Photosynthesis and the Accumulation of Proline in Response to Water Deficit

Abstract

Light increases proline accumulation in water-stressed excised leaves of barley, and the response is linked to photosynthesis. The manner in which current photosynthesis contributes to proline accumulation and the role of soluble tissue carbohydrates has been examined. Increasing the CO₂ content of the air surrounding stressed tissue had no effect on proline accumulation and reducing it to zero reduced proline accumulation only in leaves previously kept in darkness. The direct contribution of assimilated carbon to proline synthesis, assessed with labelled CO₂, was small (< 10% of accumulated proline). The potential energy supply from photosynthesis during water stress was more than adequate for proline synthesis, but the potential energy supply from carbohydrate oxidation for segments stressed in darkness became limiting in leaves incubated in darkness for 48 h before stress. Energy provision from current photosynthesis may thus contribute to light stimulation of proline accumulation. Illumination modified the soluble carbohydrate content of the leaf segments and these variations were related to the rates of proline synthesis and oxidation. However, these effects were over-ridden by stress, and the mechanism of the response to light is not fully explained.