Photosynthesis, photorespiration and proline accumulation in water-stressed pea leaves

Excised leaves of Pisum sativum L. were subjected to water stress for 24 h in the light and in darkness. Proline accumulation increased both in the light and in darkness as the relative water content decreased, but its increase was higher in the light. (CO2)-C-14 fixation in water-stressed leaves was severely inhibited. The activities of phosphoenolpyruvate carboxylase and glycollate oxidase increased and those of ribulose-1,5-bisphosphate carboxylase and phosphoglycollate phosphatase were not influenced by water stress. Photochemical reactions and electron transport in isolated chloroplasts were not significantly affected by low water potential in the leaves. Exogenously supplied glycollic and glyoxylic acids enhanced proline accumulation only in the light and had no effect in darkness. The current photosynthesis during water stress was probably not a major contributor of carbon to proline synthesis. An alternative possibility is that photorespiration might be involved in this event.