Photosynthesis at low water potentials

Abstract Low leaf water potentials result in large reductions in photosynthesis. In higher plants, the reductions are caused both by decreases in the photosynthetic activity of a unit of leaf and in the production of new leaf surface. Photosynthetic activity declines because of decreased stomatal opening and the inhibition of chloroplast activity, either of which may control photosynthesis depending on which is more limiting at low leaf water potentials. The production of new leaf area is highly sensitive to water deficits and is usually reduced before photosynthetic activity decreases. This may be attributed to the high responsiveness of leaf enlargement to turgor, which expands the cells. When low leaf water potentials are prolonged, leaf senescence often occurs and the quantity of existing leaf area may decline. There is evidence that translocation is less sensitive than photosynthesis to low leaf water potentials. Consequently, grain yield, which depends on both photosynthesis and translocation, is more likely to be limited by photosynthesis than translocation. Since substantial translocation to the grain may occur from parts of the plant other than the leaves during desiccation, the total photosynthate accumulated during the growing season is more important than that produced during the grain-filling period alone when plants have had low water potentials.