DROUGHT TOLERANCE: ROLES OF ORGANIC OSMOLYTES, GROWTH REGULATORS, AND MINERAL NUTRIENTS

Drought, the occurrence of a substantial water deficit in the soil or in the atmosphere, is an alarming constraint to crop productivity and yield stability worldwide. It is the leading environmental stress in world agriculture, causing losses in crop yield probably exceeding losses from all other causes combined. Drought stress adversely affects a variety of vital physiological and biochemical processes in plants, leading to reduced growth and final crop yield. Some plant species have evolved mechanisms to cope with the stress, including drought avoidance, dehydration avoidance, or dehydration tolerance. Such adaptive mechanisms are the results of a multitude of morphoanatomical, physiological, biochemical, and molecular changes. Osmoregulation is the most common physiological adaptation, which takes place by reducing cellular water potential via accumulation of a variety of organic and inorganic solutes in the cell. As a consequence, such plants are capable of taking up water from a low water potential medium to sustain normal or near normal physiological processes necessary for growth and development. However, most economically important crop species lack the capability of coping with this type of drought stress, precluding their cultivation under water-limited conditions. Various strategies have been proposed to facilitate crop production under drought conditions, in particular, development of new crop varieties with enhanced drought tolerance. Genetic improvement of crop plants for drought tolerance is a long-term endeavor, which requires, among other things, the availability of genetic sources of tolerance, knowledge of the physiological mechanisms and genetic controls of tolerance traits at different developmental stages, and employment of suitable germplasm screening and breeding protocols. An alternative and quicker strategy to promote plant drought tolerance is exogenous application of various compounds, including organic solutes (organic osmolytes and plant growth regulators) and mineral nutrients. Recently, this strategy has gained considerable attention because of its efficiency, feasibility, and cost- and labor-effectiveness. In this chapter, we review the roles of organic osmolytes, plant growth regulators, and mineral nutrients in plant response to drought stress and discuss their exogenous application in enhancing plant drought tolerance and alleviating the damaging effects of drought stress.