrients are important when the contents of both are near the deficiency range. Increasing the supply of only one mineral nutrient stimulates growth, which in turn can induce a deficiency of the other by a dilution effect. In principle, these unspecific interactions hold true for any mineral nutrients with contents at or near the critical deficiency contents. Optimal ratios between nutrients in plants are therefore often as important as absolute contents. For example, a ratio of nitrogen to sulfur of —17 is considered to be adequate for the sulfur nutrition of wheat (Rasmussen et al., 1977) and soybean (Bansal et al., 1983). However, optimal ratios considered alone are insufficient because they can also be obtained when both mineral nutrients are in the deficiency range (Jarrell and Beverly, 1981), as well as in the toxicity range. Specific interactions which affect CDC have been discussed in Chapters 8 and 9. Therefore, only two examples are reiterated here: (a) competition between potassium and magnesium at the cellular level, which usually involves the risk of potassiuminduced magnesium deficiency; and (b) replacement of potassium by sodium in natrophilic species, which has to be considered in the evaluation of potassium content (see Table 12.5). Specific interactions are also important in evaluating critical toxicity contents. The critical toxicity content of manganese, for example, differs not only between species

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

473

and cultivars of a species (Section 9.2), but within the same cultivar, the difference depending on the silicon supply. In leaves the critical toxicity content of manganese can increase from 100 mg kg - 1 dry matter in the absence of silicon to —1000 mg, i.e., by a factor of 10, in the presence of silicon in bean (Horst and Marschner, 1978a) and by a factor of 3-4 in different cowpea genotypes (Horst, 1983). In view of the problems arising from different CDC during plant development, and of the importance of nutrient ratios in plant analysis for diagnostic purposes, a new concept had been introduced with Beaufils's Diagnosis and Recommendation Integrated System (DRIS). This system is based on the collection of as many data as possible and plant contents of mineral nutrients (so far mainly macronutrients) and use of these data for calculations of optimal nutrient ratios - nutrient indices (norm data) for example ratios of N/P, N/K etc. (Sumner, 1977). The nutrient indices calculated through DRIS are less sensitive to changes taking place during leaf maturation and ontogenesis, but depend to some extent on location. For example, for maize earleaf tissue norm N/P ratios are on average 10.13, but 8.91 for South Africa and 11.13 for the South East of USA (Walworth and Sumner, 1988). This system requires a large number of data on both contents of different mineral nutrients in the plants of a given field, and from different locations and years. The calculated norms for ratios are thus mean values obtained from several thousand field experiments. For certain crops and under certain conditions (high yielding sites, large-scale farming) the higher analytical input might pay off by permitting a refinement in the interpretation of the data in terms of fertilizer recommendations, as has been demonstrated for sugarcane (Elwali and Gascho, 1984), maize and fruit trees (Walworth and Sumner, 1988). However, under other environmental conditions less favorable results have also been obtained with DRIS (Reuter and Robinson, 1986), and it is certainly not the method of choice in cropping systems with a diversity of annual species or for low input and small-scale farming systems. 12.3.6

Environmental Factors

Fluctuations in environmental factors such as temperature and soil moisture can affect the mineral nutrient content of leaves considerably. These factors influence both the availability and uptake of nutrients by the roots and the shoot growth rate. Their effects are more distinct in shallow-rooted annual species than in deep-rooted perennial species, which have a higher nutrient buffer capacity within the shoot. This aspect must be considered in the interpretation of both critical deficiency and toxicity contents in leaf analysis. If fluctuations in soil moisture are high, then as a rule for a given plant species the CDC of nutrients such as potassium and phosphorus are also somewhat higher in order to ensure a higher capacity for retranslocation during periods of limited root supply. The effects of irradiation and temperature on the nutrient content of leaves are described in detail by Bates (1971). For example, under high light intensity the CDC in leaves of boron and zinc are higher than under low light intensity (Sections 9.7.9). In tomato the CDC of phosphorus in mature leaves increase from 1.8 to 3.8 mg g _ 1 dry matter when the external salt concentration is increased from 10 to 100 mM (Awad etal., 1990). The physiological mechanism for this higher internal requirement of phosphorus is not clear, involvement in osmotic adjustment in the mature leaves, or restricted retranslocation to expanding leaves, might be involved.

474 12.3.7

Mineral Nutrition of Higher Plants Nutrient Efficiency

Genotypical differences in the CDC of a nutrient can also be brought about by differences in the utilization of a nutrient. In a physiological sense, this may be expressed in terms of unit dry matter produced per unit nutrient in the dry matter (e.g., mg P g _ 1 dry matter). As an example, the difference in nitrogen efficiency between C 3 and C 4 grasses is shown in Table 12.7. Much more dry matter is produced in C 4 grasses than in C 3 grasses per unit leaf nitrogen. This is a general phenomenon which is observed in comparisons of other C 3 and C 4 species (Brown, 1985). The higher nitrogen efficiency of C 4 species is presumably related to the lower investment of nitrogen in enzyme proteins used in chloroplasts for C 0 2 fixation. In C 4 species only 5-10% of the soluble leaf protein is found in RuBP carboxylase, compared to 30-60% in C 3 species (Section 5.2.4). Lower CDC in C 4 plants are of advantage for biomass production on nitrogen-poor sites, but not necessarily of advantage in view of the nutritional quality of forage (Brown, 1985). Differences in the utilization of mineral nutrients are also found between cultivars, strains, and lines of a species. These differences are a component of the nutrient efficiency in general as will be discussed in detail in Section 16.2.3. In an agronomical sense, nutrient efficiency is usually expressed by the yield differences of genotypes growing in a soil with insufficient amounts of nutrients. In most instances, high nutrient efficiency is related primarily to root growth and activity, and in some instances also to the transport from the roots to the shoots (Läuchli, 1976b). Only relatively few data indicate a higher nutrient efficiency in terms of utilization within the shoots, for example, utilization of phosphorus in bean (Whiteaker et al., 1976; Youngdahl, 1990) and maize genotypes (Elliott and Läuchli, 1985), potassium in bean and tomato (Shea et al., 1967'; Gerloff and Gabelman, 1983), and calcium in tomato (English and Barker, 1987; Behlingeifl/., 1989). In principle, higher nutrient efficiency, as reflected by lower critical deficiency contents, in one genotype compared with those in another genotype of the same species can be based on various mechanisms: Table 12.7 Relationship between Dry Matter Production and Nitrogen Content of C3 and C4 Grasses0^ Nitrogen supply (equivalent to

Dry matter (g per pot)

Nitrogen content (% dry wt)

c3

c4

c3

c4

11 20 27 35

22 35 35 48

1.82 2.63 2.77 2.78

0.91 1.18 1.61 2.00

"Based on Colman and Lazemby (1970). b C3 Grasses: Lolium perenne and Phalaris tuberosa; C4 grasses: Digitaria macroglossa and Paspalum dilatatum.

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

475

1. Higher rates of retranslocation during either vegetative or reproductive growth, for example, zinc in maize (Massey and Loeffel, 1967), nitrogen in pearl millet (Alagarswamy et al., 1988) or phosphorus in bean (Youngdahl, 1990). 2. Higher nitrate reductase activity in the leaves and thus more efficient utilization of nitrogen for protein storage [e.g., in wheat grains (Dalling et al., 1975) and potato tubers (Kapoor and Li, 1982)]. 3. Higher proportion of replacement of potassium by sodium and thus lower CDC of potassium [e.g., in tomato (Gerloff and Gabelman, 1983)]. 4. Lower proportion of nutrients which are not - or only poorly - available for metabolic processes, either due to compartmentation or chemical binding, for example of phosphorus in maize genotypes (Elliott and Läuchli, 1985). This aspect is particularly relevant for calcium, where in efficient genotypes of tomato a higher proportion of calcium is translocated to the shoot apex where more calcium also remains in the water soluble fraction (Behling et al., 1989). On the other hand, a lower CDC of 0.25% Ca in the shoot dry matter of an efficient tomato genotype compared to 0.40% in an inefficient genotype was correlated to much higher potassium contents in the shoot dry matter of the inefficient genotype (English and Barker, 1987), stressing the importance of certain physiologically based nutrient ratios for the CDC. 5. Differences in the ratio of vegetative shoot growth (source) to the growth of reproductive or storage organs (sink) or both. This aspect (Section 6.4) is probably in part responsible for the general pattern in so-called modern cultivars of many crop species with a high harvest index in which the critical deficiency contents of mineral nutrients in the leaves are usually higher than those of traditional cultivars. 12.3.8 Total Analysis versus Fractionated Extraction

Most frequently it is the total dry matter content of a nutrient that is determined in plant analysis (e.g., after ashing). The determination of only a fraction of the content - for example, that which is soluble in water or in dilute acids or chelators - sometimes provides a better indication of nutritional status. In terms of plant analysis as a basis of fertilizer recommendations this is particularly true for nitrate which is a prominent storage form of nitrogen in many plant species. In those species the nitrate content is usually a much better indicator of the nitrogen nutritional status than the total nitrogen content, and the only realistic approach for a simple, rapid method of plant analysis not only as a reliable indicator of the nitrogen nutritional status of the plants but also as a basis for recommendations of top dressing of nitrogen fertilizers. This method has been successfully used, more recently, for example, in winter cereals (Wollring and Wehrmann, 1990), irrigated wheat (Knowles et al., 1991), potato (Westcott et al., 1991), cabbage (Gardner and Roth, 1989) and other vegetable crops (Scaife, 1988). There are only a few cases where this method has not proved to be a satisfactory predictor of responsiveness of a crop to nitrogen fertilizer (Fox et al., 1989). The principal limitations of this method are set by plant species which preferentially reduce nitrate in the roots (e.g., members of the Rosaceae), or when ammonium nitrogen is supplied and taken up prior to nitrification in soils. This situation might be true for soils high in organic nitrogen with high mineralization rates during the stages of high nitrogen demand of a crop.

476

Mineral Nutrition of Higher Plants

Table 12.8 Calcium and Oxalic Acid Content of Two Cultivars of Burley Tobacco Differing in Susceptibility to Calcium Deficiency0'

Plants with Ca-deficiency symptoms Cultivar (% of total) KylO B21

0 50

Content of buds (meq g _1 dry matter)

Content of upper leaves (meq g"1 dry matter)

Ca

Oxalic acid

Ca minus oxalic acid (soluble Ca)

Ca

0.25 0.23

0.08 0.16

0.17 0.07

0.28 0.30

Ca minus Oxalic oxalic acid acid (soluble Ca) 0.11 0.15

0.17 0.15

°Based on Brumagen and Hiatt (1966). Determination of the arginine content in needles seems to be a better indicator than the total nitrogen content for assessing the nitrogen nutritional status and particularly of nutrient imbalances in Norway spruce stands with different levels of atmospheric nitrogen input (Ericsson etal. 1993). For assessing the sulfur nutritional status of plants the content of sulfate as the main storage form of sulfur, is also a better indicator than the total sulfur content. In various legume species the CDC of S0 4 -S in the dry matter of fully expanded leaves decreases during ontogenesis, for example, in alfalfa from 0.39% to 0.10% (Huang etal., 1992c). In some instances the ratio of S0 4 -S to total sulfur might be an even better indicator, for example in wheat (Freney et al., 1978) or rice (Islam and Ponnamperuma, 1982) but not in various forage grasses and legumes. There are conflicting reports on the suitability of using only the inorganic (or readily extractable) fraction of phosphorus, instead of total phosphorus, as a diagnostic criterion of the phosphorus nutritional status of plants. This approach seems to be suitable in the grapevine (Skinner et al., 1987) but not for subterranean clover (Lewis, 1992). Determination of only a defined fraction of a mineral nutrient might offer not only the possibility of better characterization of the reserves stored in plants (e.g., NO3-N; SO4-S) but also of the physiological availability of a nutrient in plant tissue. For example, extraction of leaves with diluted acids or chelators of Fe(II) for characterization of the so-called 'active iron' might improve the correlations between iron and chlorophyll content in leaves in field-grown plants (Section 9.1.5) but not necessarily so in plants grown under controlled environmental conditions in nutrient solutions (Lucena et al., 1990). Determination of water-extractable zinc in leaves might reflect the zinc nutritional status of plants better than total zinc (Rahimi and Schropp, 1984), particularly in plants suffering from phosphorus-induced zinc deficiency (Cakmak and Marschner, 1987). Another example of the importance of determination of only a defined fraction of a mineral nutrient for characterization of physiological availability is illustrated in Table 12.8. Differences in the susceptibility of tobacco cultivars to calcium deficiency were not related to the total calcium content but to the soluble fraction in the buds. These differences were caused by variations in the rate of oxalic acid synthesis and thus in the

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

477

precipitation of sparingly soluble calcium oxalate. Accordingly, the critical deficiency level of total calcium was higher in B 21 than in Ky 10. Determination of only the soluble fraction would be a more appropriate method for assessing the calcium nutritional status of the two cultivars. 12.4

Histochemical and Biochemical Methods

Nutritional disorders are generally related to typical changes in thefinestructure of cells and their organelles (Vesk et al., 1966; Hecht-Buchholz, 1972; Niegengerd and HechtBuchholz, 1983) and of tissue. Light microscopic studies on changes in anatomy and morphology of leaf and stem tissue can be helpful in the diagnosis of deficiencies of copper, boron, and molybdenum (Pissarek, 1980; Bussler, 1981a). A combination of histological and histochemical methods is useful in the diagnosis of copper (Section 9.3.4) and phosphorus deficiencies (Besford and Syred, 1979). Enzymatic methods involving marker enzymes offer another approach to assessing the mineral nutritional status of plants. These methods are based on the fact that the activity of certain enzymes is lower or higher (depending on the nutrient) in deficient than in normal tissue. Examples were given in Chapter 9 for copper and ascorbate oxidase; zinc and aldolase or carbonic anhydrase; and molybdenum and nitrate reductase. Either the actual enzyme activity is determined in the tissue after extraction or the leaves are incubated with the mineral nutrient in question to determine the inducible enzyme activities of, for example, peroxidase activity by iron (Bar-Akiva et al., 1970) and nitrate reductase by molybdenum (Section 9.5.2). For assessing the manganese nutritional status, the activity of MnSOD (Section 9.3) in leaves might be used as biochemical marker (Leidi et al., 1987) or, as a nondestructive method, the specific chlorophyll fluorescence (Kriedemann and Anderson, 1988). Biochemical methods can also be used for assessing the nutritional status of plants in relation to macronutrients. The accumulation of putrescine in potassium-deficient plants (Section 8.7) is a biochemical indicator of the potassium requirement of lucerne (Smith et al., 1982). Inducible nitrate reductase activity can be used as an indicator of nitrogen nutritional status (Witt and Jungk, 1974; Dias and Oliveira, 1987). Pyruvate kinase activity in leaf extracts depends on the potassium and magnesium content of the leaf tissue (Besford, 1978b). In phosphorus-deficient tissue, phosphatase activity is much higher, especially the activity of a certain fraction (Fraction B; isoenzyme) of the enzyme (Table 12.9). The increase in phosphatase activity in deficient tissue is an interesting physiological and biochemical phenomenon which might be related to enhanced turnover rates or to remobilization of phosphorus or both these factors (Smyth and Chevalier, 1984). In eucalyptus seedlings and 5-year-old plants, acid phosphatase activity is a more sensitive parameter for diagnosis of growth limitations by phosphorus than estimation of total phosphorus in leaves and stems (O'Connell and Grove, 1985), whereas in maize acid phosphatase activity increased distinctly only under severe phosphorus deficiency and appears to be suitable as a means of confirming visual diagnosis but is not sensitive enough to indicate latent phosphorus deficiency (Elliott and Läuchli, 1986). In principle, enzymatic, biochemical and biophysical methods can be very valuable if

478

Mineral Nutrition of Higher Plants Table 12.9 Growth, Phosphorus Content, and Phosphatase Activity of Young Wheat Plants 0

Phosphorus supply High Low

Shoot dry wt (mg per plant)

Phosphorus content of shoot (%)

Phosphatase activity (μπιοΐ NPP 6 g _ 1 fresh wt min - 1 ) Total

Fraction A

Fraction B

223 135

0.8 0.3

5.6 11.1

4.4 6.7

0.5 2.9

"From Barrett-Lennard and Greenway (1982). 6 NPP, p-nitrophenylphosphate.

the total content or the soluble fraction of a mineral nutrient is poorly correlated with its physiological availability. Whether these enzymatic, biochemical and biophysical methods can realistically be used as alternatives to chemical analysis as a basis for making fertilizer recommendations depends on their selectivity, accuracy, and particularly whether they are sufficiently simple to provide a spot test. In the case of iron and peroxidase (Bar-Akiva etal., 1978; Bar-Akiva, 1984) and copper and ascorbate oxidase (Delhaize etal., 1982), enzymatic methods seem to meet these requirements. Nevertheless, calibration of the methods remains a problem when a suitable standard (nondeficient plants) is not available and there are no visible deficiency symptoms. The potential of these methods in foliar analysis for diagnostic purposes is more in solving particular problems of nutritional disorders and to supplement total and fractionated foliar analysis rather than to replace them. 12.5

Plant Analysis versus Soil Analysis

There is a long history of controversy as to whether soil or plant analysis provides a more suitable basis for making fertilizer recommendations. Both methods rely in a similar manner on calibration, that is, the determination of the relationship between contents in soils or plants and the corresponding growth and yield response curves, usually obtained in pot or field experiments using different contents of fertilizers. Both methods have advantages and disadvantages, and they also give qualitatively different results (Schlichting, 1976). Chemical soil analysis indicates the potential availability of nutrients that roots may take up under conditions favorable for root growth and root activity (Chapters 13 and 14). Plant analysis in the strict sense reflects only the actual nutritional status of plants. Therefore, in principle a combination of both methods provides a better basis for recommending fertilizer applications than one method alone. The relative importance of each method for making recommendations differs, however, depending on such conditions as plant species, soil properties, and the mineral nutrient in question. In fruit or forest trees, soil analysis alone is not a satisfactory guide for fertilizer recommendations, mainly because of the difficulty of determining with sufficient accuracy the root zones in which deep-rooting plants take up most of their nutrients. On the other hand, in these perennial plants seasonal fluctuations in the mineral nutrient

Diagnosis of Deficiency and Toxicity of Mineral Nutrients

479

content of leaves and needles are relatively small compared with those in annual species. The nutrient content of mature leaves and needles is therefore also an accurate reflection of the long-term nutritional status of a plant. Furthermore calibrations of critical deficiency content and adequate range can be made rather precisely and refined for a specific location, plant species, and even cultivar. Therefore, in perennial species foliar analysis is in most instances the method of choice. In this instance, however, chemical soil analysis, performed at least once for a given site, is necessary for characterizing the overall level of potentially available nutrients. In pastures, plant analysis is used more frequently than soil analysis, not only because of the peculiarities of the rooting pattern in mixed pastures (deep- and shallow-rooting species) but also because of the importance of the mineral composition of pasture and forage plants for animal nutrition. In annual crops the short-term fluctuations of mineral nutrient contents place a severe limitation on plant analysis as a basis for making fertilizer recommendations. Chemical soil analysis is required for predicting the range of variation in plant nutrient content throughout the growing season. In annual crops a large proportion of the mineral nutrients are taken up from the topsoil, which makes soil analysis easier and increases its importance as a tool for making fertilizer recommendations (but see also Chapter 13). There is no doubt, however, that for various reasons plant analysis will also become appreciated more as an important guide to the nutrition of annual crops. Nutrient imbalances in plants, especially latent micronutrient deficiencies, present a problem particularly in intensive agriculture (Franck and Finck, 1980). This problem is also worldwide (Welch et al., 1991), though with consequences not only for plant yield but also for plant resistance to, and tolerance of, diseases and pests (Chapter 11) as well as for the quality for grazing animals (Kubota et al., 1987) and for animal and human nutrition in general (Chapter 9).