Abscisic Acid may Mediate the Rapid Thermal Acclimatization of Photosynthesis in Wheat

Exposing cold-grown (6°C) plants of a winter cultivar (Avalon) and a spring cultivar (Highbury) of wheat to a single warm night (18 h and 25°C) significantly increased the photosynthetic rate and stomatal conductance of fully-expanded leaves without affecting the calculated, substomatal CO2 pressure. Prior to the warm-night treatment net photosynthesis and stomatal conductance in Avalon were very small at high leaf temperatures during measurement (25–30°C). Such high temperature inhibitions were not observed in Highbury. Exposure to the warm-night treatment alleviated these inhibitions in Avalon. Consequently, the temperature response of photosynthesis was similar in the leaves of the treated cultivars and showed a relatively broad optimum. The application of exogenous abscisic acid in the transpiration stream to the leaves of treated plants caused a reduction of similar proportions in the steady-state rate of photosynthesis and in stomatal conductance and essentially reversed the effects of the warm-night treatment in a manner which depended on the cultivar. The endogenous abscisic acid content of leaves declined by 50% during the warm-night treatment. In conjunction, these data suggest that changes in the content of abscisic acid in the leaf may regulate the observed temperature acclimatization of apparent photosynthesis in the fully-expanded leaves of cold-grown wheat.