Crop Production and
Environment (1960) 177-191
R. O. Whyte
RELATIONSHIPS OF DEVELOPMENTAL PHYSIOLOGY
The physiological processes that occur in plants during vegetative growth or reproduction are of great significance in connection with the yields and nutritive values of crop plants. Although crops such as green vegetables or herbage plants are primarily manifestations of growth, they are in a varying degree also dependent upon a certain amount of development before they can reach their maximal nutritive value. The protein content of herbage is probably at its highest when a grass is in a very active state of growth, and is at the same time providing a developing ear (possibly before shooting) with building materials.
For maximum production of grains in an annual cereal, growth processes are allowed to proceed to their end, until such time as the developmental processes and building of the grain exercise a completely inhibitive effect upon growth and the plant bearing the ripe fruit dies. Where optimal environmental conditions are available for fruiting, the straw is of little value, everything having been supplied to the grain. Where conditions are below the optimum, grain yields are lower and ripening more difficult, but the straw is of higher nutritive value for livestock. Growth has continued longer owing to the lower inhibitory effect of the development, and less building material is supplied to the grain.
The striking of an appropriate balance between growth and reproduction is the basis of good management of grassland for grazing or hay production. The breeder of herbage plants produces for grazing purposes strains called 'pasture types', that is types that find the environment suitable for growth but not optimal for reproduction. The best pasture ryegrass of Great Britain (Aberystwyth S.23) is a very 'shy seeder' because it is out of its true developmental environment. This variety has, however, a high nutritive value. It presumably finds the environment suitable for just that degree of development that stimulates active metabolism and growth and produces a high nutritive value. It would appear that, if the carotenoids, carotene and xanthophyll, are taken as indicators, the maximal nutritive value is to be obtained just at the time of flowering; this has already been noted many times in studies of time of cutting hay for maximal yield and nutritive value. Murneek (1939) reports that when plants have become reproductive through exposure to an appropriate photoperiod, there is an increase in carotene and xanthophyll content of the leaves (Table 21). The concentration of these carotenoids seems to reach a maximum at the time of flowering after which there is a reduction (Table 22). The lower amounts of carotene and xanthophyll in the nodes of Biloxi soybeans when fruit is present suggest that the carotenoids are moved to the reproductive organs or changed into some other compounds (see also p. 182).
|Concentration of carotenoids in vegetative and reproductive plants at approximate time of flowering.|
|Carotene and xanthophyll in leaves of soybeans.|
Much of the herbage literature now appearing deals with the carotenoids of grasses and legumes, and the relation between this research on the production of nutritive herbage and that on developmental physiology is obvious.
Investigations on plant physiology and metabolism at different developmental phases or while exposed to controlled environmental factors have not yet reached a stage warranting definite conclusions. The scattered works to be discussed in this chapter begin to give an indication of the picture, and its possible application to the production of optimal yields of grain, or green mass for forage or green manure, and to the optimal utilization of fertilizers or other cultural measures.
Physiological, Biochemical and Anatomical Conditions in Vernalized Seeds
Since enzymatic processes are very active during the period of germination, it was natural that studies should be made of their behaviour in relation to vernalization. Whether any change which may be noted in their activity is governed ultimately by the concentration of hormones present depends upon the confirmation of Cholodny's hormonal interpretation of vernalization (see p. 161). The studies on enzymes in relation to vernalization made by Richter and his associates and Demkovskil, and on the iso-electric point by Richter, Gavrilova and others are described in the early reviews of vernalization (Imp. Agric. Bur., 1935).
Oveckin and others (1936) studied the biochemical changes in winter wheat grains during vernalization. Grains were vernalized at to 1 C. under normal air conditions, and also on a 0.003 per cent concentration of chloroform or ethylene chloride. Samples were taken on every sixth day and records made of respiration rate, sugar content, catalase activity, content of mono-amino-acids and reduced glutathione, and of the percentage of fully vernalized grains that grew when planted out of doors.
The presence of chloroform or ethylene chloride reduced the percentage of fully vernalized grains; the respiration rate, catalase activity, and content of reduced glutathione were all lower in grains vernalized in ethylene chloride. The reduction-oxidation processes are intensified during vernalization, but there is no relation between content of sugars and mono-amino-acids and vernalization.
Sapoznikova (1935) analysed vernalized seed of Lupinus angustifolius. The content of reducing sugars in seeds treated at 6 to 7 C. increases with the progress of vernalization, a fact regarded as suggesting an increasing activity of the enzymes acting on carbohydrates; however, the content of reducing sugars gradually falls in seeds vernalized at 4 to 5 C. The amount of active enzymes rises with vernalization at 6 to 7 C. to a maximum on the last day of treatment; they increase less rapidly with vernalization at 4 to 5 C, during the first 12 days and then fall. The control of active enzymes is measured by their activity at 35 C. The protease content in seeds vernalized at 6 to 7 C. was found to rise to a climax on the day of sowing, while in those vernalized at 4 to 5 C. its changes were indefinite. The activity of catalases, peroxidases and respiration varied in the different series. Marked activity of catalases and peroxidases is a feature of vernalized lupin seeds that give rise to plants with a reduced vegetative period, and this character is stated to be useful for distinguishing vernalized from unvernalized seeds.
From determinations of nitrogenous substances (total N, insoluble and soluble N, amino-N, amide-N, and ammoniacal N) in seeds and plants during vernalization and in those under conditions which prevent vernalization, Konovalov (1938) found their behaviour to vary considerably. When vernalization was prevented, the disintegration of the proteins extended to the end products, whereas during vernalization the proteins retained their form, but became more readily soluble. This worker concludes that nitrogenous substances appear to be re-synthesized during vernalization, and regards this transformation as a distinctive feature of the vernalization process.
Pasevic (1940) found that vernalization induces changes in the protein substances of the wheat germ affecting both their colloidal state and their amino-acid content.
Physiological Conditions in Plants Grown from Vernalized as Compared with Unvernalized Seeds
In some early studies on the effect of vernalization on the rate of accumulation of dry matter, Konovalov (1936) found that vernalized plants of wheat and lentils accumulated more dry matter per unit of time than the unvernalized control, and the yield of organic matter was consequently increased. More recently, Konovalov and Popova (1941) found that the synthesizing capacity of vernalized plants is higher than in the unvernalized controls. By the time of earing, vernalized plants contained 26 per cent more organic matter than those from soaked and germinated seeds and 52 per cent more than plants from seeds sown dry. Konovalov (1944) has continued his work at the Timirjazev Institute of Plant Physiology, Moscow, on the effects which vernalization of seed exerts on the growth and physiological processes in the leaf relevant to the yield of grain or seed ultimately produced. The chief concern in these experiments has been productiveness of a plant as governed by the intensity of photosynthesis, the extent of its leaf area, and the duration of activity by the leaves.
The intensity of photosynthesis was not materially affected by vernalization, but the interval between emergence of the leaf and its death was shortened. The factor most closely connected with yield was the leaf area of a plant, and it was this which was markedly affected by vernalization. The successive emergence of leaves was more rapid with than without vernalization, and each leaf reached its maximal size sooner. A growing leaf uses much of the products of metabolism for its own use; vernalization has the advantage of hastening the growth of leaves and ensuring its early completion, after which the products are released for the benefit of the embryonic ear. It was noticed that the content of nitrogenous substances in the lowest leaves was diminished after the leaves had ceased to increase in size.
The synthetic activity of the leaves reached the maximum sooner in vernalized than unvernalized plants, and, after remaining stationary for a while, gradually decreased. The accumulation of dry matter was, likewise, more abundant in vernalized plants. Consequently there was more material available for translocation to the developing ear, thus accounting for the well-being of vernalized plants which is to be especially observed during a dry season.
The yield of late-maturing wheat in Konovalov's experiments was increased by vernalization because, although tillers were reduced in number, their ears bore better and more numerous spikelets, as well as more numerous and heavier grain. In the early maturing varieties the good development of the ears could not compensate for the reduced number of tillers, and vernalization did not therefore increase the yield. A similar result is reported by Buzovir (1936), who made experiments over two seasons with varieties of winter and spring wheat and a variety of millet; at the beginning of vegetative growth in winter wheat and throughout the vegetative period in the other plants, the rate of elaboration and accumulation of dry matter was greater in vernalized plants. The accumulation of carbohydrates was also greater in vernalized plants, particularly during the period from jointing and stem elongation to milk ripeness.
In experiments made by Zaiceva (1939), with spring and winter wheats sown with vernalized and unvernalized seeds, it was found that the chlorophyll content increased as the plants advanced towards sexual maturity, reaching a maximum of over 6 mg. of crystalline chlorophyll per grm. of leaf weight in either variety, by the time of heading or thereabouts and falling rapidly thereafter. In rapidity of development, the spring wheat was somewhat ahead of the vernalized winter wheat. No such regularity was noted in unvernalized winter wheat plants that remained at the tillering phase, the chlorophyll content varying from the beginning of tillering between 3 and 4 mg. of crystalline chlorophyll per grm. of leaf weight, the highest figures being obtained in the leaves nearest to the spikes. Advance in development is considered to be the chief factor causing this conspicuous variation.
Physiology and Biochemistry in Relation to Response to Light
A considerable amount of work has been done on the physiology and biochemistry of plants in relation to the photoperiod in which they are grown; readers are referred to Burkholder's review (1936), and the literature quoted therein. As regards enzymatic activity in relation to light as distinct from the temperature relationship described earlier in this chapter, mention should be made of the experiments of Knott (1932 and 1926), who investigated the catalase in spinach before and after lengthening the photoperiod, and found a rapid response as exhibited by an increase in the enzymatic activity following a change to long days, that is to conditions favouring reproduction. In the second paper, Knott reported a decrease in catalase in the apical portion of the stem of spinach and Cosmos when the plants changed to a reproductive type of growth. If vegetative growth was resumed, a higher catalase activity was restored.
Murneek has made a detailed study of the metabolic changes associated with the photoperiodic reaction, and quotes over two hundred references to literature in his paper (see Missouri, 1937). The exposure of plants of Soja max to a 7-hour day (in comparison with a 14-hour day) resulted in induced sexual reproduction, retardation of growth in height, greater accumulation of dry matter, and higher nitrogen metabolism and nitrogen concentration; under a 14-hour day, vegetative development was continuous and no flower buds were formed. In the plants in both long and short day, the total coagulable, proteose, basic, ammonia and humin nitrogen increased upwards from the base of the stem to the tip, while nitrates, amide and amino-N decreased in the same direction. Murneek does not ascribe any specific dynamic function to any group of nitrogenous substances as regards the initiation of floral organs, and considers that the evidence available indicates the action of a specific flower-producing hormone.
The plants in short day maintained a higher rate of respiration at the time of the photoperiodic treatment than those in long day, and a change over from long- to short-day exposure also increased respiration. As a result of the curtailed growth in the plants under short day, nitrogen and carbohydrates accumulated in the vegetative parts, and at the time of full flowering there was a relatively higher accumulation of carbohydrates than nitrogen in the leaves. In plants under long day, according to Murneek, these substances are used for supporting vegetative growth, and in the plants under short day for sexual reproduction. The buds, flowers and young fruits were comparatively rich in carotene; this substance, together with xanthophyll, reached a maximum concentration in the leaves of plants under the 7-hour day at the flowering stage, and decreased subsequently.
|Carbohydrate/nitrogen ratios and photoperiodism, in the stems of Biloxi soybean, a plant reproductive in short day. Maximal flower induction period shaded (Murneek, 1939)|
FIGURES 38 & 39
|Relative catalase activity in short-day Biloxi soybean plants, expressed as log. per cent of long-day plants (Murneek, 1939)|
Parker and Borthwick (1939) studied the carbohydrate and nitrogen metabolism of soybean plants of the Biloxi variety in relation to exposure to photoperiods; the plants for these biochemical studies were first grown in a daylength which would promote the initiation of flower primordia, and were then transferred to 8, 13 and 16-hour photoperiods; the control lot had been grown in 16-hour photoperiods throughout.
One week after the transference, the total nitrogen and soluble nonprotein nitrogen were higher in the 8-hour day series than in the controls. Carbohydrates were lower than in the controls, although the starch in the leaves was higher. The total nitrogen in both leaves and stems of the 16-hour transfers became similar to that of the controls, and the 13-hour transfers approached the 8-hour ones as the season advanced. The soluble non-protein nitrogen showed the same relationship in the stems, while an abrupt rise was noted in the amount of ammonia in the leaves and stems of the 8- and 13-hour transfers at pod formation. Parker and Borthwick found that the amount of soluble carbohydrates in the transfer groups was apparently correlated with the length of the photoperiod. Starch accumulated in the leaves and stems of the 8- and 1 3-hour transfers when pods began to form.
Two groups of plants upon which flower buds had been initiated and which were subsequently grown at photoperiods just above and just below the critical showed progressive deviation from each other in their carbohydrate and nitrogen metabolism. Those grown below the critical became similar to the 8-hour transfers, while those above the critical became similar to plants that had been kept in a vegetative state by continuous cultivation in a 16-hour photoperiod.
A paper by Eremenko (1936) may be quoted as an example of a Soviet work on the relation between length of day and nitrogen metabolism, that is the C/N relationship which has already been discussed in relation to plant development on p. 22, and which Eremenko considers should not be regarded as a regulator of development, but as a function of the processes of growth and development.
In Eremenko's experiment, the vegetative period was most shortened when the soybean plants were grown in a 10-hour day; the exposure to short day of plants grown up to flowering in an 18-hour or normal day also increased their rate of development. Plant height and dry matter content were reduced by the reduction of the vegetative period. The soluble carbohydrate content (chiefly monosaccharides and less notably maltose and sucrose) was higher in plants grown in short day. A negative correlation was noted between soluble carbohydrate content and the rate of development. The total nitrogen and nitrogen of proteins fell with ageing of the plants. The highest N and protein-N content was found in plants grown in short photoperiods; consequently, the C/N ratio increased with age of plants. Development of the apical bud was retarded in plants grown in short day, although the leaves continued their functions normally and assimilated a large amount of carbohydrates. As the reproductive organs are formed early in the soybean, a considerable part of the nutrients migrate into the roots; the ratio between overground and underground parts was therefore smaller in plants grown in short day. The experiments of Katunskii (1939) are concerned with the relation between the accumulation of organic matter (weight of dry matter per plant) and the photosynthetic activity of plants during their growth and development. The long-day plants, Avena saliva var. byzantina, Phaseolus vulgaris and Hordeum vulgare, and the short-day plants, Panicum miliaceum, Soja hispida and Cannabis sativa, were grown under light conditions that were equally favourable for photosynthesis, but differed in relation to the photoperiodic reactions of the plants, namely, a 10-hour day, supplemented by light of an intensity below the critical value for photosynthesis for 0, 6, 8 and 14 hours.
Katunskii found the accumulation of dry matter to be a very complex biological phenomenon, being determined not only by the duration and intensity of the 'working hours' and energy of respiration, but also by the rapidity of development and the relative development and growth of plant organs, with which is associated the distribution of assimilates. The relative significance of these factors in relation to the amounts of organic matter ultimately produced varied considerably, being determined by the hereditary nature of the plant and the conditions of growth. In the experiments, four out of ten plants (Phaseolus and Pisum in 10- hour day plus 14-hour supplementary light, and Soja and Panicum in a 10-hour day) produced the maximum weight of dry matter under conditions of the most rapid development; this high productivity was considered to be associated with the vigour of the reproductive organs.
Photosynthetic activity and energy of respiration were also studied in relation to rapidity of development in plants that were grown under the appropriate photoperiodic treatment for 10, 20 and 30 days, and then transferred to seasonal day. Photosynthesis and respiration energy were found to increase with the rapidity of development in both long- and short-day plants, if the appropriate photoperiodic treatment was continued long enough to affect the rapidity of subsequent development. Katunskii states that these conclusions were confirmed in tests of photosynthesis and respiration during the vegetative period in Perilla, Chrysanthemum, cucumbers and potatoes, these processes increasing with the advance of plants in their development.
Nutrition and Reproduction
Most farmers and horticulturists are already quite aware of the fact that, once they have provided for the basal nutrient requirements for their crops under their own soil conditions, the balance can be tilted in one direction or another by the application of a particular fertilizer at an appropriate stage in the growth of the crop to provide better growth or reproduction.
The literature on the different rate of absorption of mineral elements by plants at different stages of growth is noted by Maximov in his Plant Physiology (1938); it is stated (p. 266) that most annuals absorb the greater part of the requisite salts before the flowering stage, while in corn all the nutrients required for further development are already accumulated in the stem at the time of flowering, when their further absorption ceases almost completely. Maximov also notes that there is a difference in the rate of absorption of individual elements during the development of plants, and describes the absorption of calcium, potassium, magnesium and phosphorus by plants such as oats and peas.
Sugar beet requires nitrogen throughout its vegetative period; a critical time is during intense development of the assimilatory apparatus (four to seven pairs of leaves). The elimination of nitrogen has a harmful effect on yields of root and leaf at all developmental stages. The supply of nitrogen during the second half of the growth period causes an increase in cell colloids. In the absence of nitrogen, the uptake of phosphates and potash is reduced (Demidenko, 1945a).
If potassium is excluded from the nutrient medium of sugar beet during early stages (four to seven pairs of leaves), yields are reduced and quality of crop impaired (Demidenko, 1945b). The period of intense sugar accumulation (seven to ten pairs of leaves) is critical as far as potash is concerned. Leaf area is reduced by potash deficiency; if it is excluded from the nutrient medium during the growing period of the plant, the amount of colloids in the cell sap is increased. If potassium is deficient during the second half of the vegetative period there is an increase in the percentage of soluble nitrogen. The period of maximum absorption does not coincide with the critical periods in potassium nutrition. Nitrogen supply and photoperiod are related in their effect on flowering, growth and stem anatomy in a number of long-day (Scabiosa atropurpurea and spinach) and short-day plants (Tithonia speciosa, Biloxi soybean, Salvia splendens and Xanthium pennsylvanicum). The external nitrogen supply is not, like photoperiod and temperature, a determining factor in determining floral initiation or vegetative growth, but has a considerable effect on time of flower bud appearance and flowering in some species (Withrow, 1945). Abundant nitrogen produces taller and heavier plants, but with the same nitrogen supply plants in a long photoperiod are taller than those in a short photoperiod. Flowering plants have a higher percentage of dry matter and a higher top-root ratio than vegetative plants; nitrogen does not alter the direction of this response. Limitation of nitrogen has a marked effect on the anatomy of the stems.
The general question of the relation of mineral nutrients to flower development has been discussed by Loehwing (1940), who reviewed the sequence of physiological events antecedent to and concurrent with flowering, which are stated to consist, first, of a change in the internal water balance, followed in turn by altered translocation and redistribution of nutrients. As far as the general progress of plant development is concerned, however, the problem is rather as expressed in a recent paper by Cailahjan and Lukovnikov (1941), who studied the limits within which the rate of development can be influenced by mineral nutrients, in order to reconcile the present conflicting views; (a) plants have been reported to show very different rates of development according to vernalization and photoperiod, and at the same time to respond only very slightly to variations in the supply of mineral nutrients, and (b) under natural conditions of growth, mineral salts have a very considerable influence on development.
Cailahjan and Lukovnikov find that, under favourable light conditions, development is not affected by varying the mineral ration; no amount of mineral matter will bring plants from a vegetative to a reproductive state. When plants are developing towards sexual maturity under favourable photoperiods, however, the conditions of mineral nutrition naturally become an important factor in plant development. At that time, the authors did not claim that this result applies equally to the respective photoperiodic groups.
Further work by Cailahjan (1944) has been concerned with nitrogen in relation to flowering and fruiting. It is stated that the oat plant (long day) when grown in the long days of Moscow flowers earlier if lower quantities of mineral nutrients are provided, while millet (short day) flowers and fruits sooner if more mineral nutrients are made available. A study was made to discover to what extent these two opposite reactions are exhibited by other plants, and which elements actually have the determining effect. Oats, blue lupin, Illini soybean and buckwheat were grown under favourable light conditions. Cailahjan finds that they are divisible into three groups:
(a) plants which start flowering sooner, the lower the mineral supply (oats);
(b) plants which start flowering sooner, the higher the mineral supply (millet and blue lupin);
(c) plants which flower simultaneously irrespective of nutrient supply (buckwheat, soybean).
Cailahjan gives some preliminary data regarding nutrition rich in nitrogen, but attempts no conclusions. Nitrogen stimulates accumulation of dry matter and growth in all plants, inhibits flowering and fruiting in some (mustard and oats), accelerates those processes in others (the short-day plants, Perilla and millet, and the long-day plants, lupin and lettuce), and has no influence on these processes in buckwheat, soybean and hemp. The following statement is proposed as governing the use of nitrogenous fertilizers in agronomic practice: 'in the case of an excess of nitrogen, nitrogenous fertilizers delay the progress of individual phases of development in some plants, and on the contrary accelerate the development of others.' The application of nitrogenous fertilizers should therefore be regulated according to the phase of development of the crop concerned.
It has now become possible, according to Cailahjan (1945), to group plants according to the reaction of flowering to changes in nitrogenous nutrition, as follows:
Development and External Morphology
The experiments of Stankov (1938) may also be quoted as an example of a claim for a direct correlation between phase of development and organ formation, and the type of mineral nutrition, and the effect adjustments in this relationship may have on the yield of grain from cereals. Stankov states that the yield of a seed plant, which depends upon number of spikes on an inflorescence, number of spikelets on a spike, and 1,000-grain weight, is a morphological expression of suitability of the environment for development, and can be co-ordinated with developmental phases as follows (Sapegin, 1938):
|Thermo-phase||Number of leaves|
|Photo-phase||Number of spikes in an ear and spikelets in a spike|
|Gametogenic phase||Number of fertile florets|
|Embryogenic phase||Number of grains in an ear and spike, and 1,000-grain weight|
The initiation and expression of these parts were investigated with deficient, optimal, and excess amounts of N and P in water cultures, with different combinations of N, P and K in soil cultures, and with different amounts of N, P and K in complete mineral nutrition under field conditions; special attention was devoted to the ratio between N and P.
It was found that the thermo-phase lasted 15 days in wheat and 6 days in barley and that its progress was not affected by type of nutrition, all variants having the same number of leaves. The length of the photophase and consequently the rate of differentiation of the apical meristem varied according to the N/P ratio, being retarded when N/P =5/250 and accelerated when N/P =250/5. This in turn affected the number of spikes, which varied in water cultures from 9.9 (N/P =5/250) to 16.2 (N/P =250/5); in soil cultures from 13 to 17.2 (lack of N) to 20.1 to 21.6 (NPK-N); and in field tests from 9.4 to 13.4 with increasing N in the nutrients supplied. The number of fully developed florets increased also with the increase of N/P, and caused a corresponding increase in the number of spikes with two and three florets.
Stankov found that the gametogenic phase depends upon the inclusion of phosphorus in the nutrition; the percentage of fertile florets increased from 66 (in absence of P) to 90 (with P in nutrition), a result regarded as indicating the importance of P in the nutrient supply towards the end of development. Maximum seed setting (86 per cent) in specially arranged water cultures was obtained when the N/P ratio was varied as follows: 5/50 during the first two phases noted above, 250/5 during the third phase, and 5/50 during the fourth phase. In spite of this conclusion, the author admits failure in finding a balance of nutrient supply which would secure at the same time the largest number of spikes in an ear and a maximum percentage of seed setting. The number of grains in an ear and spike also varied with the nutrition during the fourth phase, being eleven in the series without fertilizers, and twenty-four in the presence of N; a similar result was noted with 1,000-grain weight, which was 31.6 in absence of fertilizers and 36.0 with NPK.
Stankov notes that the location of the grain on the ear is of importance, as the protein content and vigour of derived plants varied for this reason; the correlation between shape of ear and type of nutrition has not yet been studied.
There are many more Russian papers dealing with the adaptation of fertilizer supply to developmental phase, which can be regarded as experimental confirmation of some of the practices that have long been employed by agriculturists and horticulturists to produce the maximal and optimal yield of the product in which they are concerned, whether it be cereal grain, tomatoes, lettuces or early-bite grass, at the most appropriate season of the year.