Water Regime of Plants

(exchange of water), the in-take and release of water by plants that is necessary for their life functions (metabolism, growth, development, and reproduction). It covers three continuous and intimately interrelated processes: the intake of water from the soil into the roots; the raising of the water through the roots and stems to the leaves and to the growing embryonic tissues and growth points located on the stems; and the evaporation of excess water from the leaves to the surrounding atmosphere.

The average amount of water that passes through a plant is extremely large. During the growing season in a moderately moist climate, one corn or sunflower plant loses approximately 100 liters of water, and 1 hectare of wheat gives off 2,000-3,000 cu m of water by evaporation in one summer. On the average a plant expends approximately 250-300 kg of water (approximately 500-600 kg in a dry climate) in producing 1 kg of harvested dry weight.

The water taken up by the plant from the soil is absorbed only by the young root ends—that is, the root fibrils and hairs—rather than by the whole surface area of the roots. The cells in the absorptive region of the root have a special polarity in relation to water. Their outside sucks in the water and their inside forces it into the capillaries of the root. In this manner, root pressure is created in the plant, pushing the water upward through the root and stem with a force of 2-3 and more atmospheres. The root of the plant sucks up water from the soil with approximately equal force and overcomes the resistance of soil particles, which retain water on their surfaces by the forces of adsorption and swelling of soil colloid. As the thickness of the layer of water enveloping the particles of soil decreases, the forces of adsorption that retain the water rapidly increase and become equal to, and then even larger than, the suction force of the root cells, so that the roots of plants cannot extract all of the water contained in the soil, and a certain amount of water always remains in the soil that is inaccessible to the plant. In such a case, further loss of water by the plant cannot be compensated for by taking up water from the soil; the plant’s water content drops and the plant withers.

The leaves of plants have many physiological characteristics that allow them to regulate the release of water to a considerable degree. The evaporation of water from the surface of the plant has come to be called transpiration. By lowering the water content in the cells of the leaf pulp and creating a water deficit, transpiration produces a considerable suction force, which ensures a flow of water from the vessels of the leaf veins to the cells. This causes the upward movement of water through the plant, which frequently takes place significantly faster than the pumping of water by the root fibrils. By virtue of the cohesion characteristic of water molecules, the water passing from the vessels to the living cells of the leaf pulp draws after it the whole column of water, which fills up the conduit system and reaches to the roots. As a result, water tension that promotes the intake of water from the soil through the roots is created in the vessels of the entire plant.

To ensure hardy crops and high yields, measures toward storing up reserves of moisture in the ground and decreasing its loss are extremely important (for example, snow retention, tilling of autumn plowland, early harrowing in the spring to retain moisture, and planting of protective forest strips). In arid regions, artificial irrigation of the land is used.

However, an excess of water can be harmful to plants, since when the soil is flooded there is no longer any room in the soil’s capillaries for the air necessary for root respiration and normal life functions. Furthermore, flooded soil promotes anaerobic bacterial processes, which bring about the accumulation of substances that poison the roots. Excess moisture can be eliminated by draining the soil. The optimal method is to moisten the soil so that it contains enough water and air that is accessible to the plants.

Different plants require different amounts of moisture from the soil. For example, xerophytes are adapted to living in an arid climate (in steppes, deserts, and semideserts) and in more humid climates under conditions of poor water supply. Hydrophytes and hygrophytes, on the other hand, grow in reservoirs and swamps. The intermediate position between these two extreme groups of plants is occupied by the mesophytes, which is the most numerous plant group and to which the majority of cultivated plants belong.