Effects of The Environment on The Growth of Alfalfa

EFFECTS OF THE ENVIRONMENT ON THE GROWTH OF ALFALFA K. R . Christian Division of Plant Industry. Commonwealth Scientific and Industrial Organization. Canberra. Australia

I . Introduction

.................................................. ........................ ShootGrowth ................................................. A. Mathematical Description ...................................... B . Internode Number ........................................... C. Leaf:Stem Ontogeny ......................................... D . Leaf Growth and Development .................................. RootGrowth .................................................. Environmental Factors and Vegetative Growth ........................ A. Light ..................................................... B. Temperature ................................................ C. Water ..................................................... D. Minerals ................................................... Phases in Development ........................................... A. Bud and Shoot Initiation ...................................... B . Root Carbohydrate Storage .................................... C. Regrowth Characteristics ...................................... D. Flower and Seed Formation .................................... E. Seedingandtheseed ......................................... Plant Associations .............................................. A . 1ntraspecific:Plant Density ..................................... B . Interspecific Competition ...................................... Genetic Adaptation to Environment ................................ References ....................................................

I1. Genetic Variation in Response to Environment

111.

IV . V.

VI .

VII . VIII

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I

183 185 186 186 187 188 188 189 191 191 195 199 204 209 209 210 211 212 213 214 214 215 217 219

Introduction

The importance of alfalfa in world agriculture needs no further assertion than a reference to its venerable reputation since antiquity and to its geographical distribution . The future potential and limitations of this crop are t o be seen in the volume of scientific papers emanating from every major region in which its cultivation has been considered practicable . The agronomy of alfalfa in all its aspects has recently been described. both in the United States (Hanson. 1972) 183

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and in Australasia (Langer, 1967). A certain amount of repetition of the material contained in those extensive compilations will be unavoidable in this review, which is concerned with a more limited appraisal, from a nonspecialist perspective, of the complex of factors contributing to the variability in growth under different conditions. Plant growth might be described as a genetically planned construction attuned to the environment, if such a definition did not fail to convey fully the truism that the plant has no existence apart from the environment. When one speaks of the effect of environment, it is always a departure of some sort from some other environment that is implied. This leads naturally to the proposal of a standard or reference environment, which immediately raises problems in specification. The notion of an optimal environment is open to criticism, since the requirements for maximum dry matter yield are not necessarily those which produce material of best quality or which promote highest seed yields or greatest persistence. Furthermore, the ideal environment for any one of these objectives might well involve inordinate quantities of light and nutrients, C 0 2 enrichment, and so on. Nevertheless, any study of growth requires, tacitly or explicitly, a comparison with a “normal” behavioral pattern for that genotype under circumstances where development is not unreasonably hampered by any single external parameter. It is usually not practicable to specify the environment other than in general terms, such as a set of instrument readings at a given height above the crop, which may give little impression of the changes taking place beneath. The plant inhabits the two vastly differing media of atmosphere and soil, each of which influences the other. In partaking of the tetrad of earth, air, fire in the form of radiation, and water, the plant contributes to its own microenvironment at each level of the vertical profde, and thereby to the environment as a whole. Fredricksen (1938) described marked differences in runoff, soil moisture, soil structure, air temperature, humidity, and wind movement with an alfalfa field as compared with prairie bunch grass vegetation, and similar observations have been made since. The influence of the environment extends to the response of impinging organisms, including neighboring plants, pathogens, pollinators, symbiotic microflora, and grazing animals. Disease infestation is often a secondary effect, resulting from environmentally induced hazards such as waterlogging, frost damage, high temperatures, and nutrient deficiencies, but its importance may ultimately be much greater. Alfalfa plants grown in a pathogen-free environment can be subjected to severe clipping treatment without displaying plant mortality or root necrosis (Hamlen et al., 1972). Willis et al. (1969) showed that fungicide spraying could reduce leaf drop and increase hay yields by 18% over one growing season. Management practices such as tillage, irrigation, and fertilizer application, and variations in harvesting and grazing schedules, can also modify the environment

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in many ways. In fact, it is difficult to think of aspects of growth which are not subject to environmental effects, except perhaps the genetic code itself; and even then, the interaction between genotype and environment is an important consideration. 11.

Genetic Variation in Response to Environment

The diversity and plasticity of alfalfa is illustrated by the fact that the cultivated forms cover the entire range between the extreme types of the Medicago sativa-falcata-glutinosa complex from which they were originally derived. The scope for further development of genetic material is seemingly unlimited. The strains containing Medicago falcata genes are generally characterized as winter-dormant and cold-hardy. Growth habit is somewhat prostrate, spreading, branching, and rosettelike at short day lengths, but erect and nonbranching like that of M. sativa at long day lengths. Other features of M. falcata genotypes include the tendency to form adventitious shoots on root segments (Smith, 1950), high shoot:root ratios (Heinrichs and Nielsen, 1966), and slow recovery after cutting (Tysdal and Kiesselbach, 1939). They are also thought to be more persistent, and more resistant to disease, waterlogging, and winter injury. Although it is tempting to think in terms of a “Medicago falcata syndrome,” evidence suggests that the association of these attributes is likely to be coincidental rather than due to genetic linkage. Cold-hardiness is usually regarded as an integral component of these genotypes, yet Greenham (1966) pointed out that different physiological factors are required at each critical stage, and that different genes may be responsible. It is difficult to see how these factors could not have developed contemporaneously during adaptation, but it is important to recognize that selection for only one of them may fail to achieve its overall objective. Likewise, the correlation between cold injury and growth at low temperatures has been interpreted as showing independent responses to natural selection (Daday, 1964). Winter dormancy appears to result from a qualitative difference in the form of a threshold for winter activity, rather than the expression of one extreme of a distribution of growth rates (Morley et al., 1957). Both erect, vigorous types and prostrate, slow-growing types are to be found in Mediterranean lines (Leach, 1970b). Persistence under frequent cutting may be attributable to a form of growth with broad crowns and shoots emerging close to the ground, rather than t o the M.falcara genes (Leach, 1971b; Cameron, 1974). The variability within populations illustrates the difficulty of classifying material according to genotype (Heinrichs e t al., 1969). On the other hand, classifications based on agronomic characters may be useful for making comparisons within local regions (Yamada and Suzuki, 1974), but extrapolation to other

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environments can be quite misleading. Zaleski (1954) classified as late-flowering several varieties which are regarded in the countries to which they are adapted as early-flowering, It is almost certainly because they were winter-active, and sustained such injury during more inclement conditions than those to which they were accustomed, that growth during spring was delayed because of their weakened condition. Song and Walton (1974) have suggested that breeding for late autumn growth, without the accompanying development of physiological adaptation to winter survival, can result in reduced plant vigor in the following spring. Furthermore, it is often impossible to tell how or even whether a particular phenotypic character contributes to survival. In the extreme climate of eastern Anatolia, for instance, wild forms of Medicago sativa with spreading, prostrate stems are found in the same area as forms which are perfectly erect (ChristiansenWeniger and Tarman, 1939). Citing the example of M. asiatica, which is highly resistant to leaf loss in Afghanistan, but undergoes severe leaf shedding when grown in Europe, Bennett (1 970) warns that “Except when employed for characteristics with a very high heritability (which can only be determined by prior genetic studies), or when conducted in an environment closely resembling that in which the collected material is to be utilized without further genetic manipulation, phenotypic selection is an unreliable basis for sampling.” Genetic+nvironment interactions represent possible sources of variation of uncertain magnitude to be kept in mind when discrepancies between reports are encountered. It is therefore remarkable that winter-dormant and winter-active types have been found to be not different in photosynthetic rates at different temperatures (Pearson and Hunt, 1972a) or in water-use efficiency (McElgunn and Heinrichs, 1975).

Ill. Shoot Growth

A. MATHEMATICAL DESCRIPTION

The following brief formal description indicates how one may proceed from measurements of differential growth to the quantitative assessment of total yield. The shoot is regarded as consisting of internodes, i in number, each of which contains a segment of stem of length 1 and cross-sectional area A , and two oppositely placed trifoliate leaves with thickness t and surface area S (one side only). The total aerial volume V of a plant with n shoots is then given by V=

5;

(IA t rS)

If V is made up of a proportion D of plant dry matter with density ~ ~ ( 1 . 8

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glml), according to Hundtoft and Wu (1970), W of water with density p w and void space A , the fresh weight Y is specified in appropriate units by where The formulas indicate the minimum requirements for the full interpretation of data, while the true picture will often be considerably more complex. No allowance is made for the incidence of branching, for example; nevertheless, a high correlation is likely to exist between leaf and internode numbers per stem (Liang and Riedl, 1964). Another problem is the high mortality of shoots during growth (Christian er al., 1970), representing considerable wastage of dry matter accumulation. Petioles, stipules, stem tips, flower buds, and other reproductive structures are reported to comprise a fairly constant 11% of total herbage dry matter from the bud stage onward (Fick and Holthausen, 1975). B. INTERNODE NUMBER

While the number of internodes which a plant produces may be characteristic in a given environment, the position at which flowering first occurs differs strikingly among phenotypes (Jones, 1950). Medler et al. (1955) concluded from a study of nine winter-hardy clones that flower positions at the 14th or 15th node were associated with long day length, while at short day lengths, flowers first appeared at about the 10th node. Using creeping-rooted clones under controlled conditions, Carlson (1965) found a similar relationship and noted that under long photoperiods plants continued to produce nodes even after starting to flower. In contrast, Dobrenz et al. (1965) found a high inverse correlation between minimum temperature and the number of nodes to the first raceme in a nonhardy variety “Moapa,” and to the time from cutting to floral initiation. However, Field and Hunt (1974) reported that the number of leaves and the node of first flower were unaffected by temperature at a photoperiod of 16.5 hours. It is difficult to reconcile this evidence except by postulating different responses of winter-dormant and winter-active varieties to day length and temperature. The contrasting experimental conditions may also have had some influence. Internode number is reduced by water restriction (Perry and Larson, 1974), and early transition from the vegetative to the reproductive stage may well be a response to stress, whatever its nature. The possible consequences on production of high temperatures close to dry and almost bare soil following cutting in summer warrant closer investigation.

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Lengths of internodes, and the ratios between them, are varietal characteristics in a given environment (Sheridan and McKee, 1968). In general, the length increases for the first few internodes, then steadily decreases toward the plant apex, while similar trends are apparent in leaf size. Leaf weight increases almost linearly with shoot height, with fairly constant chemical composition and uniformly high digestibility (Smith, 1970a; Christian et al., 1970). Because the stem has the function of a support, however, its cross-sectional area increases at the base, and the weight of stem dry matter increases almost as the square of the height (Christian er al., 1970). The top segments of shoots are of similar composition and of high digestibility, irrespective of shoot height, but make up only a small proportion of the total dry weight. The major weight gains during maturation are in the heavily lignified tissues of the lower stems, and hence nutritive quality falls off at an increasing rate as growth continues. The best time to harvest material of high digestibility is before severe leaf drop occurs and while the stems can still be readily cut at the base with a pair of scissors. Protein content is highly related directly to digestibility and inversely to lignin and cellulose contents. Weight increase of shoots is approximately linear over most of the period of vegetative growth (Pearson and Hunt, 1972d), although there appears to be an increase in the rate of stem thickening and elongation around the bud stage (Dent, 19.55; Nishikawa, 1966), with little increase in height or yield after the full bloom stage (Raguse and Smith, 1966). Large leaves are often associated with thick stems (Zaleski, 1954), and therefore probably with long internodes. Plants of Medicago fulcata type are usually regarded as leafier than M. sutiva types, but this may be merely due to an initially slower rate of stem development after cutting. Taken at the same stage of growth, different varieties have been found to have similar leaf stem ratios (Davies, 1960) and similar leaf and stem protein levels (Dobrenz er al., 1969). As maturity approaches, growth rates of M. falcata types become more rapid than those of M. sativa types, so that comparative yields become dependent on harvest date (Sprague and Fuelleman, 1941; Tysdal and Kiesselbach, 1939). D. LEAF GROWTH AND DEVELOPMENT

Cell division is most active during the early stages of leaf expansion, and is inversely related to the rate of cell enlargement (Koehler, 1973). Eventually, at a particular leaf length, cell division ceases, and mean cell size then increases in proportion to leaf length. Leaf growth may therefore be modified in different ways at different stages of development, depending on whichever process is

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predominant. Area, weight, and photosynthetic activity all increase at comparable rates in developing leaves when expressed as proportions of the values at full expansion (Wolf and Blaser, 1971b). The rate of development, or number of days to full expansion, is related, naturally enough, to the rate of leaf and internode appearance (Wolf and Blaser, 1971b; Field and Hunt, 1974). Turrell (1942) classified alfalfa leaves as primary, secondary, and so on, according to the origin of the shoots that bore them (main stem, axillary bud from primary leaf, etc.). Leaves of higher order formed increasingly at the lower nodes as the plant aged; they tended to become successively less in area and thickness, with thinner epidermal, palisade, and spongy mesophyll layers. The earlier and larger leaves had high interna1:external surface ratios and high intercellular volume; although stomate density was lower than in smaller leaves, pore size was greater. These factors were hghly correlated with susceptibility to damage from SOz, indicating that gas exchange rates were proportionately higher in larger leaves. Evidently the smaller and later leaves are restricted at the cell enlargement stage, and are unlikely to become as photosynthetically effective. Considerable interest has been shown over the last decade in the measurement of specific leaf weight (SLW) as a possible index of photosynthetic activity. Designating leaf dry weight as YD, SLW may be expressed in terms of the formula given earlier as

SL W = Y D / S= t p D D Clearly, SLW may vary according to leaf thickness, dry matter content, or void space, or any combination of these. In Lolium varieties, SLW may be higher in thin leaves with small mesophyll cells than in thick leaves with large cells (Wilson and Cooper, 1969). Under and conditions, the desert shrub Enceliu furinosu has leaves of very high SLW, with compact mesophyll cells and very little intercellular space (Cunningham and Strain, 1969). In alfalfa, Delaney and Dobrenz (1974) found that SLW in different genotypes was related directly to the thickness of the leaf and of the palisade tissue, and inversely to leaf area. However, Barnes et al. (1969) concluded that SLW and leaf area were under separate genetic control, with all possible combinations being encountered. IV. Root Growth

In the young seedling, root growth starts more slowly than shoot growth, and is mainly confined to the tap root, with little lateral development. Within 2-4 months, however, the shoot:root ratio declines from about 2.0 to 1.O (Gist and Mott, 1958; Matches et al., 1962). Thereafter, the ratio is likely to be governed

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to an increasing extent by environmental factors. After shoot maturation, tap root diameter and total weight continue to increase steadily (Crowder et al.., 1960; Nishikawa, 1965). Where the effective depth is 6 feet or less, maximum penetration depth may be attained within 1 year, with subsequent growth devoted to increasing the number and thickness of taproots and laterals (Upchurch and Loworn, 195 1). Thick lateral roots develop sporadically and secondary thickening extends down the whole length of the tap root (Tanaka, 1971a,b). Severing the tap root some inches below the soil surface results in increased lateral root development, with little effect on other growth (Klebesadel, 1964). Large varietal differences in the number of branch roots and in the proportion of primary roots showing branching have been observed (Smith, 1951). Little is known of the seasonal pattern of root growth. Jones (1943) distinguished the permanent or cambial roots, consisting almost entirely of secondary growth and providing transport and storage, from the transient roots, which are primary in structure and which undergo growth in spring and autumn and decay during summer. The typical pattern of root growth for many species was described by Loomis and Ewan (1936) as geotropic movement down to dry soil, followed by lateral branching in the moist layer above, without hydrotropic response. The process may be briefly described as an osmoregulated force extended by the plant (Creacen and Oh, 1972) against the mechanical resistance of the soil, which is primarily a function of bulk density and water content (Taylor and Gardner, 1963). A similar behavioral response apparently ensues when a sudden transition occurs to any inhospitable region, such as one of mineral deficiency or toxicity, or a hardpan layer. Where the restriction is less severe, growth is restricted to the tap root, and there is little or no lateral formation. The tap root always seems to develop first, even in compact soils, where branch roots eventually become more important (Carlson, 1925). On eroded slopes, the tap root may disintegrate, its functions being taken over by lateral roots (Lapinskiene, 1966). In deep sandy soils, there is little mechanical impediment, but the water-holding capacity is often low, encouraging tap root extension (Lamba et al., 1949; McCleUand, 1969). On fine textured soils, growth is often restricted by compaction, which may hamper growth by preventing the entry of root hairs or by lack of aeration (Scott and Erickson, 1964), and may be aggravated by animal traffic (Tanner and Mamaril, 1959). Increase in bulk density leads to lower plant yields (Cifford and Jensen, 1967), even when root growth is not visibly restricted (Peterson, 1971). Hence plants grown on clay soils are usually small (McClelland, 1969), with slower growth rates than on lighter soils (Dent, 1955), although the variety “Hairy Peruvian” is reported to be an exception to this rule (Rogers, 1963). Compaction generally increases with depth, which contributes to the

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frequent observation that most of the root system is contained within the first foot or so of soil (Lamba et aL, 1949; Upchurch, 1951; Bennett and Doss, 1960). Root penetration and water storage may be improved by mixing high density subsoil with topsoil (Cary e f al., 1967). In heavily compacted subsoils, tap roots follow cracks and cleavage planes (Fehrenbacher e t a l , 1965), often becoming thinner and crooked, but resuming normal growth further down (Scott and Erickson, 1964; Safta and Balan, 1971). In moist swelling subsoils, roots may follow earthworm burrows and gaps left by former decayed roots and evidently combine removal of water with root extension down the crevices left by the cracking of the soil mass (Fredricksen, 1938). The intervening blocks of soil are often bypassed, and the few lateral roots are confined to the more friable regions (Paltridge, 1955). Root growth is not limited to the vertical plane. Paltridge (1955) found that when roots were unable to penetrate a L'self-sealing'' layer, they spread laterally, up to 16 feet or until the roots of neighboring plants were reached. He affirmed that alfalfa is not necessarily or genetically a deep-rooting plant, but that its roots will grow in any plane where water is available. V.

Environmental Factors and Vegetative Growth

A. LIGHT

I . DayLength Low growth rate under short day lengths is one of the most distinctive features of winter-dormant varieties. In particular, internode elongation is greatly reduced (Carlson, 1965). Much branching takes place, presumably as a result of loss of apical dominance, since at long day lengths, adventitious stem production is inhibited (Carlson, 1965). Since even normally developed stems in these varieties are often thin, the combination of reduced supporting ability and the change in load distribution probably contributes to the typically decumbent growth habit. The effects of day length and temperature overlap, and the interaction is likely to produce different responses in different varieties, but because of the great variability between individual plants, few detailed comparisons have been carried out. At temperatures of 15"/10°C (the figures written in this form will be used to indicate day and night temperatures, respectively) and 8-hour photoperiod, Sato (1971) found that leaves of Du Puits (an intermediate type) were thicker, but much shorter and narrower, than at higher temperatures or longer day lengths. As the temperature is raised, the growth inhibition by day length disappears. The data of Schonhorst et al. (1957) show that at 15.5"C the shoot

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height of winter-dormant strains does not increase when day length is extended from 8 to 12 hours, whereas in winter-active varieties at low temperatures and in all varieties at 26.7"C, shoot height is almost proportional to photoperiod. However, the results of Iversen and Meijer (1967) suggest first that weight responses may be much greater than responses in height, and second that the complete picture may be quite a complex one. Leaf development may be more rapid at long day lengths (Wolf and Blaser, 1971b) but leaf size is apparently not affected, and since stem diameter as well as length is increased (Sato, 1974), 1eaf:stem ratios are accordingly reduced (Coffmdaffer and Burger, 1958; Sato, 1971). Growth rates are reported to be highest under continuous light (Guy et uZ., 1971). Light interruption of the dark period was found to increase plant height during cooler, shorter days, with winter-active and intermediate types producing more erect and fewer semierect stems (Massengale et UZ., 1971). However, yields were in general higher with natural daylight only, particularly during the summer months. Reports on the effect of day length on root growth differ, suggesting that stage of growth or other interactions may be involved. Coffindaffer and Burger (1958) and Carlson (1965) observed no appreciable effect on root weights, and Seth and Dexter (1958) showed that there was little effect on tap root lengths of either hardy or nonhardy varieties. Hanson (1967) recorded increases in dry matter yields of all plant parts at long day lengths, although the proportion of roots was less. Sat0 (1971) found that root:top ratios tended to be highest at a 12-hour photoperiod. On the other hand, long day lengths have been observed to accelerate thickening of tap roots and growth of the root system as a whole while short days retarded root growth (Ueno and Tsuchiya, 1968). 2. Light Composition

Winter-active varieties increase in stem length much more rapidly than winterdormant varieties under mixtures of blue and red lights or green and red lights, particularly at low temperature (Nittler and Gibbs, 1959). Inhibition of hypocotyl elongation was also greatest in winter-dormant varieties. The diversity of reactions to selected colors found by these workers may indicate one reason for conflicting results between different laboratories and between the laboratory and the field. A complete, or at least a balanced, spectrum is required for maximum growth, particularly root growth (Heinrichs, 1973). The proportion of far red (740-mm) light transmitted by the alfalfa canopy is much higher than that of red (640-mm) light (Robertson, 1966); this would favor photomorphogenic reversal at the plant bases. It may also be significant that the proportion of far red radiation in natural sky light increases at twilight, particularly when the sky is clear; the effect will assume greater importance at

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high latitudes (Robertson, 1966). In Phaseolus, internode elongation is a function of phytochrome in the far red absorbing form at the start of night, and is proportional to the number of hours of darkness. The process differs from that due to long days and becomes less important at higher internode number (Vince-Prue, 1975). It seems possible that the initiation of new growth from the plant base could be inhibited by the canopy above through selective radiation effects as well as through apical dominance. Light intensity alone is insufficient to explain the effect, since bud elongation takes place in defoliated plants even in darkness and in plants where the crowns become exposed to direct sunlight because of lodging. There seems to be no evidence to suggest that changes in spectral composition during cloudy or overcast days are large enough to modify plant growth, but the possibility should perhaps not be overlooked. 3. Light Intensity Reduced light intensity removes growth inhibition by light, and the immediate result is rapid internode elongation (Pritchett and Nelson, 1951). The formation of the layer of woody tissue inside the cambium is reduced, so that the stems are much thinner than in normal plants and have the appearance of very young, succulent internodes. The effect is transient, coming to a halt more quickly at low light intensities, so that greatest stem height is likely to be found at intermediate levels (Pritchett and Nelson, 1951). Relative growth rates are increasingly reduced at light intensities below 50-75% of full daylight, and follow the general trend of the hyperbolic lightphotosynthesis curve. SLW increases with light intensity (Cooper and Qualls, 1967). Mean leaf area is diminished to a much lesser extent (McKee, 1962), so that the net effect may be that the leaf weight:total plant weight ratio remains fairly constant (Cooper, 1966, 1967). Stem growth is proportionately increased at the expense of root growth, particularly in the early stages of seedling growth (Cooper, 1966, 1967; Pritchett and Nelson, 1951). Nodulation is even more severely affected, presumably because of inadequate carbohydrate supply (Pritchett and Nelson, 1951; McKee, 1962). As shoot length increases, shading of lower leaves becomes an increasing limitation to production. Thomas and Hill (1949) found that net assimilation in plants 48 inches tall was only twice as great as in plants 6 to 8 inches tall, even though the leaf weight was 3.6 times as great. King and Evans (1967) observed that net photosynthesis in single plants approximately doubled as the leaf area index increased from 2 to 10. It should be noted, however, that the light response curves show that shaded leaves are photosynthetically just as efficient as younger, sunlit leaves at the light levels at which they are operating and,

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because of their low respiration rates (King and Evans, 1967), they are evidently not parasitic (Wolf and Blaser, 1971a). Experiments have shown that the longevity of leaves can be increased by thinning the stand (Wolf and Blaser, 1972; Pearce et al., 1968). However, removal of a large proportion of the photosynthetic area will almost certainly divert nutrients and growth factors to the remainder. Lower leaves continue to senesce even in full sunlight, but rejuvenation occurs within a few days if the upper part of the shoot is removed (Hodgkinson et al., 1972). The importance of apical dominance was further demonstrated by the observation that high rates of photosynthesis were maintained for a longer time when new shoot buds were removed as they appeared. Net photosynthesis per unit leaf dry weight was found by Pearce and Lee (1969) to be reasonably constant, whether differences in SLW were due to differences in environment or in genotype. The experiments of Delaney and Dobrenz (1974) also show that photosynthesis per unit leaf weight is not related to SLW. In the field, however, the decline in photosynthesis with the degree of shading experienced in dense canopies was much greater than that in SLW (Wolf and Blaser, 1971a). Pearce and Lee (1969) also observed that although photosynthesis was more constant on a leaf dry weight basis than on an area basis, other factors involving senescence resulted in different relationships in the field from those in the growth chamber. Changes in light intensity have been shown to result in very substantial adaptation within 2 weeks, both in photosynthesis and in SLW, with the result that photosynthesis per unit weight was much the same, irrespective of previous light treatment (Pearce and Lee, 1969). Similar trends were observed in canopies thinned to one stem per plant (Wolf and Blaser, 1972) although the response in photosynthetic rate was less at high light and greater at low light than the response in SLW.

4. m e Diurnal Cycle Increases in starch and sugar fractions of alfalfa herbage during daylight have been reported by a number of workers. The variability in the results may be partly attributed to analytical methods as well as to stage of plant growth and to environmental effects. Lechtenberg et al. (1971) found that the starch content of leaves increased by 10%during the day while that of stems showed almost no change, accounting for most of the concurrent increase in 1eaf:stem ratio from 1.1 to 1.5. Chatterton et al. (1972) observed that the rise in total nonstructural carbohydrates in daylight hours would account for 70% of the overall change in SLW. Concentrations fell during the first few hours of sunlight and increasei thereafter, with rapid responses to changes in light intensity.

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Chatterton (1973) also found that SLW and net carbon exchange showed closely related inverse trends during the day, and suggested that the buildup of assimilates could have been inhibiting photosynthesis. However, the data could equally be used to suggest that fluctuations in photosynthesis due to other causes would bring about corresponding changes in SLW. Pearson and Hunt (1972a) reported a slight decline in net carbon exchange in alfalfa after several hours of illumination under controlled conditions, but it occurred earlier at higher temperatures, when starch would be less likely to accumulate. Although feedback mechanisms have been suggested as possible limitations to photosynthesis on a number of occasions, the changes in concentrations have not been large enough to be particularly convincing. The reverse trend-the loss of carbohydrate and dry matter from the shoots during the hours of darkness-has been the subject of controversy for some time. Knapp et al. (1973) measured a gain of about 300 kg per ha during the day, while about the same amount disappeared during the night at the late bud stage, and less than half as much at about early bloom. Starch and sucrose amounted to 70% of the weight changes. Dry matter losses tended to be greater on warm nights, but the relative proportions lost through respiration and transpiration were not determined, Tracer studies by Hodgkinson and Veale (1966) showed that assimilated 14C is rapidly translocated in the form of soluble carbohydrate to the stems and, to a greater extent, to the roots, where it is steadily converted into insoluble forms. In the light, however, the formation of insoluble carbohydrate in stems is greater than in the dark, and there is considerable accumulation of insoluble carbohydrate in the leaves, while transfer to the roots is slowed down. Evidently, assimilate which has recently been formed is not used for respiration during the light (Ludwig and Canvin, 1971). The magnitude of the variations in carbohydrate concentrations creates uncertainty concerning the trends which have been observed in other constituents. Changes in nitrogen fractions, although significant, have been reported to be small in relation to pool sizes, and intermediates do not normally accumulate from rate-limiting steps (Youngberg et al., 1972). No day-to-day weather effects were found. B. TEMPERATURE

Many reports have shown the importance of air temperature on shoot development. The most consistent effect is that of rate of maturation or time to first flower, ranging from more than 40 days at day temperatures of 20°C or below to 20 days or less at day temperatures of 30°C or above (Jensen et al., 1967;

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Nelson and Smith, 1969; Marten, 1970; Smith, 1970b; Pearson and Hunt, 1972c; Smith and Struckmeyer, 1974). The differences appear to be greater up to the bud stage than between the bud stage and first flower (Greenfield and Smith, 1973), and in general are less at high temperatures. At 35"C, inhibition of floral initiation may cause delay in flowering (Pearson and Hunt, 1972~). Maturation is linked with the rate of node formation, and also possibly with node number, as mentioned in Section III,B. Field and Hunt (1974) reported that the Qlo for node formation was 2.03, with nodes being formed every 3.38 days at 15"/1O"C and every 1.19 days at 30"/25"C. Pearson and Hunt (1972~) observed a similar trend, with a slightly slower rate of 35"/30", attributed to possible water stress. They also found that the rate was faster for regrowth of seedlings than for the primary growth. The effect is paralleled by the more rapid rate of leaf expansion and maturation as the temperature is increased (Wolf and Blaser, 1971b). Dry matter yields at first flower are higher as a rule at lower temperatures because of the extended growing period (Nelson and Smith, 1969; Marten, 1970; Smith, 1970b; Lee and Smith, 1972b; Smith and Struckmeyer, 1974). Growth rates appear to be highest when daylight temperatures are in the region of 20"-25"C (Smith, 1970b; Ueno and Smith, 1970a; Guy ef al., 1971; Bula, 1972). The preceding experiments were all carried out under controlled environment conditions. In the field, Evenson and Rumbaugh (1972) reported that when wheat straw mulch was applied, soil temperatures were lowered by as much as 9"C, plant maturity was delayed, and yields increased by more than lo%, under conditions where it was considered that reradiation and soil moisture differences were of no importance. Plant height, and therefore internode length, do not appear to be greatly affected by temperature (Cowett and Sprague, 1962; Nelson and Smith, 1969), although there are indications that plants in cool regimes tend to be slightly taller than those in warm to hot conditions (Smith, 1970b; Ueno and Smith, 1970a; Vough and Marten, 1971). Although stem and leaf weight and leaf area are all increased at low temperatures, stem growth is proportionately greater than at higher temperatures. Nelson and Smith (1969) found that the total yield at 18"/1O"C was three times that at 32"/24"C, while the leaf weight was only twice as much. Marten (1970) reported that the 1eaf:stem ratio at first bloom was lower at 16"/10"C than at 27"/2 1°C. Stem diameter is greater under cooler conditions (Vough and Marten, 1971), and the weight of stem per unit length of internode declines at an increasing rate as the temperature rises (Field and Hunt, 1974). Leaf area at full expansion is greatly influenced by temperature, reaching a maximum in the vicinity of 20". It decreases gradually as the temperature is lowered (Wolf and Blaser, 1971b; Sato, 1974), and more rapidly as the tempera-

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ture is raised (Nelson and Smith, 1969; Wolf and Blaser, 1971b; Bula, 1972; Pearson and Hunt, 1972c; Sato, 1974). The rate of increase in leaf area is equivalent to the area at full expansion divided by the rate of leaf appearance, and there is some evidence to suggest that this is highest at intermediate temperatures (Nelson and Smith, 1969; Wolf and Blaser, 1971b; Sato, 1974). SLW was reported by Smith and Struckmeyer (1974) to be almost twice as great at first flower under a 21"/12"C regime as compared with 32"/24"C, and this was associated with starch concentrations of 40% and 6%, respectively. Microscopic examination showed that under cooler conditions, chloroplasts accumulated so much starch that cell lumina were not discernible. Leaflets had highly thickened sclerenchyma and phloem cell walls, and were 30% thicker than leaflets at the high temperature regime, with more compact palisade and spongy parenchyma cells. When expressed on a starch-free basis, SLW showed much less variation with treatment. Sato (1974) also found that SLW, leaf thickness, and mesophyll thickness were all greater at 15°/100C than at higher temperatures, while palisade cell diameter decreased steadily with temperature. Stomata1 and epidermal cell densities were lowest and intercellular volume greatest at 20'11 5°C. In contrast, Bula (1972) observed a tendency in three different varieties for SLW to increase from 25" to 35°C. Leaflets grown at 20" and 25°C had larger cells, particularly in the xylem and bundle sheath parenchyma, and more intercellular spaces. There were no obvious trends in leaf thickness. Pearson and Hunt (1972d) found that SLW was lower at 20°/15"C than at 30"/25"C, while Field and Hunt (1974) reported that the decline in SLW was rapid between 15"/lO"C and 20"/15"C, but became more gradual as the temperature increased, with little difference between 25"/20"C and 30"/25"C. These results indicate that starch accumulation is mainly responsible for changes in SLW below about 20°C but that at higher temperatures different rates of cell division and development modify the leaf anatomy, altering cell size, the proportion of void space, and also possibly the dry matter content. It also appears from measurement of carbohydrate levels that leaflets are just as able to adapt themselves to changes in temperature (Greenfield and Smith, 1973) as they are to changes in light intensity. The temperature coefficient for photosynthesis is close to unity over the normal range (Thomas and Hill, 1949; Stanhill, 1962; Pearson and Hunt, 1972a), with a rapid decline below 5°C and above 30°C (Murata et al., 1965). However, when Pearson and Hunt (1972b) raised the temperature in steps over the day from 10" to 40"C, they found that net carbon intake decreased from 20 to about 5 mg per dm2 per hour, suggesting that treatment interactions of some kind may have been involved. Pearson and Hunt ( 1 9 7 2 ~ )found that the root:shoot ratio of seedlings increased more rapidly with time as the temperature was raised, but that the

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asymptotic value finally reached was lower at the time of 50% flowering. The nature of the curves indicated a possible reason for inconsistencies among other results obtained at different stages of growth. However, Smith (1970b) reported that, as temperature increased, plants harvested at first flower had lower total yields but higher root:shoot ratios. It is usually assumed that in a controlled environment chamber, soil temperature follows air temperature, with a lag period depending on the size of the container. However, such factors as radiation level, air movement, and soil moisture content can produce large temperature gradients in the region of greatest root density close to the surface. Using controlled soil temperatures, with air temperatures fluctuating between 15°C and 32"C, Heinrichs and Nielsen (1966) found for a wide range of cultivars that herbage growth was higher at a root zone temperature of 27°C than at lower temperatures, whereas root growth was greater at 12°C than at higher temperatures, and much greater than at 5°C. Despite considerable temperature X variety interactions, the fmdings are consistent with other indications that while fairly low temperatures are suitable for root growth because of reduced respiration requirements, top growth i s restricted as a result of inadequate supplies of nutrients and possibly of water. It seems that root temperature has no appreciable effect on shoot maturation (Nielsen et al., 1960; Heinrichs and Nielsen, 1966; Jensen et al., 1967). Dermine et al. (1967) found that whereas a change from 15.6'/4.4"C to 26.7"/15.6"C resulted in an immediate shoot growth response, the reverse change took 2-3 weeks for growth to slow down, with no comparable reduction in root growth. This could be interpreted as the result of reduced mineral uptake at the lower temperature regime, with a depletion of the reverse already present. The effect of variation between day and night temperatures has received little attention. Steinke (1968) found no significant differences in yield between plants grown at 18"/1OoC and 18"/4"C. Robison and Massengale (1969) concluded that high night temperatures might have been partly responsible for a decline in vegetative growth, carbohydrate reserves, and plant vigor, but other environmental differences between plants grown in the greenhouse and in the field may well have had greater effects. Smith and Struckmeyer (1974) found that plants grown at 3Oo/3O0C not only had yields similar to those at 32"/24"C but were higher in leaf carbohydrate content. Pearson and Hunt (1972a) inferred from their results that translocation from shoots to roots during darkness may have been less at higher temperatures, but more direct evidence would be valuable, Dark respiration increases almost linearly with temperature, corresponding roughly to a Q l o of 2.0 over the lower part of the range (Thomas and Hill, 1949; Murata et al., 1965; Pearson and Hunt, 1972b). There is evidence of possible acclimation (West and Prine, 1960; Pearson and Hunt, 1972b), al-

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

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though its significance is not clear. Comparing successive harvests in the field over an entire season, Delaney et al. (1974) found an inverse relationship between temperature and leaf respiration, which they suggested might be due to lack of substrate during the summer months. However, the results may be associated with the rate of growth and degree of maturation of the leaves at the time of harvesting. Although respiration is often stilI thought of as wasteful dissipation of assimilate, it is in fact concerned with two vital processes: the repair and maintenance of existing tissues and the provision of energy for synthesis and growth. The deleterious effects of exposure to high atmospheric temperature have been demonstrated by Pulgar and Laude (1974). Plants subjected to treatments of 52°C for 2.5 hours or 46°C for 6.5 hours showed reductions in shoot number and height within 7 days. The effect persisted during the next regrowth period, and shoot lengths were 20% lower than in the controls even at 70 days after treatment. Although roots were not visibly damaged, root dry weights were slightly reduced.

C . WATER

I . Root Growth Plant roots will not grow for any appreciable distance into dry soil, and the extent to which they develop in moist soils depends on the supply of assimilate. Janson (1975b) showed that, in a climate subject to drought, herbage yield, root weight, and root depth during the establishment year were linearly related to the amount of irrigation water, irrespective of time, frequency, or rate of application. The results indicate first that in the coarse free-draining shingle used in this trial, root growth is stimulated by moist, not dry, conditions, and second that the plant is able to take full advantage of water supplies as they become available. With established plants grown in a fine sandy loam of bulk density 1.61, Bennett and Doss (1960) obtained no consistent relation between root weight and soil moisture, but found that effective rooting depth was greater at low soil moisture levels. It is evident that no simple generalization is possible, and that such factors as soil penetrability and water-holding capacity must be taken into consideration.

2. Water Uptake In considering water extraction by the plant, it is useful to start from the premise that uptake from the soil is directly proportional to the root density and to the difference in water potential between root xylem and soil in any given

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region (Bahrani and Taylor, 1961;Kohl and Kolar, 1976). In addition, soil water conductivity may be expected to become limiting at low water potentials. There remains considerable doubt concerning the measured values of plant water potential, and it has been recently reported that levels below -20 bar may commonly occur in alfalfa (Cary and Wright, 1971; Kohl and Kolar, 1976). In fine textured soils, the great majority of the roots are in the top 6-12 inches, in the best position to encounter incoming water from rainfall and irrigation. It is therefore usually observed that soil water in this region is removed much more rapidly than at greater depths (Bennett and DOSS,1963; Lucey and Tesar, 1955). Alfalfa is able to deplete the surface layers just as rapidly as other species, but because of its deeper rooting system it extracts water from lower levels during dry periods (Chamblee, 1958b; Van Riper, 1964). In sandy soils, water is likely to be more evenly extracted at all depths. Under moderate to high saline conditions, alfalfa removes water at depth at tensions well below -15 bar (Brun and Worcester, 1975). At low temperatures, water uptake may become limiting. Ehrler (1 963) found that, compared with rates above 20"C, water absorption was reduced by 20% at 10" and by 70% at 5°C. There were no interactions between contrasting varieties and temperatures. 3. Plant Growth and Water Use

If water uptake were the only consideration, plant growth would be greatest when the soil was at field capacity (100% water availability), but would show little reduction until the soil water potential reached a value of about -2 bar (Kemper and Amemiya, 1957). Since this figure may correspond to less than 25% water availability in sand and more than 75% in clay, it is hardly surprising that trials in which stands have been irrigated at various levels of soil water availability have yielded such different results (Bourget and Carson, 1962; Hobbs et al., 1963; Bezeau and Sonmor, 1964; Peyremorte et al., 1971). The additional complication of excess water supply will be discussed later. Alfalfa has acquired the reputation of being an extravagant consumer of water, despite considerable evidence that this is an unbalanced verdict. Trials in a number of regions have shown that daily consumptive use is similar to that of other crops in which full ground cover is established (Fredricksen, 1938; Halkias et al., 1955; Krogman and Lutwick, 1961; Peck et aL, 1958; Bennett and Doss, 1963; Szeicz et aL, 1969). Various workers have concluded that annual water use depends not so much on species as on length of growing season, proportion of ground cover, rooting depth, and crop yield (Chamblee, 1958b;Krogman and Lutwick, 1961; Sonmor, 1963; Tadmor et al., 1966). During the dry summer months, water consumption by actively growing alfalfa may be similar to that of dormant or semidormant species (Snaydon, 1972~).

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

20 1

During the spring, water demands are comparatively low, and it is usually only during periods when potential evaporation rates are high and growth rates low that water losses become excessive. Yields often decline with successive harvests during the season to a much greater extent than evapotranspiration, with water requirement (weight of water used per weight of dry matter produced) being almost doubled (Cohen and Strickling, 1968; Vorhees and Holt, 1969; Daigger et al., 1970). Hanson (1967) reported that both yields and water-use efficiencies were higher on frequently irrigated plots, and that consumptive use was greatest when irrigation was delayed until essentially all moisture had been depleted to a depth of 6-12 inches. Gifford and Jensen (1967) found that water-use efficiency declined at lower soil water availability, but Lucey and Tesar (1965) reported that it was independent of irrigation regime. High evaporative demand alone does not seem to be responsible for wastage, since Tadmor et al. (1966) showed that under arid conditions, water requirement was 820 in one year and 740 in the next, compared with the mean of 859 established by Schantz and Piemeisel (1927). The general impression is therefore that alfalfa uses water wastefully because it continues to function during periods when water stress restricts its growth and when other plants remain dormant, neither growing nor using water. This places the crop at a further disadvantage. In summer, the deep green mass of an alfalfa field often presents the appearance of an oasis among the surrounding areas, and for this reason becomes subject to the “oasis effect.” Advective winds brought across dry, bare ground supply latent heat flux energy to the crop, increasing evapotranspiration rates by up to 40% (Blad and Rosenberg, 1974), or even more (Hudson, 1965; de La Sayette, 1967). Where moisture supply is adequate and advection is absent, evapotranspiration from a crop which has established full ground cover is close to potential evaporation as governed by meterological factors, particularly incoming radiation (Jackson, 1960; Nicholaichuk, 1964; Krogman and Hobbs, 1965; Hobbs and Krogman, 1966). As the soil dries out, evaporation rates decline according to the gap between the transpirational demand arid the water resources available to the root system. Van Bavel (1967) found that stomata1 conductance started to decrease when the soil water potential reached -4 bar. At -11 bar, evapotranspiration was less than 0.2 of potential evaporation and was controlled by the plant such that it did not exceed 20 mm per hour, regardless of demand; at this stage, the crop was emitting heat to the atmosphere. Numerous trials have shown that evapotranspiration is least in the week following defoliation, despite the higher daytime temperatures above bare ground, and is usually one-quarter to one-half of the rate at full bloom. Where plants remain dormant following harvest during summer drought, water use may be as low as 0.1-0.3 mm per day (Tadmor e l al., 1966). Transpiration increases not only as full ground cover is attained (Krogman and Hobbs, 1965) but also as

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crop height increases, exposing greater leaf surface area and creating greater air turbulence by increasing the aerodynamic roughness of the canopy. The evapotranspiration rate from a tall crop can be twice that from a short one (Hafeez and Hudson, 1965; Grassi and Chambouleyron, 1965). A stand cut weekly is likely to produce somewhat less dry matter than one allowed to reach maturity, but may use only half as much water (Sprague and Graber, 1938). Water use becomes greatest at full flower and during seed formation and ripening (Ktidrev et al., 1970), and it is during this time that differences between cultivars in water requirement become most apparent (Cole e t a l , 1970). Genotypes within cultivars show considerable differences in water-use efficiency (Cole and Dobrenz, 1970), which is associated with htgh forage production, particularly of stems, but is not related to palisade cell density or leaf thickness (Dobrenz et al., 1971). 4. Water Deficit

Bauman (1957) distinguished five phases in moisture stress effects on growth, related to the osmotic potential of the plant during a drying cycle: above -10 bar, optimal growth; -10 to -12 bar, little effect on growth; -12 to -17 bar, growth very slow; -17 to -32 bar, no growth; and below -32 bar, dry weight loss. Stem growth is most affected by water deficit; stems per plant, internodes per stem, internode length, and branching are all reduced (Perry and Larson, 1974), and eventually elongation ceases (Lucey and Tesar, 1965). Under mild deficits, the lower yields may be of higher quality than in plants with adequate soil moisture (Jensen et a l , 1967), but further desiccation leads to general plant deterioration. Shoot growth appears to be slightly more affected than root growth (Bourget and Carson, 1962; Janson, 1975b), presumably because of the water potential gradient within the plant, and growth appears to be diverted to carbohydrate storage in the root (Willard, 1951 ; Cohen et al., 1972). Striking changes in leaf morphology under arid conditions have been described by Gindel (1968). Mean leaflet size was only 1 cm2 during the dry season, compared with 4 cmz at the end of the wet season. Stomate and epidermal cell densities were somewhat greater in the smaller leaves, but it appeared that cell expansion was reduced more than cell division. Small cells, having proportionately less volume reduction when desiccated, and high negative osmotic values are characteristic of drought-hardy plants (Russell, 1959). Alfalfa has no more ability than many other plants to remove water from soil before permanent wilting occurs (Briggs and Shantz, 1912). Murata et al. (1966) found that respiration was reduced when the soil water content was reduced to 45%, photosynthesis at 35%, and leaf water content at 25%. All three values

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

203

were reduced by about 40% at a soil water potential of -4.5 bars. While these soil water availabilities cannot be interpreted in terms of plant water status, they do suggest that vapor loss may continue long after growth has been brought to a halt by insufficient turgor. Loper (1972) reported relative turgidities of less than 50% in wilted alfalfa leaves, and it is difficult to see in what way this could be of benefit to the plant. It does not appear to be characteristic of other drought-resistant species (Sanchez-Diaz and Kramer, 1973). Further studies are required of this most important and inexplicably neglected aspect of alfalfa physiology. Night opening of stomata under arid conditions was observed by Loftfield (1921), but no explanation for its occurrence in alfalfa has been put forward, although it is familiar in plants with crassulacean acid metabolism. It can result in measurable transpiration losses, particularly following hot days, but the effect on the total water economy of the plant is likely to be small (England, 1963; Van Bavel, 1966), except under conditions of severe advection (Abdel-Aziz et al., 1964; Rosenberg, 1969).

5. Excess Water Alfalfa is highly susceptible to waterlogging (Finn e t al., 1961). Differences in tolerance between cultivars have been reported (Rogers, 1974). The initial effects are usually attributed to lack of oxygen in the root zone, resulting in the formation of toxic substances which produce root xylem necrosis and yellowing and wilting of leaves. Symptoms appear sooner and with greater severity as the root zone temperature increases (Erwin et al., 1959; Finn e t al., 1961; Heinrichs, 1972; Cameron, 1973), presumably since higher root respiration rates deplete the oxygen supply more rapidly; air temperature has no effect (Cameron, 1973). Damage is greatest in recently defoliated plants (Erwin et aL, 1959; Rai et aL, 1971; Cameron, 1973), which is probably due to the small amount of leaf area available for gas exchange and to a lack of root activity. The more transpiration is restricted, the longer the plant takes to obtain relief from the conditions causing the problem. In seedlings, excess soil water can also lead to subsequent impairment in mineral absorption (Andrew, 1966). Waterlogging may induce manganese toxicity in certain soil types, with possible lasting effects (Graven et al., 1965). Primary symptoms are apparently not associated with pathogens (Cameron, 1973), but wet soils enable Phytophthora oospores to germinate and release zoospores that infect the alfalfa root (Lueschen et al., 1976). Frosheiser and Barnes (1973) found that Medicago sativa, M. falcata, and M. lupulina were the only forage legumes susceptible to Phytophthora megasperma in the field, and they concluded that alfalfa can tolerate wet soils in the absence of plant pathogens. But although infection ceases when the soil dries out, tap roots may

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be partly or completely rotted off, and the shallow adventitious lateral shoots which subsequently develop are usually inadequate to restore full plant vigor (Lueschen et al., 1976). In the seeding year, Phytophthora may increase in incidence and severity because of stress factors such as high soil moisture, frequent cutting, and high seeding rates (Pulli and Tesar, 1975). Wahab and Chamblee (1972) found that irrigation at 50% water availability initially increased yields, but eventually reduced them, with loss of the stand. The time taken for damage due to various organisms to become evident vaned from the first harvest to 2 years, depending on variety. Hobbs et al. (1963) reported that irrigation at 75% water availability gave higher yields than at lower levels only for first-year stands. Lehman et aZ. (1968) carried out experiments on two different soil types which indicated that the effect of irrigation on yield depended greatly on soil water permeability and subsurface drainage. The results suggest that rate of irrigation may have even more important consequences than timing, and since the rooting depth will often closely correspond with depth of infiltration, the range of optimal water supply may lie within fairly narrow limits. I). MINERALS

1. Root Growth Root penetration depends on the nutrient supply at each level. Roots grow vigorously in fine sand or silt where phosphorus is available but do not move into regions of moist gravel or coarse sand, even when phosphorus fertilizer is applied (Fox and Lipps, 1955a). There is a lack of nodulation and root branching in phosphorus-deficient soils (Fox and Lipps, 1955b). In strongly acid soils, the taproot grows down to the limed-unlimed interface and stops abruptly, sending out a number of lateral roots (Buss et al., 1975b). At slightly higher pH levels, taproots do not develop normally, and there is less proliferation of fibrous roots (Pohlman, 1946; Schmehl et al., 1952; Moschler et al., 1960). Liming at depth can stimulate root growth even when the surface layers have been limed (Pohlman, 1946). On the other hand, when calcium is obtained from lower depths, nodules may develop on roots in the acid topsoil (Fox and Lipps, 1955b). In addition to phosphorus and calcium, boron is also needed for the growth of fine roots (Simpson and Lipsett, 1973).

2. Uptake by Roots At low temperatures, mineralization and rates of equilibration of sulfur and phosphorus in the soil are greatly reduced, and responses to fertilizer application

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

205

are more likely to be obtained (Jones, 1970; Sutton, 1969). Restricted water absorption by roots below 10°C may also limit mineral intake. Phosphorus content of alfalfa herbage has been shown to increase with temperature, with accompanying effects on yield (Nielsen et al., 1960; Parsons and Davis, 1960; Levesque and Ketcheson, 1963; Heinrichs and Nielsen, 1966). With potassium, the evidence is less clear. Nielsen et al. (1960) found that uptake was proportional to growth and that herbage concentration at final harvest showed no definite trends with root temperature. D. Smith (1971) reported low herbage concentrations and deficiency symptoms at cooler temperatures, but herbage dry weight was correspondingly higher. Plant calcium and magnesium levels show a complementary decline as temperature increases (Nielsen et al., 1960; D. Smith, 1970a, 1971). Soil moisture stress may limit the uptake of phosphorus, Several workers (Kilmer et al., 1960; Younis et al., 1963; Snaydon, 1972b) have reported that the phosphorus content of herbage increased linearly with soil water availability, although Bourget and Carson (1962) found no effect of water regime. Although phosphorus absorption may be influenced by soil moisture, it need not parallel water absorption. Lipps et al. (1957) reported that even where a water table was present, phosphorus was not removed from the subsoil when surface soil moisture was high. Little uptake may occur from the drier region between the surface and the region of capillary rise (Lipps and Fox, 1956; Campbell et al., 1960; Simpson and Lipsett, 1973). In many circumstances, it may be difficult t o tell whether growth is restricted by phosphorus or water supply. Sorensen et al. (1968) suggested that high levels of sulfur might accumulate in the plant under water restriction. Kilmer et al. (1960) did not find that growth response to additional water supply was accompanied by increases in the herbage content of minerals other than phosphorus. However, boron deficiency can be induced in plants of high boron content by a sharp reduction in soil moisture (Buss et al., 1975a), since the roots cannot absorb the element from dry soil, even if moisture is present in a deficient subsoil (Hobbs and Bertramson, 1950). The detrimental effects of low pH, apart from those on nodulation, are attributed to aluminum and manganese toxicity. Alfalfa is highly susceptible to aluminum toxicity (Andrew et al., 1973), which may be related to its high cation exchange capacity (Vose and Randall, 1962). Compared with other temperate legumes, alfalfa is highly inefficient at removing calcium from deficient soils, although appreciable amounts can be taken up when the deficiency is not extreme because of the extensive root system (Andrew and Norris, 1961). The beneficial effects of adding lime on base saturation, soil exchangeable aluminum and manganese levels, and herbage yields are highly correlated (Moschler ei a f , 1960; Munns, 1965; Hutchinson and Hunter, 1970; Helyar and Anderson, 1971; John etal., 1972; Buss et al., 1975b; Janghorbani et al., 1975).

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The range of pH over which amelioration occurs is naturally dependent on soil characteristics, and when both high acidity and low phosphorus availability are encountered, interactions between lime and phosphate applications are to be expected (Munns, 1965; Janghorbani et al., 1975). Excessive liming not only may have little beneficial effect, but also may induce deficiencies of boron (Mannetje, 1967) or manganese (Bear and Wallace, 1950). Visual symptoms of aluminum toxicity in shoots resemble those of phosphorus deficiency, while those in roots show similarities with those of calcium deficiency (Andrew and Vanden Berg, 1973). Toxicity is associated with high concentrations of aluminum or manganese in the tops, rather than in the roots (Ouelette and Dessureaux, 1958; MacLeod and Jackson, 1964; John et al., 1972; Andrew er aZ., 1973), and is evidently overcome by the uptake of calcium in sufficient quantities to reduce the ionic concentration of the toxic element to the point where phosphorus is not precipitated or otherwise immobilized in the roots. Manganese ions can also reach toxic levels in the soil either when extremely hot, dry conditions prevent oxidation to unavailable compounds or when waterlogging causes reductions of higher oxides by anaerobic bacteria (Graven et al., 1965; Simon er aL, 1974). 3. Nitrogen Fixation Nitrogenase activity of nodules on Medicago sativa roots is high between 2" and 38OC, with a maximum at 35°C (Dart and Day, 1971), and Rhizobium melibti is reported to be capable of withstanding high temperatures (Brockwell and Phillips, 1964). However, nitrogen furation and assimilation and total yield may be greatly reduced when soil temperatures of 40°C are frequently exceeded (Rogers, 1969), and Godley (1968) reported that the frequency of functional nodules was impaired above 27°C. At low temperatures, it has been suggested (Leach, 1968b) that winter-active varieties may be capable of faster growth than the symbiotic source can sustain. Water deficits may reduce the frequency and duration of functional nodules (Codley, 1968), and have major effects on nitrogen fixation (Imangaziev and Patakhov, 1968). Lack of aeration leads to abnormal functioning of nodules. BeIow pH 5.5, root hairs are prevented from curling and becoming infected. The initiation of infection is the most calcium-dependent and acid-sensitive stage of nodulation, and once it is completed, a pH of 4.4 does not hamper further development (Munns, 1968). Other effects are likely to be direct consequences of failure in carbohydrate supply by the host plant. Shading depresses nodulation proportionately more than root growth, and nodules which are formed may not be functional (Pritchett and Nelson, 1951; McKee, 1962). Clipping is reported to have little

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

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effect on nodulation (Godley, 1968), although repeated short cutting may result in weak root development with few and discolored nodules (Langer and Steinke, 1965). The loss of transient roots, whether from drought, flowering, winterkilling or other causes must lead to the decay of the attached nodules, the contents of which are liberated into the soil, where they would be readily available for further cycling (Holford, 1968). The quantities involved may be of little consequence, since fmation products appear to be transferred immediately to the host plants, and the roots and nodules do not act as substantial storage organs (Pate, 1958). Alfalfa leaflets have a high nitrate reductase activity at young vegetative stages, indicating their ability to supplement fixation with soil nitrogen when demands are high (Eskew et al., 1973). However, additional nitrogen supply does not fully compensate for light restriction and tends to inhibit nodule development (Schertz and Miller, 1972), even to the extent of subsequently inducing symptoms of nitrogen deficiency (Kunelius, 1974). In established stands, increases in yield and stem nitrogen content produced by nitrogen fertilizer application may be too small to be worthwhile (Lee and Smith, 1972a). However, where very large amounts of forage are being removed (over 20 tons per ha per year) supplementary nitrogen may substantially increase yields (Vartha, 1972; Hoglund et al., 1974), suggesting that the supply of symbiotic nitrogen is becoming limiting. Yield responses to applied nitrogen at low temperatures have also been reported (Hamilton, 1970). Plant nitrogen content is reported t o increase with temperature (Heinrichs and Nielsen, 1966; Lee and Smith, 1972b), and the rapid fall in nitrogen content in roots during early spring (Bula and Smith, 1954; Bowren et al., 1969) may also be associated with inadequate fixation. 4. Mineral Interrelationships Plant mineral nutrition is dominated in many respects by nitrogen, and deficiencies in other elements often become manifest through the inability to carry out normal protein synthesis. MacLeod and Suzuki (1967) found changes in the amino acid pool that indicated a shift from nitrogen catabolism to anabolism as the K:N ratio of the nutrient solution increased, with maximum yields at a ratio of unity. Both potassium and sulfur deficiencies increased the aspartic acid:glutamic acid ratio, but produced contrasting changes in other amino acids (Adams and Sheard, 1966). The buildup of arginine and asparagine in sulfur and phosphorus deficiencies suggests alternative nitrogen storage when further synthesis is blocked (Mertz et al., 1952; Gleiter and Parker, 1957). From another aspect, the direct effect of potassium upon photosynthetic activity (Cooper et al., 1967) may be sufficient to account for most of the observed increases in leaf area expansion, persistence of lower leaves, stem

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growth (D. Smith, 1971), and root carbohydrate storage (Matches et al., 1963; MacLeod, 1965b; Reid et aZ., 1965). Higher leafstem ratios are to be found in plants whose growth is restricted by a deficiency of potassium (Bear and Wallace, 1950) or of phosphorus (Gleiter and Parker, 1957), and nitrogen values are therefore likely to be higher than in normal plants. The impairment of nitrogen assimilation by mineral deficiencies will have the opposite effect, however, and nitrogen concentrations have been found to be slightly lower in sulfur deficiency (Caldwell et al., 1969). The ionic environment of the plant root can seldom be evaluated satisfactorily by extraction procedures, and changes in plant composition are commonly used to interpret nutrient status. However, supplying a deficient element is likely to increase yield without substantially raising plant concentrations (Larson et al., 1952; Andrew and Robins, 1969a,b), while inconsistent yield increases at high potassium levels suggest that either potassium is needed to maintain parity with high nitrogen levels or it is influencing the uptake of some other element. Mineral uptake is largely governed by the tendency to maintain a stable balance between major inorganic cations and anions and by the ionic balance in the soil. Plant concentrations of calcium and potassium, for example, are more dependent on the Ca:K ratio in the soil than on the absolute values (Wallace, 1952), and an increase in one often results in a complementary reduction in the other. Simple anion substitution is probably responsible for increases in plant sulfur and decreases in phosphorus and boron produced by sulfur application (Caldwell et al., 1969). Saline soils have little effect on herbage cation content, since sodium uptake is always low, and chloride is accumulated at the expense of other anions (Brown, 1958). The addition of ions to the soil involves an equivalent number of ions of opposite charge and equal availability, and the extent to which these are taken up by the plant will depend on the ability of ions already present to compete with them. Gervais et al. (1962) found that when responses to both phosphorus and potassium were obtained, the addition of either to the soil produced a reduction in plant concentration of the other. The results of D. Smith (1971) are of interest, since they show that the application of potassium chloride or sulfate resulted in much lower phosphorus levels, particularly at low temperatures. Further progress in evaluating nutrient requirements might result most rapidly from a realization of the need for more comprehensive plant analysis and systematic examination of the results, viewed in the context of quite basic considerations. Nyatsanga and Pierre (1973) pointed out that although legumes obtain most of their nitrogen from the air, bases are derived from the soil, and they calculated that an annual yield of 10 tons per ha would produce soil acidity equivalent to 600 kg per ha2 of CaC03. In a greenhouse experiment, they

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measured an amount of nitrogen fixed by alfalfa over 167 days sufficient to lower the soil pH from about 5.8 to about 4.9. The result has implications concerning the soil environment and plant growth which require further investigation.

VI.

Phases in Development

A. BUD AND SHOOT INITIATION

In cold-tolerant varieties, dormancy of crown buds is evidently established at the time of physiological changes associated with the hardening process (Grandfield, 1943), and the next season's growth originates from these overwintering buds as well as from buds developing during the early spring (Nelson and Smith, 1968). In winter-active types, buds continue to develop at colder temperatures, although at a reduced rate, and they are vulnerable to frosts and grazing. During the growing season, buds commonly appear at the base of the plant at about the time when the first floral buds become visible (Nelson and Smith, 1968; Janson, 1975a). If the plants are left uncut, elongation starts at about full bloom, with the old stems subsequently lodging and drying. Both the size and the number of basal shoots increase as the plant enters the flowering stage (Cowett and Sprague, 1962; Leach, 1968b; Janson, 1975a). The appearance of new shoots is often regarded as a sign of herbage maturity, even where growth is continuous and flowering sporadic, as in the cool Andean regions (Crowder et aZ., 1960). However, Bartels (1956) in Victoria, Australia, observed a wide distribution of shoot lengths, with little indication of distinct growth cycles; no information was given to indicate what environmental factors may have been involved. New crown growth can also be induced by restraining the older shoots in a horizontal position close to the ground (Tysdal, 1946). The earlier cutting is carried out, the more gradual is the start of bud elongation and the higher the proportion of shoots originating from the crown (Leach, 1968a), and shading plants before defoliation further delays the start of growth (Leach, 1969). Once growth starts, rates of elongation and weight gain are largely independent of cutting treatment (Leach, 1968a). If sufficient stubble leaf remains, photosynthetic activity may be high enough to reduce carbohydrate mobilization from the roots (Hodgkinson, 1973), and crown temperatures may be lowered somewhat (West and Prine, 1960), but little regrowth is likely to originate from stubble nodes more than an inch or so above the crown (Leach, 1970a). Growth is initially more rapid at higher temperatures (Leach, 1971a), but exposures to air temperatures above 45" reduces shoot numbers (Pulgar and Laude, 1974).

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The number of stems that develop after defoliation is correlated with crown width, weight of previous harvest, and competition from neighboring plants (Miller et al., 1969), a finding which is consistent with the conclusion of Cowett and Sprague (1962) that environmental factors affect tillering indirectly through their effects on plant growth and vigor. B. ROOT CARBOHYDRATE STORAGE

Although an association obviously exists between shoot growth initiation and root carbohydrate storage, causal relationships have yet to be unequivocally established. Adequate root carbohydrate levels are not in themselves responsible for shoot initiation, although they evidently have an important bearing on the success of subsequent growth. Chatterton et al. (1974) found that the dry weights and total nonstructural carbohydrate (TNC) concentrations in herbage, crowns, and roots of plants which tillered and flowered early were higher than those in plants which matured later. Cowett and Sprague (1962) observed that even where roots were small as a result of moisture stress, yield and stem number were comparable with those of unstressed plants when grown at adequate levels of soil moisture. In contrast, Ueno and Smith (1970b) found that stem number was directly related to root and crown weight at harvest. They also found that the proportion of TNC lost from the roots was similar for plants of different root size. Quite possibly the respiration requirements of the extensive root system are at least partly responsible for the greater dependence of alfalfa on these reserves than is true of other species. The effects of temperature on rates of depletion and renewal of carbohydrate in the roots reflect those on shoot maturation. The fall in concentration is more rapid at higher temperatures (Singh and Winch, 1974), but the leaf area needed to restore self-sufficiency is also attained earlier (Silva, 1968). The subsequent peak in concentration is reached at about the time of first flower, which is much earlier at high temperatures, but the level is likely to be lower than in cooler conditions (Nelson and Smith, 1969). Moderate shading may make little impression on root carbohydrate concentrations (Matches et al., 1963), but low light levels accentuate the effect of severe and repeated defoliations (Steinke, 1968). Very little root growth occurs in plants grown in the dark (Smith and Silva, 1969). Prolonged dry periods lead to virtual dormancy of shoots and crown buds, and grazing at such times is unlikely to seriously harm subsequent production (Wilman, 1965; McAuliffe, 1967). However, Snaydon (1972a) reported that irrigation water applied at the rate of 5 mm every 8 days gave higher yields during a dry summer period, but lower yields in the following autumn, than the same total amount applied in larger aliquots less frequently. Since such small

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

21 1

additions could hardly have made any difference in the soil moisture content, he suggested that each watering might have stimulated translocation from the taproot. Cohen et al. (1 972) found that when plants were irrigated at the time of cutting, the rate of regrowth was twice as great as when irrigation was withheld for 10 days, but the fall in TNC was 8 times as great. Other evidence of a more general nature suggests that alfalfa may not be well suited to moist environments. Willard (1951) stated that root weights can be twice as great at the end of a dry year as following a wet season, and that in dry regions alfalfa could be cut at much earlier stages than in humid regions. Unfortunately, the distinction between humid and high-rainfall climates is not always made, and no experiments to determine the effects of atmospheric moisture on alfalfa growth have been described. Reports have indicated the difficulty in maintaining stands in such diverse high-rainfall areas as Florida (Prine, 1966), Hawaii (Wilsie and Takahashi, 1937), and Ireland (Farragher, 1969). Such factors as waterlogging and disease may of course contribute to lack of persistence; but the results of Weir et al. (1960) in California, for example, indicate that frequent cuttings with the aid of irrigation in a semiarid climate cause no lasting damage to plant stands. Willard (1951) also noted that, in humid regions, shoots very seldom appear uniformly, and may be present before full bloom. At the low radiation levels experienced beneath a stand of high leaf area index, it would appear that vigorously growing new shoots must be parasitic and in competition with the roots for assimilate. Whether these shoots eventually died or were mowed before they developed fully, they would contribute little to total yields, but might well constitute a continuing drain on plant reserves.

C. REGROWTH CHARACTERISTICS

As might be expected from general seasonal trends, regrowths display a number of features observed in controlled eiivironment studies at elevated temperatures. Regrowths are in general shorter and take less time to reach maturity than the first growth (Dent, 1959). They usually have a higher leafstem ratio, and the stem diameter is reported to decline with each cut (Mowat et al., 1967). However, reports vary as to whether stems become more lignified or less digestible (Lagowski et al., 1958; Jensen el al., 1967; Mowat et al., 1967). Rates of accumulation of dry matter and leaf area index may be either slower than for the first growth (Hunt et al., 1970), or comparable, but with a lower maximum (Greub and Wedin, 1971). The decline and recovery in acid-hydrolyzable carbohydrate concentrations occur more rapidly with each successive cutting, corresponding with rates of floral initiation (Dobrenz and Massengale, 1966).

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D. FLOWER AND SEED FORMATION

Insufficient information is available to show whether any clear relationship exists between floral and vegetative development. Maturation and flowering take place more rapidly as the temperature is increased (Nittler and Kenny, 1964; Sato, 1971; Section V,B), and plants may even fail to flower at temperatures around 20°C (Roberts and Struckmeyer, 1939; Sato, 1971). Flowering is promoted at greater day lengths, and is earliest and most prolific under continuous light (Nittler and Kenny, 1964; Murray, 1967; Guy et al., 1971); although the effect is primarily photoperiodic, higher plant growth rates may also contribute. The detrimental effect of low light intensities (Nittler and Kenny, 1964) is probably a direct consequence of reduced plant vigor. However, the mechanism of induction at low temperatures (Roberts and Struckmeyer, 1939) remains obscure. Interactions between pollinating insect, plant, and environment are of great importance, but not well understood (Heinrichs, 1965; Kauffeld et al., 1969). High temperatures may increase the number of inflorescences pler plant but reduce the number of flowers on each inflorescence (Guy et al., 1971), while high night temperatures are reported to cause flowers to abscise (Roberts and Struckmeyer, 1939). In comparisons of soil moisture and spacing treatments, Tysdal (1946) showed that a strong inverse relationship existed between forage and seed production. Soil moisture stress is likely to reduce the number of flowers per plant (Grandfield, 1945; Tysdal, 1946). However, seed production is reported to be maximized by maintaining a soil moisture potential between -2 and -8 bar after the start of blossoming (Taylor et al., 1959), while high humidities can seriously reduce the percentage of flowers setting pods, particularly at temperatures around 27°C (Grandfield, 1945). Lodging also affects the proportion of seeds set, rather than the number of flowers (Tysdal, 1946). It would seem that conditions which favor the elongation of new shoots can reduce the supply of assimilates available for seed formation, and high carbohydrate reserves in the roots may be of value for this reason (Grandfield, 1945). Undei severe stress conditions, such as dry soils or temperatures above 30°C,a high proportion of seeds either abort or do not develop normally (Taylor et al., 1959; Dane and Melton, 1973). The effect of increased spacing depends largely on whether the plants are able to take full advantage of the soil volume and the irradiated area, and whether the additional racemes per stem more than compensate for the smaller number of stems per unit area (Nittler and Kenny, 1964; Dovrat et aL, 1969). If full ground cover is not attained, radiation levels and temperatures are likely to be higher within the canopy, and humidities lower.

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

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E. SEEDING AND THE SEED

Self-seeding rarely occurs to any great extent in alfalfa, and stands usually become thinner and less productive with age. It is therefore of interest that Medicago falcata is reported to increase and become a dominant component of swards in which it is oversown (Rabotnov, 1969). A number of reports have shown that alfalfa is able to regenerate if the stand is rested during mild weather following good rainfalls (Young et al., 1958; Clinton, 1968; Cameron and Mullaly, 1970; Campbell, 1974; Douglas, 1974). Seed produced under cool (6"-21"C) temperatures is reported to be heavier than under hot (16"-32°C) conditions, giving rise to more vigorous seedlings in the early stages of growth (Walter and Jensen, 1970). The microenvironment of the seed is critical for successful germination. Whereas grass radicles tend to enter the soil at angles between 45" and 90", the legume radicle generally grows along the soil surface and enters at obtuse angles, with consequent exposure to desiccation (Campbell and Swain, 1973). The diameter of the root apex is greater than for grasses, and the legume seedling lacks coleorhizal hairs which provide root anchorage to the soil surface. In contrast to grass seeds, entry by secondary roots after the death of the radicle is rarely successful. Consequently, the time for radicle entry is often delayed, and mortality may be high, particularly on smooth or fine structured soils of high bulk density. Establishment is encouraged if sufficient dead vegetation or surface roughness is present to restrain seed movement during penetration (Dowling et al., 1971). Exposure to direct sunlight is clearly to be avoided. Miles (1969) observed that seeds that fell into frost cracks and were covered with soil by later rains germinated well. Since the greatest emergence force is developed within 1-2 days, crusting of the soil surface may prevent penetration (Jensen et al., 1972). Damage to the unprotected emerging cotyledons may cause prolonged reduction in vigor and growth rate (Bignoli, 1950). The temperature requirement for initial development appears to be higher than that for older plants. Pearson and Hunt ( 1 9 7 2 ~ )concluded from their results that optimal growth rates during establishment were highest at 20"/1 5"C, but later shifted to lS"/lO"C. Smoliak et al. (1972) found that the weight and lengths of both tops and roots at 4 weeks from germination increased considerably with each temperature increment from 7" to 27°C. The data of Heinrichs and Nielsen (1966) indicate that the time from seeding to first flower was much greater at a root temperature of 5°C than at higher temperatures, whereas in subsequent cuts, the time to reach maturity was unaffected by root temperature. McElgunn (1973) showed that cold temperatures reduced the rate of germination, while alternating temperatures of 13"/2"C also reduced total germination.

214

K.R. CHRISTIAN VII. Plant Associations

A. 1NTRASPECIFIC:PLANTDENSITY The individual plant perceives its neighbors only as environmental boundaries, regions into which expansion is restrained or curtailed by the lack of light, water, or nutrients. Survival is contingent upon meeting the minimum requirements for viability within these confines, including the readiness to occupy less favorable areas, At the interfaces, its success in direct competition depends on the extent to which it can deplete one or another of the resources to a level below that which can be endured by neighboring specimens. Even in a monospecific sward, wide ranges in plant vigor and physiological tolerance are superimposed on random variations in spacing and soil conditions. The main effect of increased density is a general reduction in size and dry weight of individual plants (Miller et al., 1969; Takasaki et al., 1970; Roufail, 1975). Roots tend to be lighter, of smaller diameter, and with less branching (Hansen and Krueger, 1973; Radei, 1974; Roufail, 1975), while nodulation may be depressed (McKee, 1962). Stems are usually thinner (Mowat et al., 1967) and somewhat shorter (Chisci, 1966), and therefore of lower weight (Takasaki, 1972), with less branching and lower dry matter content (Dovrat et al., 1969). There are fewer stems per plant (Marten et al., 1963; Chisci, 1966; Miller et al., 1969; Roufail, 1975), and presumably less leaves, although the effect of plant density on leaf area has apparently not been studied. In addition, relationships between yield, crown width, and stem length and number alter with plant spacing (Rumbaugh, 1963). Although the seasonal rhythms of production in different varieties are similar for spaced plants and broadcast swards, the relative yields are not comparable (Chisci, 1966). Because of the inverse relationship between plant size and density, correlations between yield and stand density are likely to be unreliable (Ronningen and Hess, 1955; Gross et al., 1958). High seeding rates may increase production in the establishment year by providing more complete ground cover (Moline and Robison, 1971; Takasaki, 1972), but the gaps tend to close up as time goes on (Kramer and Davis, 1949). A number of workers have observed that very different rates of seeding result in similar yields after the establishment year, while plant densities move toward a common lower limit (Marten et al., 1963; Jacquard et aZ., 1967; Takasaki et al., 1970; Scateni, 1972; Takasaki, 1972; Palmer and Wynn-Williams, 1976). The proportion of seeds producing plants declines with seeding rates and, after establishment, deaths are density-dependent; higher seeding rates apparently do not improve persistence (Palmer and Wynn-Williams, 1976). Plants which die are mainly below the mean in height, stem number, and dry weight at previous harvest (Takas&, 1971). It has been suggested (Palmer and Wynn-Williams,

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

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1976) that satisfactory yields could be maintained at 15 plants per m2. According to Chamblee (1958a), most of the competition of alfalfa with itself is above ground, but this might not be the case if water or nutrients become limiting, and on shallow soils, at least, higher densities would probably be needed. B. INTERSPECIFIC COMPETITION

The value of mixed swards lies in complementary growth, but competition is always present. Roberts and Olson (1942) found no cases when both legume and grass benefited or were injured by association, and the root weights of both are likely to be reduced (Aberg et al., 1943; Langille et al., 1965). In a critical review of the literature, O’Connor (1967) concluded that alfalfa-grass mixtures do not provide large margins in yield over the pure legume, except where the grass fills seasonal gaps, although considerations other than those of dry matter production may often be important. Nevertheless, successful maintenance of mixed swards requires a much keener appreciation of the growth patterns and responses of the component species than is needed for a pure stand. Conditions during establishment under a cover crop are often crucial. If soil moisture is inadequate, the cover crop will use it all, and the alfalfa will die from desiccation; Peters (1961) found that oats had a severe effect on establishment during a dry summer. On the other hand, prolonged rain or irrigation may shade the young alfalfa plants for an excessive length of time (Janson and Knight, 1973; Vartha and Allison, 1973). Buxton and Wedin (1970) considered that a suitable cover crop was one such as oats, which was aggressive enough to smother weeds initially, but which had a more open canopy than that of natural weeds. They found that extreme shading, with less than 2% incident radiation at ground level for up to 1 month, did not impair alfalfa survival. Under such conditions, however, the growth of shade-tolerant weeds is encouraged. Klebesadel and Smith (1960) reported that cutting the oats at maturity had little more effect on subsequent alfalfa production than early cutting. Frequent clipping to reduce weed competition is detrimental to the young alfalfa stand, while no clipping at all is even worse (Sprague er al., 1963; Janson, 1971). Provided survival is adequate, the method of establishment has apparently little carry-over effect on subsequent performance (Barker et aL, 1957; Sprague et al., 1963; Hansen and Krueger, 1973; Nordquist and Wicks, 1974). If severe winter conditions prevent growth of the grass species, the grass is more likely to replace lucerne production during the remainder of the year than to augment it (Douglas and Kinder, 1973). Because alfalfa displays little activity at low temperatures it is apt to become smothered by grasses before it starts spring growth. In semi-arid conditions, species which start growth early in the season also tend to become dominant by using all the available soil moisture (Kilcher and Heinrichs, 1966).

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K. R.CHRISTIAN

Alfalfa competes with tropical legumes for soil moisture (Yates et at., 1971), and during long dry summer periods it may eliminate grasses from the sward by both competing for moisture near the surface and obtaining further supplies at depth (Dann, 1962; Vartha, 1972). But heavy clay soils with waterlogging tendencies encourage the takeover by summer-active grasses (Edye and Haydock, 1967). Spatial variations in mixed-sward microclimate were demonstrated by Larson and Willes (1957) in sowings between 80-inch corn rows running east-west. The soil temperature of the band on the sunlit side of the rows was 4"-7"C higher than on the shady side, and soil moisture was significantly lower. Stand counts of alfalfa increased dramatically from the sunlit side to within 12 inches of the shaded side of the next corn row. At low soil pH, clovers tend to become the dominant legumes, while the addition of lime shifts the balance in favor of alfalfa (Griffith et al., 1966; Helyar and Anderson, 1971). Because the cation exchange capacity of alfalfa is roughly twice that of the highest values among the cultivated monocots, alfalfa is unable to compete strongly for soil potassium (Drake et al., 1952). Alfalfa herbage contains much more calcium and less potassium than associated grass or weed species (Comstock and Law, 1948; Bear and Wallace, 1950; Lawton and Tesar, 1958). The uptake of potassium is lower when competing grasses are present (Coffmdaffer and Burger, 1958; Langdle et al., 1965), and is still further reduced by nitrogen application (MacLeod, 1965a). Accordingly, the addition of potassium is usually essential to maintain alfalfa in the stand, while the necessity to add calcium to reduce soil acidity means that alfalfa is unable to exert any corresponding nutrient restriction on competing plants. Phosphorus is taken up competitively from the topsoil by alfalfa (Massey and Sheard, 1970), although there is some suggestion that phosphorus application favors the growth of other species (Markus and Battle, 1965; Harris et al., 1966). Again, because of its strong root system, alfalfa may gain advantage over its rivals by obtaining access to minerals in short supply near the surface (Blakemore et d.,1969; Jones, 1970). Nitrogen fertilizer invariably leads to a higher proportion of grasses and weeds at the expense of alfalfa (Bear and Wallace, 1950; Gewig and Ahlgren, 1958; Carter and Scholl, 1962; MacLeod, 1965a; Cooke et al., 1968; Peters and Stritzke, 1970; Chan and MacKenzie, 1971; Kust, 1971). Where the legume becomes suppressed, total yields may decline still further because the dominant grass species experiences nitrogen deficiency (Hamilton et a!., 1969). The beneficial influence of alfalfa upon the growth of adjacent plants is evidently dependent on environmental conditions as well as on management practices, but no consistent pattern has emerged (Roberts, 1946; Tewari and Schmid, 1960; Simpson, 1965; Cameron and Mullaly, 1969). Despite the large amounts of

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217

nutrients in alfalfa roots, Whitehead (1970) considered that root decomposition would release little nitrogen into the soil because of the hgh C:N ratio, and that little phosphorus or sulfur would be either mineralized or immobilized. Regulating the time intervals between grazings or cuttings affords perhaps the most effective means of controlling pasture composition. Frequent defoliation almost always reduces the proportion of alfalfa in the sward, while the longer it can maintain a closed canopy and dry out the surface soil layers, the better its chances of eliminating competition. Maintaining the vigor of the grass species may depend on its morphological stage of growth at cutting, particularly on whether or not it has reached the heading stage and whether new basal or axillary tillers are available for immediate growth (Parsons, 1958; Smith et al., 1973). Frequent cutting of cocksfoot (Dactylis glornerutu), for example, stimulates tillering and allows rapid recovery and leaf production from the basal storage organs (Williams, 1950; Barker et al., 1957). Yields of grasses relative to that of alfalfa are likely t o be increased by greater stubble height (Wolf et al., 1962). Selective grazing may destroy emerging alfalfa shoots and buds and encourage weed growth, while the effect of urine on pasture composition may depend on the relative amounts of nitrogen and potassium (Whitear et al., 1962; Cuykendall and Marten, 1968). V I I I.

Genetic Adaotation to Environment

The richness and diversity of the alfalfa gene pool is such that its extension to new areas would seem to depend only on the effort devoted to appropriate breeding programs. Problems involving pests and diseases have been overcome in regions where the value of the crop has already been firmly established and recognized; there is less demand for its adaptation to places where it is now considered either unsuitable or only marginally useful, but the potential may be just as great. Natural adaptation to environment has been observed when populations of seed were increased in warmer or cooler climates (Smith, 1961; Zaleski, 1962). However, Simon et al. (1974), working with seed composites, found no s i g nificant yield differences as a result of multiplications in different regions, and the shifts they observed in growth-type means were regarded as minor. Yet adaptation is undoubtedly an important and continuous process, although its detailed effects are often by no means obvious. In Australia, for instance, the cultivar “Hunter River,” developed locally over many years, has consistently been shown to give greater long-term productivity than imported cultivars under many of the widely contrasting climates in which it is grown in this continent; yet it has not displayed any remarkable superiority in other countries. Experi-

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K. R. CHRISTIAN

ments carried out by Chisci and Lessells (1960) support the hypothesis that varieties of local origin possess advantages which, other things being equal, result in the production of higher yields than those of outside varieties. Because of the great variability of plants within cultivars, it may even be unnecessary in many cases to seek unadapted genetic material outside the local region to obtain improved types. Mixtures of contrasting types are unlikely to be complementary, since the spaces previously occupied by plants that have died are taken by weeds rather than by the remaining alfalfa plants (Jackobs and Miller, 1973). The decline in stand density is for the most part an irreversible process, and cultural practices enabling self-seeding or interseeding would perhaps enhance production more than would any other single factor, not only by preventing stand deterioration but also by propagating genotypes most adapted to the environment. Various studies have illustrated the fact that genotypic variation is adequate for selection of specific characters. However, it has yet to be convincingly demonstrated that improvement in any one physiological character is not likely to be accompanied by compensatory changes that would negate any net effect on productivity. As examples, selection for higher phosphorus and lower calcium would probably at the same time increase potassium and reduce magnesium levels (HiU and Jung, 1975); fewer and thicker shoots might reduce shoot mortality, but could also result in more highly lignified herbage of lower digestibility; and, despite between-leaf relationships, SLW is poorly related to plant yield (Hart et al., 1972; Porter and Reynolds, 1975; Song and Walton, 1975). An integrated program of worldwide collection, recombination, and mild selection has been advocated by Hanson et al. (1972) to conserve and improve germplasm resources. On a more modest scale, many promising advances are already being made. Selective modification toward acclimatization has resulted in the development of a strain with exceptional tolerance to subarctic conditions (Klebesadel, 1971). T. J. Smith (1971) in a single cycle of natural selection under field conditions in a warm, humid region of Virginia obtained experimental strains from the interpollinated seed of “DU h i t s ” plants equal in persistence to those of the control variety “Williamsburg.” Hanson et al. (1972) used recurrent phenotypic selection for vigor and general appearance in developing multiple pest resistance. After three cycles of selection in the field, Frosheiser and Barnes (1973) obtained a 63% increase in resistance to Phytophthora megasperma, correlated with forage and root production in wet soils. Simpson (1974) has shown how the genetic potential to produce roots in low-calcium soils may be sought within cultivars by using clones propagated from cuttings which are grown in soils whose calcium availability gradually declines with depth.

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Although a better understanding of environmental effects is essential for the intelligent and economically viable upgrading of agricultural practices and techniques, our ability to modify the environment as a means of obtaining more favorable conditions for plant growth is limited and likely to become increasingly costly. On the other hand, our ability to modify plants to suit new environments, with the minimum expenditure of resources, may offer considerable opportunity.

REFERENCES Abdel-Aziz, M. H., Taylor, S. A., and Ashcroft, G. L. 1964. Agron. J. 56,139-142. Aberg, E., Johnson, I. J., and Wilsie, C. P. 1943. J. Am. SOC.Agron. 35, 357-369. Adams, C. A., and Sheard, R. W. 1966. Can J. Plant Sci 46,671480. Andrew, C. S. 1966. Aust. J. Agric. Res. 1 7 , 6 1 1 4 2 4 . Andrew, C. S., and Vanden Berg, P. J. 1973. Aust. J. Agric. Res. 24, 341-351. Andrew, C. S., and Norris, D. 0. 1961. Aust. J. Agric. Res. 12,4&45. Andrew, C. S., and Robins, M. F. 1969a. Aust. J. Agric. Res. 2 0 , 6 6 5 4 7 4 . Andrew, C . S., and Robins, M. F. 1969b. Aust. J. Agric. Res. 20, 999-1007. Andrew, C. S., Johnson, A. D., and Sandland, R. L. 1973. Aust. J. Agric. Rex 24, 325-339. Bahrani, B., and Taylor, S. A. 1961. Agron. J. 53, 233-237. Barker, M. G., Hanley, F., and Ridgeman, W. J. 1957. J. Agric. Sci. 48,361-365. Barnes, D. K., Pearce, R. B., Carlson, G. E., Hart, R. H., and Hanson, C. H. 1969. Crop Sci. 9,421-423. Bartels, L. F. 1956. J. Dep. Agric., Yirtoria, Aust. 54,595-597. Bauman, L. 1957. Ber. Dtsch. Bot. Ges 70, 67-78. Bear, F. E., and Wallace, A. 1950. N J . Agric. Exp. S t . , Bull. 748. Bennett, E. 1970. I n “Genetic Resources in Plants-Their Exploration and Conservation” (0.H. Frankel and E. Bennett, eds.), p. 166. Blackwell, Oxford. Bennett, 0. L., and Doss, B. D. 1960. Agron. J. 52, 204-207. Bennett, 0. L., and Doss, B. D. 1963. Agron. J. 55, 275-278. Bezeau, L. M., and Sonmor, L. G. 1964. Can. J. Plant Sci 44, 505-508. Bignoli, D. P. 1950. J. Br. Grassl. SOC.5, 281-286. Blad, B. L., and Rosenberg, N. J. 1974. Agron. J. 66,248-252. Blakemore, L. C., Ludecke, T. E., Leamy, M. L., and Metson, A. J. 1969. N.Z. J. Agric. Res. 12,333-351. Bourget, S. J., and Carson, R. B. 1962. Can J. Soil Sci. 42,7-12. Bowren, K. E., Cooke, D. A., and Downey, R. K. 1969. Can. J. Plant Sci. 4 9 , 6 1 4 8 . Briggs, L. J., and Schantz, H. L. 1912. Bot. Gaz, (Chicago) 53, 229-235. Brockwell, J., and Phillips, L. J. 1964. Aust. J. Sci 27, 332-333. Brown, B. A. 1958. Agron. J. 50,445-447. Brun, L. J., and Worcester, B. K. 1975. Agron. J. 67,586-589. Bula, R. J. 1972. Crop Sci 12,683486. Bula, R. J., and Smith, D. 1954. Agron. J. 46, 397-401. Buss, G. R., Lutz, J. A., and Hawkins, G. W. 1975a. Crop Sci 1 5 , 6 1 4 4 1 7 . Buss, G. R., Lutz, J. A., and Hawkins, G. W. 1975b. Agron. J. 67,331-334. Buxton, D. R., and Wedin, W. F. 1970. Agron. J. 62,93-97.

220

K. R. CHRISTIAN

Caldwell, A. C., Seim, E. C., and Rehm, G. W. 1969. Agron. J. 61,632-634. Cameron, D. G. 1973. Aust. J. Agn'c. Res. 24, 851-861. Cameron, D. G. 1974. Proc. Int. Grassl. Congr., 12th 1, 30Ck308. Cameron, D. G., and Mullaly, J. D. 1969. Queensl. J. Agric. Anim. Sci. 26, 35-40. Cameron, D. G., and Mullaly, J. D. 1970. Proc. Int. Grassl. Congr., 11th pp. 597400. Campbell, M. H. 1974. Aust. J. Exp. Agric. Anim. Husb. 14,507-514. Campbell, M. H., and Swain, F. G. 1973. J. Br. Grassl. SOC.28,41-50. Campbell, R. E., Larson, W. E., Aasheim, T. S., and Brown, P. L. 1960. Agron. J. 52, 437-441. Carlson, F. A. 1925.J. Am. Soc. Agron. 17, 336-345. Carlson, G. E. 1965. Crop Sci. 5, 248-250. Carter, L. P., and Scholl, J. M. Agron. J. 54,161-163. Cary, E. E., Horner, G. M., and Mech, S. J. 1967. Agron. J. 59, 165-168. Cary, J. W., and Wright, J. L. 1971. Agron. J. 63, 6 9 1 4 9 5 . Chamblee, D. S. 1958a. Agron. J. 50,434437. Chamblee, D. S . 1958b. Agron. J. 50,587-589. Chan, W.-T., and MacKenzie, A. F. 1971. Agron. J. 63,667-669. Chatterton, N. J. 1973. Crop Sci 13, 284-285. Chatterton, N. J., Lee, D. R., and Hungerford, W. E. 1972. Crop Sci. 12, 576-578. Chatterton, N. J., Carlson, G. E., Hart, R. H., and Hungerford, W. E. 1974. Crop Sci. 14, 783-787. Chisci, G. C. 1966. Acta Agric. Scand., Suppl. 16, 268-274. Chisci, G. C., and Lessells, W. J. 1960. Euphyticu 9, 81-88. Christian, K. R., Jones, D. B., and Freer, M. 1970. J. Agric. Sci. 75,213-222. Christiansen-Weniger, F., and Tarman, 0. 1939. Herb. Rev. 7, 59-69. Clinton, B. H. 1968. A g i c . Guz. N.S. W. 79, 282-286. Coffindaffer, B. L., and Burger, 0. J. 1958. Agron. J. 50, 389-392. Cohen, 0. P., and Strickling, E. 1968. Agron. J. 60,587-591. Cohen, Y.,Bielorai, H., and Dovrat, A. 1972. Crop Sci. 12,634436. Cole, D. F., and Dobrenz, A. K. 1970. Crop Sci 1 0 , 6 1 4 3 . Cole, D. F., Dobrenz, A. K., Massengale, M. A., and Wright, L. N. 1970. Crop Sci. 10, 237-240. Comstock, V. E., and Law, A. G. 1948. J. Am. SOC.Agron. 40,1074-1083. Cooke, D. A., Beacom, S. E., and Dawley, W. K. 1968. Can. J. Plant Sci. 48,167-173. Cooper, C. S . 1966. Crop Sci. 6 , 6 3 4 6 , Cooper, C . S. 1967. Crop Scl. 7, 176-178. Cooper, C. S., and Qualls, M. 1967. Crop Sci 7,672473. Cooper, R. B., Blaser, R. E., and Brown, R. H. 1967. Soil Sci. Soc. Am. Proc. 31, 231-235. Cowett, E. R., and Sprague, M. A. 1962. Agron. J. 54, 294-297. Crowder, L. V., Vanegas, J., and Silva, J. 1960. Agron. J. 52, 128-130. Cunningham, G. L., and Strain, B. R. 1969. Ecology 50,400-408. Cuykendall, C. H., and Marten, G. C. 1968. Agron. J. 60,404-408. Daday, H. 1964. Heredity 19, 173-179. Daigger, L. A., Axthelm, L. S., and Ashburn, C. L. 1970. Agron. J. 62,507-508. Dane, F., and Melton, B. 1973. Crop Sci. 13, 753-754. Dann, P. R. 1962. Agric. Guz. N.S.W. 73, 120-121. Dart, P. J., and Day, J. M. 1971. Planf Soil,Spec. Vol. pp. 167-184. Davies, W. E. 1960. J. Br. Crassl. Soc. 15, 262-269. Delaney, R. H., and Dobrenz, A. K. 1974. Crop Sci. 14,444-447. Delaney, R. H., Dobrenz, A. K., and Poole, H. T. 1974. Crop Sci. 14,58-61.

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

22 1

de La Sayette, P. 1967. Inst. Natl. Rech. Agron. Tunis., Ann. 40(4), 18 pp. Dent, J. W. 1955. J. Br. Grassl. Soc. 10, 330-340. Dent, J. W. 1959. J. Br. Grassl. Soc. 14, 262-271. Dermine, P., Hidiroglou, M., and Hamilton, H. A. 1967. Can. J. Planf Sci. 47, 523-531. Dobrenz, A. K., and Massengale, M. A. 1966. Crop Sci. 6 , 6 0 4 4 0 7 . Dobrenz, A. K., Massengale, M. A., and Phillips, W. S. 1965. Crop Sci. 5 , 572-575. Dobrenz, A. K., Schonhorst, M. H., and Thompson, R. K. 1969. Prog. Agric. Ariz. 21,4-5. Dobrenz, A. K., Cole, D. F., and Massengale, M. A. 1971. Crop Sci 11, 124-125. Douglas, J. A. 1974. Proc. Int. Grassl. Congr., 12th 1, 6 5 0 4 5 9 . Douglas, J. A., and Kinder, J. W. 1973. N.Z. J. Exp. Agric. 1, 23-27. Dovrat, A., Levanon, D., and Waldman, M. 1969. Crop Sci. 9, 33-34. Dowling, P. M., Clements, R. J., and McWiUiam, J. R. 1971. Ausf. J. Agric. Res. 22, 61-74. Drake, M., Vengries, J., and Colby, W. G. 1952. Soil Sci. 72, 139-147. Edye, L. A., and Haydock, K. P. 1967. Aust. J. Agric. Res. 18, 891-901. Ehrler, W. L. 1963. Agron. J. 55,363-366. England, C. B. 1963. Agron. J. 55,239-242. Erwin, D. C., Kennedy, B. W., and Lehman, W. F. 1959. Phytopafhology 49,572-578. Eskew, D. L., Schrader, L. E., and Bingham, E. T. 1973. Crop Sci. 13,594-597. Evenson, P. D., and Rumbaugh, M. D. 1972. Agron. J. 64, 154-157. Farragher, M. A. 1969. Ir. J. Agric. Res. 8, 221-227. Fehrenbacher, J. B., Ray, B. W., and Edwards, W. M. 1965. Soil Sci. Soc. A m . , Proc. 29, 591-594. Fick, G. W., and Holthausen, R. S. 1975. Crop Sci. 15, 259-262. Field, T. R. O., and Hunt, L. A. 1974. Proc. Int. Grassl. Congr., 12th 1, 357-365. F u n , B. J., Bourget, S. J., Nielsen, K. F., and Dow, B. K. 1961. Can. J. Soil Sci. 41, 16-23. Fox, R. L., and Lipps, R. C. 1955a. Soil ScL Soc. Am., Proc. 19,468473. Fox, R. L., and Lipps, R. C. 1955b. Agron. J. 4 7 361-367. Fredricksen, M. T. 1938. A m M i d . Nat. 20,641-681. Frosheiser, F. I., and Barnes, D. K. 1973. Crop Sci. 13, 735-738. Gervais, P., Dionne, J. L., and Richardson, W. S. 1962. Can. J. Planf Sci 42, 1-10. Gerwig, J. L., and Ahlgren, G. H. 1958. Agron. J. 50, 291-294. Gifford, R. O., and Jensen, E. H. 1967. Agron. J. 59,75-17. Gindel, I. 1968. Physiol. Plant 21, 1287-1295. Gist, G. R., and Mott, G. 0. 1958. Agron. J. 50,583-586. Gleiter, M. E., and Parker, H. E. 1957. Arch. Biochem. Biophys. 7 1 , 4 3 W 3 6 . Godley, M. Q. 1968. Diss.Abstr. B 29, 18. Grandfield, C. 0. 1943. J. Agric. Res. 67, 33-47. Grandfield, C. 0. 1945. J. Agric. Res. 70, 123-132. Grassi, C. J., and Chambouleyron, J. 1965. Rev. Fac. Cienc. Agrar., Univ. Nac. Cuyo 12, 12-23. (Herb. Abstr. 38, 824.) Graven, E. H., Attoe, 0. J., and Smith, D. 1965. Soil Sci. Soc. Am., Proc. 29, 702-706. Greacan, E. L., and Oh, J. S. 1972. Nature (London), New Biol. 235, 24-25. Greenfield, P. L., and Smith, D. 1973. Agron. J. 65, 871-874. Greenham, C. G. 1966. Can. J. Bor. 44, 1471-1483. Greub, L. J., and Wedin, W. F. 1971. Crop Sci. 11, 341-344. Griffith, W. L., Feuer, R., and Musgrave, R. B. 1966. Proc. Inf. Grassl. Congr., 9th 1, 4 7 9 4 83. Gross, H. D., Wilsie, C. P., and Pesek, J. 1958. Agron. J. 50, 161-164. Guy, P., Blondon, F., and Durand, J. 1971. Ann. Amelior, Plant. 21,409422. Hafeez, A. T. A., and Hudson, J. P. 1965. Exp. Agric. 1,99-106.

222

K. R. CHRISTIAN

Halkias, N. A., Veihmeyer, F. J., and Hendrickson, A. H. 1955.Hilgardia 24,207-232. Hamilton) H. A. 1970. Can. J. Plant Sci 50,401-409. Hamilton, R. I., Scholl, J. M., and Pope, A. L. 1969.Agron. J. 61,357-361. Hamlen, R. A., Lukezic, F. L., and Bloom, J. R. 1972. Can. J. Plant Sci. 52,633-642. Hansen, H. L., and Krueger, C. R. 1973.Agron. J. 65, 755-759. Hanson, C. H., ed. 1972. “Alfalfa Science and Technology.” Am. SOC.Agron., Madison, Wisconsin. Hanson, C. H., Busbice, T. H., Hill, R. R., Hunt, 0. J., and Oakes, A. J. 1972. J. Environ. Qual. 1,106-1 11. Hanson, E. G. 1967.N.M. Agric. Exp. S t n , Bull. 514. Harris, P. B., McNaught, K. J., and Lynch, P. B. 1966.N.Z. J. Agric. Res. 9,653-690. Hart, R. H.. Carlson. G. E., Hungerford, W. E., and Thompson, A. J. 1972.Agron. Abstr. p.

54.

Heinrichs, D. H. 1965. Can. J. Plant Sci 45, 177-183. Heinrichs, D.H. 1972. Can. J. Plant Sci 52, 985-990. Heinrichs, D. H. 1973. Can J. Plant Sci 53,291-294. Hehrichs, D.H., and Nielsen, K. F. 1966.Can. J. Plant Sci. 46,291-298. Heinrichs, D.H., Troelsen, J. E., and Warder, F. G. 1969.Can. J. Plant Sci. 49,293-305. Helyar, K. R., and Anderson, A. J. 1971. Aust. J. Agric. Res. 22, 707-721. Hill, R. R., and Jung, G. A. 1975. Crop Sci 15,652-657. Hobbs, E. H., and Krogman, K. K. 1966. Can. J. Plant Sci. 46,271-277. Hobbs, E. H., Krogman, K. K., and Sonmor, L. G. 1963. Can. J. Plant Sci. 43,

441-446.

Hobbs, J. A,, and Bertramson, B. R. 1950.SoilSci. Soc. Am., Proc. 14, 257-261. Hodgkinson, K. C. 1973.Aust. J. Agric. Res. 24,497-510. Hodgkinson, K. C., and Veale, J. A. 1966.Aust J. Biol. Sci 19, 15-21. Hodgkinson, K. C., Smith, N. G., and Miles, G. E. 1972. Aust. J. Agric. Res. 23, 225-238. Hoglund, J. H., Dougherty, C. T., and Langer, R. H. M. 1974. N.Z. J. Exp. A g r k 2, 7-11. Holford, I. C. R. 1968.Aust. J. Exp. Agric. Anim. Husb. 8,555-560. Hudson, J. P. 1965. Exp. Agric. 1,23-32. Hundtoft, E. B., and Wu, S. M. 1970. Trans. Am. Soc. Agric. Eng. pp. 181-186. Hunt, L. A., Moore, C. E., and Winch, J. E. 1970. Can J. Plant Sci 50,469-474. Hutchinson, F. E.,and Hunter, A. S. 1970.Agron. J. 62,702-704. Imangaziev, K. I., and Patakhov, M. I. 1968. Vestn. Skh. Nauki (Alma-Ata) pp. 2W26. (Herb. Abstr. 38, 1794.) Iversen, C. E., and Meijer, G . 1967. In “The Lucerne Crop” (R. H. M. Langer, ed.), pp. 74-84. A. H. & A. W. Reed, Wellington, New Zealand. Jackobs, J. A., and Miller, D. A. 1973.Agron. J. 65,222-225. Jackson, E. A. 1960.Aust. J. Agric. Res. 11, 715-22. Jacquard, P., Guy, P., and Genier, G. 1967. Int. Symp. Prod. Uril. Lucerne, Brno, Czech. (Herb. Abstr. 38, 1478.) Janghorbani, M., Roberts, S., and Jackson, T. L. 1975.Agron. J. 67, 350-354. Janson, C. G. 1971.N.Z. J. Agric. 122(6), 51-53. Janson, C. G. 1975a.N.Z. J. Exp. Agric. 3,63-69. Janson, C. G. 1975b.N.Z. J. Exp. Agric. 3,223-228. Janson, C . G.,and Knight, T. L. 1973.N.Z. J. Exp, Agric. 1, 243-51. Jensen, E. H., Massengale, M. A., and Chilcote, D. 0.1967. Nev., Agric. Exp. Stn., Bull. T9. Jensen, E. H., Frelich, J. R., and Gifford, R. 0. 1972.Agron. J. 64,635-639. John, M. K., Eaton, G. W., Case, V. W., and Chuah, H. H. 1972.Plant Soil 37, 363-374. Jones, F. R. 1943.J. A m Soc. Agron. 35,625-634.

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

223

Jones, F. R. 1950. Agron. J. 42,432-433. Jones, R. M. 1970. Aust. J. Exp. Agric. Anim. Husb. 10,749-754. Kauffeld, N. M., Sorenson, E. L., and Painter, R. H. 1969. Crop Sci 9, 225-228. Kemper, W. D., and Amemiya, M. 1957. Soil Sci. Soc. Am., Proc. 21,65760. Kilcher, M. R., and Heinrichs, D. H. 1966. Can. J. Plant Sci 46, 177-184. Kilmer, V. J., Bennett, 0. L., Stahly, V. F., and Timmons, D. R. 1960. Agron. J. 52, 282-285. King, R. W., and Evans, L. T. 1967. Aust. J. Biol. Sci. 20,623635. Klebesadel, L. J. 1964. Agron. J. 56, 359-361. Klebesadel, L. J. 1971. Crop Sci 11,609-614. Klebesadel, L. J., and Smith, D. 1960. Agron. J. 52,627-630. Knapp, W. R., Holt, D. A., Lechtenberg, V. L., and Vough, L. R. 1973. Agron. J. 65, 41 3 4 17. Koehler, P. G. 1973. Ann. Bot. (London) 37, 65-68. Kohl, R. A., and Kolar, J. J. 1976. Agron. J. 68, 536-538. Kramer, H. H., and Davis, R. L. 1949. Agron. J. 41,470-473. Krogman, K. K., and Hobbs, E. H. 1965. Can. J. Plant Sci. 45, 309-313. Krogman, K. K., and Lutwick, L. F. 1961. Can. J. Plant Sci. 4 1 , 1 4 . Kudrev, T., Petkov, V., and Radeva, V. 1970. Rastenievud. Nauki 7, 33-45. (Herb. Abstr. 41,483.) Kunelius, H. T. 1974. Agron. J. 66,806-809. Kust, C. A. 1971. Agron. J. 63, 394-396. Lagowski, J. M., Sell, H. M., Huffman, C. F., and Duncan, C. W. 1958. Arch. Biochem. Biophys. 76,306-316. Lamba, P. S., Ahlgren, H. A., and Muchenhirn, R. J. 1949. Agron. J. 41,451-458. Langer, R. H. M., ed. 1967. “The Lucerne Crop.” A. H. & A. W. Reed, Wellington, New Zealand. Langer, R. H. M., and Steinke, T. D. 1965. J. Agric. Scf. 64, 291-294. Langille, J. E., MacLeod, L. B., and Warren, F. S. 1965. Can. J. Plant Sci. 45, 383-388. Lapinskiene, N. 1966. Tr. Akad. Nauk Litov. SSR, Ser. B 3, 15-24. (Herb. Abstr. 38, 2042.) Larson, W. E., and Willes, W. 0. 1957. Agron. J. 49,422-426. Larson, W. E., Nelson, L. B., and Hunter, A. S. 1952. Agron. J. 44, 357-361. Lawton, K., and Tesar, M. B. 1958.Agron. J. 50, 148-151. Leach, G. J. 1968a. Aust. J. Agric. Res. 19, 517-30. Leach, G. J. 1968b. Aust. J. Exp. Agric. Anim Husb. 8, 323-326. Leach, G . J. 1969. Aust. J. Agric. Res. 20,425434. Leach, G . J. 1970a. Aust. J. Agric. Res. 21, 583-591. Leach, G. J. 1970b. Aust. J. Exp. Agric. Anim Husb. 10, 53-61. Leach, G. J. 1971a. Aust. J. Agric. Res. 22,49-59. Leach, G . J. 1971b. Aust. J. Exp. Agric. Anim. Husb. 11, 186-93. Lechtenberg, V. L., Holt, D. A,, and Youngberg, H. W. 1971. Agron. J. 63, 719-724. Lee, C. T., and Smith, D. 1972a.Agron. J. 64,527-530. Lee, C. T., and Smith, D. 1972b.J. Sci. FoodAgric. 23, 1169-1181. Lehman, W. F., Richards, S. J., Erwin, D. C., and Marsh, A. W. 1968. Hilgardia 39, 2 77-295. Levesque, M., and Ketcheson, J. W. 1963. Can. J. Plant Sci, 43,355-360. Liang, G . H. L., and Riedl, W. A. 1964. Crop Sci. 4,394-396. Lipps, R. C., and Fox, R. L. 1956. Soil Sci. SOC.Am., Proc. 20,28-32. Lipps, R. C., Fox, R. L., and Koehler, F. E. 1957. Soil Sci 84, 195-204. Loftfield, J. V, G . 1921. Carnegie Inst. Washington, Publ. 314.

224

K. R. CHRISTIAN

Loomis, W. E., and Ewan, L. M. 1936. Bot. Gaz. (Chicago) 97,728-743. Loper, G. M. 1972. Crop Sci. 12,459-461. Lucey, R. F., and Tesar, M. B. 1965. Agron. J. 57,519-523. Ludwig, L. J., and Canvin, D. T. 1971. Plant Physiol. 48, 712-719. Lueschen, W. E., Barnes, D. K., Rabas, D. L., Frosheiser, F. I., and Smith, D. M. 1976. Agron. J. 68, 281-285. McAuliffe, J. D. 1967. J. Agric. South Aust. 71, 99-105. McClelland, V. F. 1969. Aust. J. Exp. Agric. Anim. Husb. 9,428431. McElgunn, J. D. 1973. Can. J. Plant Sci. 53,797-800. McElgunn, J . D., and Heimichs, D. H. 1975. Can. J. Planr Sci. 55,705-708. McKee, G. W. 1962. Pa., Agric. Exp. Stn., Bull. 689. MacLeod, L. B. 1965a. Agron. J. 57,129-134. MacLeod, L. B. 1965b. Agron. J. 57,345-350. MacLeod, L. B., and Jackson, L. P. 1964. Can. J. SoilSci 45,221-234. MacLeod, L. B., and Suzuki, M. 1967. Crop Sci. 7,599-605. Mannetje, 't.L. 1967. Trop Grass. 1, 9-19. Markus, D. K., and Battle, W. R. 1965. Agron. J. 57,613-616. Marten, G. C. 1970. Proc. Int. Grassl. Congr., 11th pp. 506-509. Marten, G. C., Wedin, W. F., and Hueg, W. F. 1963. Agron. J. 55,343-344. Massengale, M. A., Dobrenz, A. K., Brubaker, H. A., and Bard, A. E. 1971. Crop Sci. 11, 9-12. Massey, D. L., and Sheard, R. W. 1970. Can. J. Soil Sci. 50,9-16. Matches, A. G., Mott, G. O., and Bula, R. J. 1962. Agron. J. 54, 541-543. Matches, A. G., Mott, G. O., and Bula, R. J. 1963.Agron. J. 55,185-188. Medler, J. T., Massengale, M. A., and Barrow, M. 1955. Agron. J. 47, 216-21 7. Mertz, E. T., Singleton, V. L., and Garey, C. L. 1952. Arch. Biochem. Biophys. 38, 139-145. Miles, A. D. 1969. J. RangeManage. 22,205-207. Miller, D. A., Shrivastava, J. P., and Jackobs, J. A. 1969. Crop Sci. 9,440-443. Moline, W. J., and Robison, L. R. 1971. Agron. J. 63,614416. Morley, F. H. W., Daday, H., and Peak, J. 1957. Aust. J. Agric. Res. 8,635451. Moschler, W. W., Jones, G. D., and Thomas, G. W. 1960. Soil Sci. Soc. Am., Proc. 24, 507-509. Mowat, D. N., Fulkerson, R. S., and Gamble, E. E. 1967. Can. J. Plant Sci 47,423426. Munns, D. N. 1965. Aust. J. Agric. Res. 16,757-766. Munns, D. N. 1968. Plant Soil 28, 129-146. Murata, Y., Iyama, J., and Honma, T. 1965. Proc. Crop Sci. SOC.Jpn. 34, 154-158. Murata, Y., Iyama, J., and Honma, T. 1966. Proc. Crop Sci. SOC.Jpn. 34,385-390. Murray, G. A. 1967. Diss. Abstr. B 28, 1316-1317. Nelson, C. J., and Smith, D. 1968. Crop Sci. 8, 25-28. Nelson, C. J., and Smith, D. 1969. Crop Sci. 9, 589-591. Nicholaichuk, W. 1964. M.S. Thesis, Univ. of Saskatchewan, Saskatoon, Canada. Nielsen, K. F., Halstead, R. L., MacLean, A. J., Holmes, R. M., and Bourget, S. J. 1960. Proc. Int. Grass. Congr., 8th pp. 287-292. Nishikawa, K. 1965. Proc. Crop Sci. SOC.Jpn. 34,52-59. Nishikawa, K . 1966. Sci Rep. Hyogo Univ. Agric. (Agric. Hortic.) 7, 129-134. (Herb. Abstr. 38, 383.) Nittler, L. W., and Gibbs, G. H. 1959. Agron. J. 51, 727-730. Nittler, L. W., and Kenny, T. J. 1964. Crop Sci. 4, 187-190. Nordquist, P. T., and Wicks, G. A. 1974. Agron. J. 66,377-380.

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

225

Nyatsanga, T., and Pierre, W. H. 1973. Agron. J. 65,936-940. O’Connor, F. J. 1967.Zn “The Lucerne Crop” (R. H. M. Langer, ed.), pp. 163-176. A. H. &. A. W. Reed, Wellington, New Zealand. Ouelette, G. J., and Dessureaux, L. 1958. Can. J. Plant Sci 38, 206-14. Palmer, T. P., and Wynn-Williams, R. B. 1976. N.Z. J. Exp. Agric. 4,71-77. Paltridge, T. B. 1955. Aust., C.S.I.R.O., Bull. 274. Parsons, J. L. 1958. Agron. J. 50,593-594. Parsons, J. L., and Davis, R. R. 1960. Agron. J. 52,441443. Pate, J. S. 1958. Aust. J. Biol. Sci 11,496-515. Pearce, R. B., and Lee, D. R. 1969. Crop Sci 9, 791-794. Pearce, R. B., Carlson, G. E., Barnes, D. K., Hart, R. H., and Hansen, C. H. 1969. Crop Sci 9,423426. Pearson, C. J., and Hunt, L. A. 1972a. Can. J. Bot. 50, 1377-1384. Pearson, C. J., and Hunt, L. A. 1972b. Can. J. Bot. 50, 1925-1930. Pearson, C. J., and Hunt, L. A. 1972c. Can. J. Plant Sci. 52, 1007-1015. Pearson, C. J., and Hunt, L. A. 1972d. Can. J. Plant Sci. 52,1017-1027. Peck, N. H., Vittern, M. T., and Miller, R. D. 1958. Agron. J. 50, 109-112. Perry, L. J., and Larson, K. L. 1974. Crop Sci 14,693-496. Peters, E. J., and Stritzke, J. F. 1970. Agron. J. 62, 259-262. Peters, R. A. 1961.Agron. J. 53, 195-198. Peterson, G. A. 1971. Soil Sci Soc. A m . Proc. 35,294-296. Peyremorte, P., Plancquaert, P., and Chambon, J. 1971. Fourrages 46, 29-50. Pohlman, G. G. 1946. Soil Sci. 62,255-266. Porter, T. K., and Reynolds, J. H. 1975. Agron. J. 67, 625429. Prine, G. M. 1966. Proc. Soil Crop Sci Soc. Fla. 26,217-226. Pritchett, W. L., and Nelson, L. B. 1951. Agron. J. 43, 172-177. Pulgar, C. E., and Laude, H. M. 1974. Crop S c i 14,28-30. Pulli, S. K., and Tesar, M. B. 1975. Crop Sci. 15, 861-864. Rabotnov, T. A. 1969. Herb. Abstr. 39,269-277. Radei, I. 1974. Proc. Int. Grassl. Congr., 12th 1, 823-830. Raguse, C. A., and Smith, D. 1966.J. Agric. Food Chem. 14,423426. Rai, S. D., Miller, D. A., and Hittle, C. N. 1971.Agron. J. 63,331-332. Reid, D. J., Lathwell, D. J., and Wright, M. J. 1965. Agron. J. 57,434437. Roberts, J. L., and Olson, F. R. 1942. J. Am. Soc. Agron. 34,695-701. Roberts, R. H. 1946. J. A m . Soc. Agron. 38,947-953. Roberts, R. H., and Struckmeyer, B. E. 1939. J. Agric. Res. 59,699-710. Robertson, G. W. 1966. Ecology 47,640-643. Robison, G. D., and Massengale, M. A. 1969. J. Arizona Acad. Sci. 5, 227-231. Rogers, V. E. 1963. Fld, Stn. Rec,, CSZRO, Div. Plant Ind. 2(2), 35-40. Rogers, V. E. 1969. Fld. Stn. Rec., CSIRO, Div. Plant ind. 8 , 3 7 4 4 . Rogers, V . E. 1974. Aust. J. Exp. Agric. Anim. Hush. 14,520-552. Ronningen, T. S., and Hess, A. G. 1955. Agron. J. 47, 92-93. Rosenberg, N. J. 1969. Agron. J. 61,879-886. Roufail, A. 1975. Aust. J. Exp. Agric. h i m . Husb. 15, 64-68. Rumbaugh, M. D. 1963. Crop Sci 3, 423424. Russell, M. B. 1959. Adv. Agron. 11, 70-73. Safta, I., and Balan, C. 1971. Wirtschaftseigene Futter 17, 49-54. (Herb. Abstr. 42, 1931.) Sanchez-Diaz, M. F., and Kramer, P. J. 1973. J. Exp. Bot. 24,511-515. Sato, K. 1971. Proc. Crop Sci Soc. Jpn. 40,120-126. Sato, K . 1974. Proc. Crop Sci. Soc. Jpn. 43,59-67.

226

K. R. CHRISTIAN

Scateni, W. J. 1972. Queensl. J. Agric. A n i m Sci 29,41-50. Schertz, D. L., and Miller, D. A. 1972. Agron. J. 64,660-664. Schmehl, W. R., Peech, M., and Bradfield, R. 1952. SoilSci 73, 11-21. Schonhorst, M. H., Davis, R. L., and Carter, A. S. 1957. Agron. J. 49, 142-143. Scott, T. W., and Erickson, A. E. 1964. Agron. J. 56,575-576. Seth, J., and Dexter, S. T. 1958. Agron. J. 50, 141-144. Shantz, H. L., and Piemeisel, L. N. 1927. J. Agric. Res. 34, 1093-1190. Sheridan, K. P., and McKee, G. W. 1968. Crop Sci 8,289-290. Silva, J. P. 1968. Diss. Abstr. 29, 1906B. Simon, U.,Kastenbauer, A., and Garrison, C. S . 1974. Crop Sci 14,682686. Simpson, J. R. 1965. Aust, J. Agric. Res. 16,915-926. Simpson, J. R. 1974. Proc. Int. Grassl. Congr., 12th 1, 330-7. Simpson, J. R., and Lipsett, J. 1973. Aust. J. Agric. Res. 24, 199-209. Singh, Y., and Winch, J. E. 1974. Can. J. Plant Sci 54,449. Smith, D. 1950.Agron. J. 42,398-401. Smith, D. 1951. Agron. J. 43,573-575. Smith, D. 1961. Can. J. Plant Sci 41,244-251. Smith, D. 1970a. Agric, Food Chem 18,652-656. Smith, D. 1970b. Agron. J. 62,520-523. Smith, D. 1971. Agron. J. 63,497-500. Smith, D., and Silva, J. P. 1969. Crop Sci 9,464467. Smith, D., and Struckmeyer, B. E. 1974. Crop Sci. 14,433436. Smith, D., Jacques, A. V. A., and Balasko, J. A. 1973. Crop Sci 13,553-556. Smith, T. J. 1971. Crop Sci 11, 27-28. Smoliak, S., Johnston, A., and Hanna, M. R. 1972. Can. J. Plant Sci 52,757-762. Snaydon, R. W. 1972a. Aust. J. Agric. Res. 23, 239-252. Snaydon, R. W. 1972b. Aust. J. Agric. Res. 23,253-256. Snaydon, R. W.1972c. Agric. Meteorof. 10,349-363. Song, S . P., and Walton, P. D. 1974. Crop Sci 14,663666. Song, S. P., and Walton, P. D. 1975. Crop Sci 15,649-652. Sonmor, L. G. 1963. Can. J. Soil Sci 43, 287-297. Sorensen, R. C., Penas, E. J., and Alexander, U. U. 1968. Agron. J. 60,20-23. Sprague, M. A., and Fuelleman, R. F. 1941. J. Am. Soc. Agron. 33,437447. Sprague, M. A., Hoover, M. M., Wright, M. J., MacDonald, H. A., and Brown, B. A. 1963. N.J., Agric. Exp. Stn., Bull. 804. Sprague, V. G., and Graber, L. F. 1938. J. Am . SOC.Agron. 30,986-997. Stanhill, G . 1962.Neth. J. Agric. Sci 10,247-253. Steinke,T. D. 1968.S. Afr. J. Agric. Sci 11,211-217. Sutton, C. D. 1969. J. Sci Food Agric. 20,l-3. Szeicz, G., Endrsdi, G., and Tajchman, S . 1969. Water Resour. Res. 5,380-394. Tadmor, N. H., Cohen, 0. P., Shanan, L., and Evenari, M. 1966. Proc. Int. Grassl. Congr., 10th pp. 897-906. Takasaki, Y. 1971. Proc, Crop Sci Soc. J p n 40,4&44. Takasaki, Y. 1972. Proc. Crop Sci Soc. Jpn. 41,205-212. Takasaki, Y., Takahashi, N., and Yokoyama, K. 1970. Proc. Crop Sci Soc. Jpn. 39, 144-150. Tanaka, T. N. 1971a. Proc. Crop Sci Soc. Jpn. 40,69-74. Tanaka, T. N. 1971b. Proc. Crop Sci Soc. Jpn. 40,306-310. Tanner, C. B., and Mamaril, C. P. 1969. Agron. J. 51,329-331. Taylor, H. M., and Gardner, H. R. 1963. SoilSci. 96, 153-156.

ENVIRONMENTAL EFFECTS ON ALFALFA GROWTH

227

Taylor, S . A., Haddock, J. L., and Pedersen, M. W. 1959. Agron. J. 51,357-360. Tewari, G. P., and Schmid, A. R. 1960. Agron. J. 52,267-269. Thomas, M. D., and Hill, G. R. 1949. In “Photosynthesis in Plants” (J. Franck and W. E. Loomis, eds.), pp. 19-52. Iowa State College Press, Ames, Iowa. Turrell, F. M. 1942. Am . J. B o t 29,400-415. Tysdal, H. M. 1946. J. Am . SOC.Agron. 38,515-535. Tysdal, H. M., and Kiesselbach, T. A. 1939. J. A m . Soc. Agron. 31, 513-519. Ueno, M., and Smith, D. 1970a. Agron. J. 62, 764-767. Ueno, M., and Smith, D. 1970b. Crop Sci. 10, 396-399. Ueno, M.,and Tsuchiya, S. 1968. J. Jpn. SOC.Grassl. Soc. 14, 188-192. Upchurch, R. P. 1951.Agron. J. 43,552-555. Upchurch, R. P., and Loworn, R. L. 1951. Agron. J. 43,493-498. Van Bavel, C. H. M. 1966. WaterResour. Res. 2 , 4 5 5 4 6 7 . Van Bavel, C. H. M. 1967. Agric. Meteorol. 4, 165-176. Van Riper, G. E. 1964. Agron. J. 56,45-50. Vartha, E. W. 1972. N.Z. J. Exp. Agric. 1, 29-34. Vartha, E. W., and Allison, R. M. 1973. N.Z. J. Exp. Agric. 1, 365-368. Vince-Prue, D. 1975. “Photoperiodism in Plants.” McGraw-Hill, New York. Vorhees, W. B., and Holt, R. F. 1969. Bull. Univ. Minn. Agric. Exp. Sin. 494. Vose, P. B., and Randall, P. J. 1962. Nature (London) 196, 85-86. Vough, L. R., and Marten, G. C. 1971. Agron. J. 6 3 , 4 0 4 2 . Wahab, H. A., and Chamblee, D. S. 1972. Agron. J. 64,713-716. Wallace, A. 1952. Agron. J. 4 4 , 5 7 4 0 . Walter, L. E., and Jensen, E. H. 1970. Crop Sci. 10,635438. Weir, W. C., Jones, L. G., and Meyer, J. H. 1960. J. Anim. Sci. 19,5-19. West, S . H., and Prine, G. M. 1960. Proc. Crop Sci. SOC.Fla. 20,93-98. Whitear, J. D., Hanley, F., and Ridgman, W. J. 1962. J. Agric. Sci. 59,415428. Whitehead, D. C. 1970. J. Br. Grassl. Soc. 25, 236-241. Willard, C. J. 1951. Adv. Agron. 3, 93-112. Williams, W. 1950. J. Br. Grassl. Soc. 5, 113-129. Willis, W. G., Stuteville, D. L., and Sorensen, E. L. 1969. Crop Sci. 9,637-640. Wilman, D. 1965. J. Agric. Sci, 65, 293-294. Wilsie, C. P., and Takahashi, M. 1937. J. Am. SOC.Agron. 29, 236-241. Wilson, D., and Cooper, J. P. 1969. New Phytol. 68,645455. Wolf, D. D., and Blaser, R. E. 1971a. Crop Sci. 11,55-58. Wolf, D. D., and Blaser, R. E. 1971b. Crop Sci. 11,479482. Wolf, D. D., and Blaser, R. E. 1972. Crop Sci. 12, 23-26. Wolf, D. D., Larson, K. L., and Smith, D. 1962. Crop Sci. 2,363-364. Yamada, T., and Suzuki, S. 1974. Proc. Znt. Grassl. Congr., 12th 3, 1016-1022. Yates, J. J., Russell, M. J., and Fergus, I. F. 1971. Aust. J. Exp. Anim. Husb. 11, 6 5 1 4 6 1 . Young, N. D., Fox, N. F., and Burns, M. A. 1958. Queensl. J. Agric. Sci. 16,199-215. Youngberg, H. W., Holt, D. A., and Lechtenberg, V. L. 1972. Agron. J. 64, 288-291. YouNs, M. A., Stickler, F. C., and Sorensen, E. L. 1963. Agron. J. 55, 177-1 82. Zaleski, A. 1954.J. Agric. Sci. 44,199-220. Zaleski, A. 1962. J. Br. Grassl. SOC.17,7-16.