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Water stress is one of the most important environmental factors that regulate a plant’s growth and development. In agronomic practice the effects of water stress are translated into low yield and/or reduced quality. Abscisic acid (ABA) sprays (1 mM) were applied to wheat plants at different phenological stages and the effects on several physiological variables and on yield were evaluated under field conditions at different water regimes. Studies were conducted in the field across three consecutive winter-spring seasons. ABA treatments were applied at the beginning of shoot enlargement and repeated at anthesis. Exogenous ABA increased shoot dry weight and maintained a high concentration of photosynthetic pigments for a longer period of time during grain growth and maturation. Although ABA applications increased stomatal closure immediately after its application, the longer-term effect was to allow for a greater ostiolar opening of the stomatal pore which resulted in increased conductance of gases and water vapor. ABA also improved the transport of photoassimilates from the leaves and stem to the developing grains, that is, it effectively increased the sink strength of the grains. This correlated with a yield increase without significantly changing the protein quality in the grains. Thus, elevated ABA levels from exogenous application or genetic selection could help improve agricultural production of grains in arid areas where irrigation is not possible.