In recent years, the definition of the term stress (meaning "strain" in English) has been extended, so that it is now used in reference to higher and lower plants and even microorganisms, as opposed to the original usage implying humans and animals [1][2][3]. The Canadian physiologist Hans Selye, who coined the term in the 1930s [4], believed that the ability of an organism to adaptation is related to concentration and exertion, in the sense of strenuous efforts undertaken to mobilize this organism's defense systems. Similarity of stress reactions in bacteria (the most ancient inhabitants of Earth) [3] to those occurring in contemporary representatives of eukaryotes [1, 2, 4] is currently a welldocumented fact.The most frequent stress effect experienced by plants and their photosynthetic apparatus is water deficiency. The greenhouse effect (which becomes more and more pronounced), increased evaporation and reduction of water basins, elevation of mean annual temperature, and augmented contamination of the atmosphere by anthropogenic microadulterants (first of all, methane and oxides of carbon and nitrogen) [5, 6] may disrupt gas exchange, dark and light reactions of photosynthesis [7][8][9], and the unified hydrodynamic system of water supply, transport, and transpiration, which ensures normal water homeostasis in plants [10].Photosynthesis is particularly sensitive to effects of water deficiency and increased temperature. Even a small decrease in the water potential of a plant causes its stomata to close, which entails ëé 2 deficiency in photosynthesizing tissues and decreases the intensity of photosynthetic assimilation of CO 2 (which is the integral parameter of the process). As the degree of hydration of the tissues decreases, the photosynthetic apparatus of plants undergoes functional changes, which further develop into disruption of its structure [7-9, 11].Studies of the resistance of agricultural plants to water deficiency are of great importance for the understanding of the mechanism of disrupting effects of stress. Such studies are also necessary for developing scientifically well-grounded methods for restoring the operation of the photosynthetic apparatus and increasing its stability (which in turn increase the harvest. However, alterations of the photosynthetic apparatus (and structural and functional characteristics of chloroplasts in particular), taking place under the conditions of water deficiency, are not as well studied as those occurring under the effect of other stress types (e.g., decreased or increased temperatures, deficiency or excess of nitrogen or other nutritive minerals, and amount and quality of light).Plant resistance to water deficiency is related to metabolic changes and functional and structural rearrangements of photosynthesizing tissues. These changes are coordinated by regulatory systems, of which the most important for processes of plant adaptation are those involving cytokinins and other phytohormones. Cytokinins, which regulate multiple functions in plant cells, were shown to ...
