The Role of Endogenous Abscisic Acid in the Response of Plants to Stress

Abstract

When a continuous stream of warm air (38°C) was directed on to the leaves of dwarf bean seedlings they wilted and then gradually regained turgor. This process of adaptation was accompanied by an increasing abscisic acid (ABA) level in the leaves and an increase in leaf resistance (R_L). It is suggested that the leaf-water deficit induced by the warm-air treatment caused the increase in ABA level and that the latter was responsible for stimulating stomatal closure, enabling the plants to regain full turgor. A similar type of adaptation, brought about by an increased level of ABA in the leaves, is believed to occur in tomato, dwarf bean, and wheat plants when they are flooded. Predictably, in rice, a species adapted to a flooded environment, seedlings showed no increase in ABA level as a result of flooding. It is proposed that adaptation may involve the formation of an equilibrium between ABA and its conjugate form (i. e. the glucose ester). The ABA-conjugate was observed to disperse slowly from leaves recovering from a water deficit and therefore it may act as a metabolic ‘back-stop’, enabling the ‘free’ ABA level to remain high for a period even when the leaves have regained turgor. Abscisic acid appears to be responsible for alleviating the effects of water stress in plants, making it possible for plants to pass through periods of stress with little harm.