The Situation in Biological Science (1948)
Periodicity of Starch Formation in Potatoes
B. A. Rubin
(Bach Institute of Biochemistry of the Academy of Sciences of the U.S.S.R.).
One of the central problems in the theory of evolution is the interrelation of the organism with its environment. Academician T. D. Lysenko devoted much attention to this problem in his address, and he quite rightly stressed the immense importance of a correct, Michurin conception of the nature of these interrelations for agronomical scientists and for workers in all branches of biology. For, indeed, when trying to grasp the inner meaning of the chemical properties of an organism, the properties of the metabolic processes in the organism, the biochemist cannot ignore the influence which the changes of external conditions exercise on these processes. It is here that lies the key to an understanding of how the adaptive reaction of the organism to external influences is effected, a reaction which ensures normal development of the organism and, what is of most interest to the biochemist, the synthesis of definite organic substances.
Until now, the attention of biochemists working on this problem was concentrated on the so-called geographical, or climatic, variability of the chemical composition of plants, which furnishes an example of inconstant adaptation in ontogenesis.
The work, chiefly of Russian, Soviet scientists, has established many valuable and important facts relative to the dependence of the chemical composition of plants on conditions of the external environment. At the same time, these researches have not dealt with the nature of the influence exercised by the environment on the metabolic, processes, with the causes which determine the specific character of the reaction of plants of different groups to external influences. The data obtained in these researches, highly valuable though they are, did not touch upon the important question of the inherent nature of the adaptive reactions of plants to environmental conditions. Proceeding from the teachings of A. N. Bach and A. I. Oparin on the biological role of enzymes in the plant organism, it is necessary when studying this problem to give prime attention, not to ascertaining what particular chemical compounds are contained in the tissue of plants, but to investigate the processes which lead to the synthesis of these substances.
It must also be borne in mind that it is not only when the plant organism is transplanted from one geographical zone to another that it encounters altered external conditions. The plant encounters constantly changing conditions of existence all through its life cycle and in the course, of every, diurnal period.
The characteristic thing about these changes in plant organisms is that they occur in a regular succession, that they have a definite rhythm. Obviously, the plant organism can normally exist and develop only if the metabolic processes are suitably adapted to the constant change of external conditions. These are the basic ideas guided by which we, in the Bach Institute of Biochemistry, have been studying the biochemical nature of the adaptive reactions of plants.
In our researches, major attention is devoted to the leaf. I listened with the greatest attention to the speech of Academician P. N. Yakovlev, who, when telling of his experiments in training hybrid seedlings, stressed the immense influence which the leaf has on the properties of the forming organism, often determining the definite generic and specific character of the latter. Our researches over a period of many years fully corroborate this view. More, we have reason to believe that many also of the varietal characteristics of plants of one and the same species—such as early-maturation, storage qualities, yield (accumulation of reserve substances), etc.—may with a great degree of accuracy be characterized as biochemical properties relating to the leaf. In the case of many vegetable and fruit plants, this method may be effectively used by making a special selection for biochemical features.
The leaf is of special interest to the biochemist also because it is the organ in which are concentrated, not only diverse but even contradictory, antagonistic, functions. It is sufficient to recall that it is in the leaf that the primary act of formation of organic matter occurs, and, consequently, the leaf must be adapted to the processes of accumulation of assimilates. At the same time, this organ must be adapted to supplying the plastic substances of all the other parts of the plant. The concentration within one organ of functions so antagonistic in character permits us a priori to presume the existence within the leaf of a very refined, adjusted, and at the same time very dynamic system, under the influence of which alone the simultaneous performance of the functions of accumulation and translocation is possible. There can, be no doubt that the functioning of this system is closely dependent upon the conditions of the organism's existence.
Our researches have completely confirmed these opinions. Particularly interesting is the data we have obtained pointing to regular rhythmic fluctuations in the activity of the leaf enzymes both in the course of the vegetative period and in the course of the diurnal period.
This problem was placed on an experimental footing for the first time by Academician A. N. Bach more than twenty-five years ago. A. N. Bach considered that the diurnal fluctuations of the activity of the enzymes in one and the same organism are connected with changes in its physiological state, and he emphasized the deep scientific interest of this problem.
In 1936, N. M. Sisakian established the existence of seasonal rhythms in the action of the enzymes which regulate the carbohydrate exchange in the leaves of the sugar beet, in 1937, we demonstrated that the activity of these enzymes does not remain constant throughout the twenty-four hour day, but experiences quite regular fluctuations.
Further researches elicited that the diurnal course of the enzymatic processes in the leaf are of very great importance, inasmuch as the intensification of the processes of synthesis in the daytime and of the processes of decomposition in the nighttime are suitably adapted to enable the leaf to perform the functions of assimilation and translocation.
These researches further showed that the diurnal rhythm of the action of the enzyme is not a direct reflection of the existing conditions, since it was preserved in plants which were placed in abnormal conditions (e. g., under light at nighttime, and in darkness in the daytime).
The diurnal rhythm of the action of enzymes is an example of phylogenetic adaptation, bearing the features of conservatism. But at the same time it was ascertained that the rhythm of enzymatic action cannot be regarded as an autonomous property, since when the plant organism is subjected to abnormal conditions for a sufficiently long time, this rhythm is substantially disturbed.
Here we thus have a confirmation of Darwin's opinion that the rhythmic movements in plants are a hereditary feature, although, as he stressed, this periodicity may be disturbed by applying appropriate influences.
Our further work was concerned with a study of the infernal mechanism of the adaptation of plants to the regular temperature changes taking place during the period of vegetation.
The study of this question was begun by V. E. Sokolova and myself in 1945. The chief object of observation was the potato, whose development, according to the work of Academician T. D. Lysenko, has an extremely close dependence on the temperature factor.
The processes of starch formation were chosen as the biochemical index, since they are most indicative of the specific directiveness of metabolism in the potato plant. The work consisted in studying the temperature curves of the processes of enzymatic formation and decomposition of starch in the leaves and tubers at various stages of the ontogenesis of the potato.
The chief conclusion to be drawn from these studies is that the temperature optima for the action of the enzymes are not constant. As, the plant develops, these optima change, and the direction of the changes harmonizes very well with the course of the temperature changes in the plant's surrounding milieu. At first, roughly for two-thirds of the vegetative period, the temperature optimum for the formation of starch in potato leaves shifts towards higher temperatures, and in the last period towards lower temperatures. In the tubers, the starch-formation optima shift towards lower temperatures, apparently because the development of the tubers takes place in the latter half of the summer.
Consequently, the temperature optima for the action of one and the same enzymes in different organs of the plant are dissimilar, and their changes in the course of the vegetative period are likewise dissimilar.
In spite of the fact that the experiments were conducted in years when the weather conditions varied considerably, the direction of the shifts of the temperature optima for enzyme action as a rule remained the same.
Hence the temperature curves of enzyme action, like the diurnal rhythm, cannot be regarded as a direct reflection of the conditions of existence of the plant organism. This characteristic was elaborated in the course of a long process of evolutionary adaptation, under the influence of that rhythm of temperature changes in which the plant's evolution proceeded.
These facts accord with Academician T. D. Lysenko's theory of phasic development of plants, according to which the demands made by a plant on its environment entirely depend upon its preceding evolutionary history, upon the environment in which the plant was formed.
The data we obtained also helped in the elucidation of the mechanism by which the mutual coordination of the functions of the various organs of the plant is attained. Experiments show that in the process of development of the potato plant there is not only a fluctuation of the temperature optima, but also changes in the temperature zones of starch synthesis and decomposition. Fluctuation of the optima towards higher temperatures is, as a rule, accompanied by a contraction of the zone of starch synthesis and a considerable expansion of the zone of starch decomposition. For example, in the beginning of July starch synthesis in the leaves took place already at a temperature of 15°, whereas at the end of August this process was to be observed only beginning with 28°.
The predominance at fairly high temperatures of the starch decomposition processes over the synthetic processes should assist the release by the leaves of a certain part of the assimilates accumulated in their tissues during the daytime. This would well explain the earlier observations of Chesnokov and Bazyrina, who discovered that in the first half of the vegetative period of the potato the curve of translocation of the assimilates from the leaves has a double peak, and that one of the maxima occurs in the earlier half of the day.
Widening of the zone of starch synthesis in the course of the development of the potato was also observed in experiments on tubers. For instance, in the tissues of young tubers selected at the beginning of August, starch synthesis occurred only in the high temperature interval, outside of which the processes of starch decomposition predominated.
Hence, in the early period of development of the tubers, when the functions of growth predominate and the storage of starch is not the main process, starch formation is timed to occur in a very limited temperature interval, outside of which the processes of starch decomposition predominate. The biological significance of this regularity evidently consists in the fact that when soluble and easily mobilized forms of carbohydrate predominate in the tissues, the growth of these tissues proceeds more effectively. At a later period, when the growth of the tubers is less intense, and the storage of starch predominates, we observe a considerable expansion of the temperature zone of starch synthesis.
Further researches made it possible to establish that the optima for the action of the starch-forming enzymes in one and the same organ of the plant does not remain constant either in the vegetative or in the diurnal period.
It follows from the data we have obtained for tubers that the intensity of starch synthesis in their tissues is greater at nighttime, and, further, that this process coincides in time with lower temperatures, especially in the later stages of the potato's development.
A reverse picture was established in experiments on leaves, in which the processes of starch synthesis coincide with the higher temperatures. Further, both the general intensity of the process and its coincidence with the high temperature intervals leave no doubt that starch formation in the leaves, in contradistinction to the tubers, is a daytime process. It follows from this that the photoperiodic reactions of plants must under no circumstances be considered in divorcement from the thermoperiodic reactions, with which they seem to be most closely connected.
Our data likewise show that temperatures which favour the processes of starch synthesis in the tubers simultaneously stimulate the processes of starch decomposition in the leaves.
It is appropriate to point out that the so-clearly manifested coordination between the functions of the overground and underground organs of the plant is due to the regulating action of the enzymes, which are specifically adapted to a definite state of the temperature factor.
Thus the periodicity observed in the biochemical activity of plants is a result of the development of the heredity of the organism under the influence of definite external conditions.
The data I have quoted is only part of the material at our disposal. We do not claim that they give an in any way full explanation of the problem under discussion. They only show that the specific peculiarities of metabolism are as closely connected with the conditions of existence of the organism as are the form and structure of the organism.
Academician T. D. Lysenko is constantly emphasizing that the forms of living bodies were created, and are created, solely by their conditions of life. Hence, changes of vegetable and animal forms can be directed only by skilfully directing the conditions of life of the plants and animals. A splendid example of such direction is the summer planting of potatoes suggested by T. D. Lysenko. Our data, in full accord with T. D. Lysenko's conclusions, also show that in this way the fullest correspondence is obtained, between the temperature to which the potato is adapted at the various stages of its development and the actual temperature of its environment. The decisive factor in the degeneration of the potato observed in the South is the high temperature of the soil at nighttime, which far exceeds the temperature level to which the tuber-formation processes in the potato are adapted. Here we have manifested the dual nature of the potato, as a plant which, it is true, originated in the South, but in areas of high elevation. As we know, one of the distinguishing features of the latter is the wide amplitude between day and night temperatures, which is particularly characteristic of the last third of the vegetative period, when the tuber-formation process in the potato is most intense.
In full accordance with the theory of phasic development of plants, our data show that plant metabolism is adapted, not to a constant but to a successively changing temperature. The regular changes of temperature must embrace not only the entire life cycle of the plant organism, but also the separate diurnal periods.
We therefore see that metabolism in a plant not only reflects its definite biological peculiarities, but at the same time performs the important role of creating a unity between the organism and the conditions of its life, that dialectical unity which, as Academician T. D. Lysenko has rightly observed, lends a body the properties of animation and renders it fundamentally different from inanimate bodies.