Plant Water Deficits and Physiological Processes

The plant component of the soil-plant-atmosphere system is an organized body or structure permeated by water. When we speak of a plant as an organization we refer to evidence of the integrated unity that is apparent throughout its entire phenologic cycle ( 186) . In the view of Needham (1 12) the plant (as a micro-universe) can be con­ sidered as having a number of levels of organization and complexity from the atomic to the whole organism. Laws applying to one level may not apply to other levels. Each level requires its own concepts. Any study of more than one level at a time must involve an attempt to determine the intrinsic rela­ tionships between one level of complexity and the others, and, finally, the organizations perceived at each level of complexity must be integrated. These same considerations should govern our concepts of the relation between the plant and the soil and atmospheric environmental factors affect­ ing its life, with water recognized as a vital factor that is both intrinsic and extrinsic. Water is essential to the life and growth of plants, being a major constituent of living green tissues and often comprising as much as 90 to 95 per cent of the fresh weight. Water provides plants with their mechanical strength through the mechanism of cell turgor, since most vegetative tissues are composed of cells with non-lignified walls. Such plastic bodies become mechanically stable under the influence of the hydrostatic pressure developed by water inside the cell. Water is a raw material in metabolism and in the synthesis reactions that occur in living plants, even though the amounts involved here are rather small compared to the total amount of water that passes through the plant. Large amounts of water move into the roots, through the stem, into the leaves, and out into the atmosphere in the form of vapor, in what is known as the transpiration stream. Growth itself is an expression of cell enlargement brought about by the action of water. Many of the metabolic reactions of plants are conditioned by the degree of hydra­ tion of certain of the reactants or enzymes (23, 185). To understand this complex system we must approach it with the realiza­ tion that it is highly dynamic and composed of many interlocking and inter­ dependent phenomena. One of the most difficult problems encountered in understanding the behavior of water in the soil-plant system arises because observed reactions involving water have to be integrated both over time and through volume (141) .