Plant Physiology

a biological science concerned with the general patterns governing the life processes of plants. Plant physiology studies the ways in which plants absorb minerals and water, grow and develop, and flower and bear fruit. It also deals with mineral nutrition and photosynthesis, respiration, and biosynthesis and the accumulation of substances which together enable plants to grow and reproduce themselves. By revealing the dependence of the life processes on environmental conditions, plant physiology serves as the theoretical basis for increasing the total productivity of plants, improving their nutritional value, and raising the quality of their tissues and organs for use in industry. Research in plant physiology provides a scientific basis for the rational planting of crops in regard to soil and climatic conditions.

The range of problems investigated by plant physiologists is largely determined by the specific characteristics of their object of study—green plants. Green plants are distinguished from all other forms of living matter by their ability to carry out photosynthesis, that is, derive energy from sunlight and convert it into the chemical (free) energy of organic compounds. Photosynthesis enables green plants to obtain nutrition from inorganic compounds lacking in significant supplies of readily mobilizable free energy. In the course of photosynthesis, plants energize the mineral compounds they absorb and convert; thus they synthesize high-energy organic matter and provide a source of food and energy for all other forms of life on earth. This is the fundamental trait distinguishing green plants from animals and other organisms lacking chlorophyll, such as fungi and bacteria, which require ready organic compounds to survive.

The specific properties of plants are closely related to their general anatomical and morphological structure. Unlike animals, with their characteristic compact structure, most plants have a large surface, owing to the branching of their aboveground and underground organs. Their structure enables them to obtain nutrition from fairly large volumes of soil and air. Moreover, plants continue to grow almost throughout their lives because, along with old tissues, they have young ones (meristems) that retain the capacity to form new cells. Another specific characteristic of green plants is the variability of certain internal factors, for example, tissue temperature and oxygen and carbon dioxide concentrations. Thus, plants and animals adapt to environmental change in fundamentally different ways.

Historical sketch. Plant physiology initially evolved as a branch of botany concerned chiefly with the mineral nutrition of plants. The first experiments dealing with the formation of plant tissues were conducted by the Dutch naturalist Jan van Helmont in 1629. After growing a willow branch in a pot filled with a weighed quantity of soil for five years, he found that the weight of the branch increased by 30 times, while that of the soil scarcely changed at all. Van Helmont concluded that water, rather than soil, was the main source of plant nutrition. Although the conclusion was false, the experiment was highly significant because it was the first to use a quantitative method (weighing). The sexual differentiation of plants was discovered in the late 17th century. In 1727 the Englishman S. Hales observed that solid matter and water pass through tissues.

The English chemist Joseph Priestley in 1771 discovered that green plants alter the composition of air in the course of their life processes and restore its capacity to permit combustion and the respiration of animals. This was the most important factor in the subsequent development of plant physiology and of the natural sciences in general. The phenomenon was later called photosynthesis. The first to suggest the possibility of the aerial nutrition of plants was M. V. Lomonosov (1753), who, observing that healthy trees growing on nutrient-poor sand cannot obtain the nutrients they require through their roots, concluded that plants obtain nutrients through their leaves, from the air.

Important studies on photosynthesis were conducted by the Dutch naturalist J. Ingenhausz (1779) and especially by the Swiss scientists J. Sénebier and N. T. de Saussure (late 18th and early 19th centuries), the German scientist J. R. Mayer, and the French agricultural chemist J.-B. Boussingault (1868). These scientists helped elucidate certain aspects of photosynthesis, in particular the absorption of carbon dioxide and water and the simultaneous release of oxygen, which must occur in the presence of light. The work of the French scientist A. L. Lavoisier on the chemistry of combustion and oxidation (1774–84) led to considerable advances in plant physiology. Growth movements (tropisms) were noted in plants in the early 19th century and were later described in detail by C. Darwin.

Research in the soil nutrition of plants progressed rapidly. The German scientist A. Thaer formulated the humus theory (1810–19), which ascribed a decisive role in plant nutrition to organic matter. In the 1840’s the humus theory was superseded by the mineral theory of the German chemist J. Liebig, who emphasized the role of soil minerals in the root feeding of plants. With his mineral theory, Liebig made a significant contribution to physiological research and helped introduce inorganic fertilizers into agriculture. Boussingault used Liebig’s pot method to study the entry of nitrogen and other mineral elements into plants. Boussingault and the German scientist H. Hellriegel discovered that legumes are nitrogen fixers, and the Russian botanist M. S. Voronin in 1866 demonstrated that the root nodules of these plants are bacterial in nature.

Other scientists who made notable accomplishments in plant physiology in the 19th century included the Germans J. von Sachs and W. Pfeffer, the Austrians J. Wiesner and H. Molisch, and the Czechs B. Nĕmec and J. Stoklasa.

In the second half of the 19th century, K. A. Timiriazev conducted important studies on the role of chlorophyll in photosynthesis. After demonstrating that the law of conservation of energy could be applied to plants, Timiriazev substantiated and elaborated the concept that green plants play a role of cosmic significance by performing the unique function of photosynthesis, thus serving as a link between life on earth and solar energy.

Other major contributions to plant physiology, in particular to the study of photosynthesis, were made by the Soviet botanists A. A. Rikhter, who discovered the phenomenon of adaptive changes in the qualitative composition of photosynthesis pigments; E. F. Votchal, who made a thorough study of the relationship between photosynthesis and water metabolism; and F. N. Krasheninnikov, who, using colorimetric methods, was the first to demonstrate that chemical compounds other than carbohydrates are formed during photosynthesis.

Votchal was one of the founders of the Ukrainian school of plant physiologists. Other prominent figures in the school included V. R. Zalenskii, who discovered the role of suction in regulating the water balance in plants; V. V. Kolkunov, who established that the anatomical structure of the beet root is related to its sugar content; and V. N. Liubimenko, who proved that the chlorophyll in chloroplasts is not free, but is bound to proteins.

In the second half of the 19th century and in the early 20th century, major discoveries were made in plant metabolism and energy. Plant physiology and biochemistry also became very closely related. The term “metabolism” was first applied to plants by the Russian botanist A. S. Famintsyn (1883). Intensive research was undertaken in the late 19th century on the mechanisms of respiration—the biological oxidation of organic matter without external energy sources.

In 1896–97, the Russian biochemist A. N. Bakh formulated the peroxide theory of biological oxidation, which is now the basis of the modern theory of radicals. The peroxide theory provided the impetus for a thorough study of the chemism and enzymology of respiration.

V. I. Palladin (1912) substantiated the concepts that held biological oxidation, based on dehydrogenation, to be a principal stage of respiration. These concepts were subsequently elaborated by the German scientist H. Wieland. S. P. Kostychev made an important contribution to the study of respiration and other processes. The German biochemist O. Warburg discovered the role of iron as a structural element of the enzymes associated with biological oxidation. Soon afterward the British scientist D. Keilin discovered cytochromes, a very important group of compounds involved in electron transport both in photosynthesis and in respiration. The Soviet physiologist V. O. Tauson was the first to investigate the parameters of energy in respiration.

Science is indebted to the Soviet agricultural chemist D. N. Prianishnikov for his thorough study of the metabolism of nitrogenous substances in plants, which led to a radical change in the use of nitrogenous fertilizers. The most important studies of Prianishnikov and his school dealt with the phosphorus and potassium nutrition of plants, the liming of soils, and various other aspects of the physiology of mineral nutrition.

Prianishnikov’s students also did significant work. G. G. Petrov made a detailed study of nitrogen metabolism in relation to light. I. S. Shulov devised several variations of the pot method, including the methods of flowing solutions and sterile cultures, which he employed to demonstrate the ability of plant roots to assimilate organic compounds, including some protein compounds. F. V. Chirikov investigated the physiological characteristics of crops in regard to their varying capacity to absorb poorly soluble forms of soil phosphates. N. A. Maksimov did basic research in water metabolism and drought resistance. As a result of the work done on the physiology of microorganisms, notably S. N. Vinogradskii’s discovery of chemosynthesis (1887), it has become possible to discern patterns in the cycle of individual elements in nature, to determine the role played in this process by plants, and to establish the symbiotic relations between plants and soil-dwelling microflora.

Current status and achievements. One of the most important advances in plant physiology was the elucidation of the subtle processes that regulate energy metabolism in green plants. Photosynthesis and respiration were found to be two related aspects of the same function—the metabolism of nutrients and energy. It was learned that the biochemical processes involved in respiration are the source of the intermediate substances used by cells to synthesize the main structural and physiologically active constituents of protoplasm. Under certain conditions, respiration fulfills the functions of photosynthesis. When photosynthesis does not take place, plants can assimilate nutrients only as a result of the oxidation-reduction changes that occur during respiration.

Progress has been made in the study of physicochemical and biochemical processes involved in the absorption of light energy, the transformation of light energy into chemical energy, and the storage of chemical energy in the form of high-energy compounds that function as biological fuel. A number of Soviet and foreign scientists have conducted important research in this area, for example, the Germans O. Warburg and H. Wieland, the British biologist D. Keilin, the Swede A. H. Theorell, the British biochemist H. A. Krebs, the Americans A. Szent-Györgyi, and M. Gibbs, and the Soviet scientists la. O. Parnas and D. M. Mikhlin.

Of fundamental importance are the advances made in the study of the structure, physicochemical properties, and biosynthesis of photosynthetic pigments, as well as the study of their metabolism and the mechanisms underlying the functions they perform. These advances include the discovery of cyclical, noncyclical, and pseudocyclical photophosphorylization by the American scientist D. I. Arnon and others. The mechanisms of the primary stages in the absorption of light quanta were clarified by the Soviet scientist A. N. Terenin, the American B. Chance, and the Dutch biochemist L. N. M. Duysens. Modes of chlorophyll biosynthesis were researched by the Soviet scientist T. N. Godnev and the American E. Rabinowitch. Dark reactions and other biochemical aspects of photosynthesis were the subject of research by the American M. Calvin, the Australians M. D. Hatch and S. R. Slack, and the Soviet Iu. S. Karpilov.

The work of these scientists has considerable theoretical significance because it confirms the principle of alternation and reciprocity, the organizational basis of all physiological functions and regulatory systems in plants. The relations between cyclical and noncyclical or pseudocyclical photophosphorylation in ontogeny depend on external conditions, such as light. One- and two-quantum mechanisms of photosynthesis have been found to exist, and the existence of a three-quantum mechanism is also likely. Along with anaerobic oxidative energy metabolism (glycolysis), which is the oldest from the evolutionary point of view, other means of aerobic oxidation also exist, notably the Krebs cycle, the glyoxylate cycle, and the pentose phosphate pathway. The relation between glycosis and aerobic oxidation is inconstant and depends on the given plant species and the conditions under which it develops, for example, partial pressure of O₂ in the atmosphere, temperature, and light.

An important event in modern plant physiology was the discovery of photorespiration, a specialized function of energy metabolism in green plants consisting in the light-induced absorption of oxygen by a green cell, accompanied by the release of CO₂. The efficient use of light, the net productivity of photosynthesis, and the total productivity of a plant are apparently related largely to photorespiration.

The study of the development of an individual plant (its ontogeny) and of the factors that regulate it show that the plant is greatly influenced not only by environmental conditions but also by hormones present in its tissues, mainly auxins, gibberellins, and cytokinins. The discovery of these substances stimulated a new approach to the study of plant growth and an emphasis on the reproductive rather than the vegetative functions of plant organisms. Root systems, the tissues of which synthesize gibberellins and cytokinins, have been found to play a major role in regulating the general course of plant development. In addition to stimulants, plants contain compounds that inhibit growth and development, for example, abscisic acid, which regulates the germination of seeds and the dormancy of wintering buds.

It has also been found that several physiological processes are regulated by phytochrome, for example, the germination of seeds, the elongation and strengthening of the hypocotyl, the formation of leaf rudiments, and the differentiation of primary leaves, elements of the xylem, and stomata. Phytochrome was found to induce the biosynthesis of the enzymes involved in the formation of chlorophyll, chloroplasts, and all bodies involved in photosynthesis. Also of significance was the discovery of several members of the phytochrome group that apparently regulate phototropism, photoperiodism, and certain other functions. This research has revealed important aspects of the regulatory functions of light in plant life.

Highly significant information on root feeding has also been revealed. The study of the absorption of minerals by roots and of the transformation of these minerals indicates that root systems can synthesize such important compounds as amino and nucleic acids, vitamins, and auxins. Roots can also synthesize chlorophyll independently, without relying on the chlorophyll synthesis processes in leaves.

The mechanisms regulating the absorption capacity of root systems and the relations between mineral nutrition and water exchange have also been made clear. Valuable findings have been made concerning the role of individual minerals in plant metabolism, particularly that of several trace elements involved in the construction of many enzyme systems. Productive research has been conducted on cell physiology and the functions of protoplasmic organelles, as well as on cell membranes and their structure and function in the absorption, transport, and release of ions. Studies of great practical importance have revealed physiological aspects of the resistance of plants to a variety of harmful abiotic factors, including extremely high and low temperatures, drought, excess moisture, and salinization, and to such biotic factors as immunity to disease and harmful insects. Findings are used in selective breeding and in the development of methods to increase plant resistance and hardiness.

In addition to advances in general plant physiology, increasing attention is being paid to the physiology of individual crop species and varieties. Such specialized research is important because crop yields and the ability of cultivated plants to make efficient use of nutrients, moisture, and light depend on the interrelations of all plant functions in the different stages and conditions of development. Thus, specialized studies in plant physiology have practical as well as theoretical value.

Methods and tasks. Plant physiology was originally the science of the soil nutrition of plants. After the discovery of photosynthesis and the laws of conservation of matter and energy, however, specialists became increasingly concerned with air and light as the principal material and energy sources of plant life. Until the early 20th century, physiological processes were investigated chiefly by means of analytical, quantitative methods. For example, the quantity of absorbed CO₂ and released O₂ served as criteria in the study of photosynthesis. In research on respiration, attention was focused on determining the quantity of O₂ absorbed and CO₂ released. Studies on root nutrition entailed determining the quantity of minerals absorbed and the effect that the concentration of inorganic and organic compounds in the soil had on these processes.

During the course of several decades, however, research on soil nutrition was conducted without reference to aerial nutrition, and nutrient metabolism was dissociated from energy metabolism. Experimental studies in plant morphology were conducted in a similar manner, without reference to nutrient and energy metabolism.

K. A. Timiriazev, basing his approach on Darwin’s theory of evolution, constantly emphasized that all processes in the living plant are interdependent and must be studied comprehensively. Because plants lack constant internal conditions, their ability to develop in an ever-changing environment is due to regular and strictly controlled metabolic changes, which occur in response to changes in the external environment. Since environmental conditions change in the course of evolution, the properties of a species can be revealed only by studying them from a historical point of view. Only by integrating the experimental and historical methods is it possible to ensure the successful development of plant physiology and of biology in general. These ideas were first formulated by Timiriazev in his book The Historical Method in Biology (1922).

In the first half of the 20th century, plant physiology became more closely linked with biochemistry and biophysics and made more extensive use of such physicochemical methods as spectral analysis and mass spectrometry, ultraviolet electron microscopy, differential centrifugation, chromatography, and isotope indication. These methods enabled scientists to conduct research at the cellular, subcellular, and molecular levels and obtain fundamentally new data on the mechanisms regulating the entire complex of life processes and the way in which they function as integral systems. Progress was facilitated in the mid-20th century by the invention of controlled-environment facilities.

In studying complex biological phenomena, plant physiologists today make extensive use of models of simpler elements. Such models help lead to the discovery of new patterns in the absorption and assimilation of inorganic matter and water; in the absorption, transformation, and storage of solar energy; and of the subsequent use of energy in biosynthesis, growth, development, and movement. Starting at the molecular and subcellular levels, plant physiologists study the cell, the organs, the organism, and ultimately various communities—the phytocoenosis (plant community), biocoenosis (biotic community), and biogeocenosis (ecosystem).

Using modern methods and taking into account findings made in other sciences, plant physiology in the broad sense deals with two main topics. First, it studies the plant organism as a system of interacting morphologically and physiologically active protoplasmic elements. Second, it deals with the interaction of the plant organism with biological and physicochemical environmental conditions, including the range of the plant’s varying functions, the plant’s capacity to maintain its characteristic metabolism, and the systems that determine the ways in which the plant reacts to external factors.

Modern findings in plant physiology have implications for such important practical agricultural problems as acclimatization, the introduction of new species, selective breeding, hybridization, the development of heteroses, the regionalization of varieties, arrangement of crops, agricultural technology, fertilization, and artificial irrigation.

Scientific organizations and periodicals. In the USSR, research in plant physiology is carried out in the Institute of Plant Physiology of the Academy of Sciences of the USSR, the Institute of Botany of the Academy of Sciences of the USSR, the Siberian Institute of Plant Physiology and Biochemistry of the Siberian Division of the Academy of Sciences of the USSR, the Institute of Plant Physiology of the Academy of Sciences of the Ukrainian SSR, and other institutes of the academies of sciences of the USSR and the Union republics.

Research is also conducted at the Institute of Horticulture of the V. I. Lenin All-Union Academy of Agricultural Sciences and at subdepartments of universities and agricultural institutes.

Foreign research centers include the Institute of Plant Physiology in Gatersleben (German Democratic Republic), the Metodii Popov Institute of Biology (Bulgaria), the Institute of Experimental Botany of the Czechoslovak Academy of Sciences in Prague, the botany department at Durham University (Great Britain), the Science Center in Gif-sur-Yvette (France), and the department of plant physiology and anatomy at the University of California (USA).

The principal periodicals published in the USSR are Fiziologiia rastenii (since 1954; published in English translation in the USA as Soviet Plant Physiology) and Fiziologiia i biokhimiia kul’turnykh rastenii (Physiology and Biochemistry of Cultivated Plants; Kiev, since 1969). Articles on plant physiology are also published in Doklady AN SSSR (Reports of the Academy of Sciences of the USSR, since 1922), Uspekhi sovremennoi biologii (Advances in Modern Biology, since 1932), Biokhimiia (since 1936), Biofizika (since 1956), Agrokhimiia (since 1964), Sel’skokhoziaistvannaia biologiia (Agricultural Biology, since 1966), and Vestnik sel’-skokhoziaistvennoi nauki (Review of Agricultural Science, since 1956). Data on general plant physiology and specialized branches are published in Fiziologiia sel’skokhoziaistvennykh rastenii (Physiology of Crops, vols. 1–12; 1967–71).

The principal foreign periodicals include Photochemistry and Photobiology (Oxford-New York-Braunschweig, since 1962), Photosynthetica (Prague, since 1967), Physiologia plantarum (Copenhagen, since 1948), Physiological Plant Pathology (London-New York, since 1971), Physiologie Végetale (Paris, since 1963), Plant and Cell Physiology (Kyoto, since 1950), Plant Physiology (since 1926), Plant Science Letters (Amsterdam, since 1972), and Planta (Berlin, since 1925).

Some botanical journals that have shifted entirely to subjects relating to plant physiology changed their names. For example, Zeitschrift für Botanik has been called Zeitschrift für Pflanzenphysiologie since 1965, and Flora oder allgemeine botanische Zeitung (part A) has been called Biochemie und Physiologie der Pflanzen since 1970. Survey articles on specialized topics in plant physiology are included in Annual Review of Plant Physiology and Fortschritte der Botanik (Berlin, since 1931).