Drought Resistance

(of plants), the ability of plants to tolerate substantial dehydration of their tissues and organs as well as overheating. Xerophytes are the most drought resistant, and mesophytes growing in arid, sunny places are similar to them in their ability to tolerate dehydration.

Drought resistance is determined mainly by the hereditary properties that developed in the course of evolution, although plants may adapt to drought as they mature. Transpiration, mineral nutrition, photosynthesis, and other physiological processes are important factors in the development of drought resistance. During drought the relative humidity decreases sharply and the temperature rises. By midday the plant becomes overheated, and daytime water deficiency follows, causing the plant to wilt. This is accompanied by complete hydrolysis of proteins, decomposition of the cytoplasm, and impairment of the phosphorylation of sugars and, consequently, of energy metabolism. Respiration that is inadequate from the standpoint of energy somewhat facilitates hydration of the biopolymers in the cell (owing to so-called metabolic water), but in this manner the plant can replenish in one hour only about 15 percent of its water content (if its total content is taken to be 100 percent). Dehydration also causes some disturbances in the colloidal properties of the cytoplasm; its degree of dispersion and its capacity to retain adsorbed compounds are altered. Water deficiency and related metabolic disorders slow down or halt the growth of plants, reduce their productivity (yield), and may cause them to die. High elasticity of the cytoplasm and the capacity to withstand compression of the cells during dehydration are characteristic of drought-resistant plants.

Drought resistance is studied under conditions both of naturally occurring and of artificially induced drought: in field experiments in arid regions and during artificially induced soil drought in a drought installation as well as in plant containers and phytotrons.

Drought resistance usually increases as the plant develops, but it decreases sharply as soon as the generative organs begin to form, a phenomenon discovered by the Russian re-searchers I. P. Pul’man (1898) and P. I. Brounov (1912) and studied in detail by F. D. Skazkin (1961) and his co-workers. They regard the phenomenon as a manifestation of the biogenetic law in plants: the ancestors of flowering plants had emerged from water, and at a critical time (from the appearance of the maternal cells of the pollen until the completion of fertilization) they cannot tolerate a deficiency of water. The drought resistance of plants may be increased if they are hardened before transplanting. The stability of the respira-tory enzymes and protein synthesis increases as the plant adapts to drought. Protein synthesis declines sharply both during overheating and during dehydration because adenosine triphosphatase is activated. This enzyme ruptures the strands of messenger ribonucleic acid (RNA) on which the poly somes that synthesize protein are found. The polysomes are then broken down into ribosomes and subunits. In hardened plants there is more RNA, the adenosine triphosphatase is less active, and the polysomes begin to break down later. In these plants the generative organs are much more resistant to drought, metabolism is more active, and the cytoplasm colloids are more viscous and elastic. All this increases resistance to drought.

Hardening prior to transplanting is a practical method for increasing the drought resistance of plants with fine seeds. In the case of other plants it can be effective in seed growing and plant breeding. Drought resistance can also be increased by the efficient use of fertilizers, especially those enriched with trace elements (or by treating seeds with them). In addition to breeding plants for high drought resistance, such farming practices as the selection of drought-resistant crops, mulching, snow retention, fertilization, and correct crop rotations are of great practical value.