The relationship between phosphate status and photosynthesis in leaves : Effects on intracellular orthophosphate distribution, photosynthesis and assimilate partitioning.

Photosynthesis, assimilate partitioning and intracellular distribution of orthophosphate (Pi) in barly (Hordeum vulgare L.) leaves were measured in plants grown with either 25, 1 or 0 mmol· 1(-1) nutrient phosphate supply. Phosphate deficiency resulted in a significant decrease in the leaf Pi, diminished rates of photosynthesis and a decrease in the sucrose/starch ratio in the leaves. Changes in the cytoplasmic Pi content were relatively small in comparison with the large variations in vacuolar Pi. The cytoplasmic Pi concentration was slightly higher in the leaves of plants grown at 25 mmol·l(-1) Pi than in those grown at 1 mmol·l(-1) Pi and was decreased in the phosphate-deficient plants in which photosynthesis was inhibited. With barley plants grown in phosphate-deficient media, very little, if any, Pi was present in the vacuole. All of the cellular Pi was in the cytoplasm. Barley, spinach (Spinacia oleracea L.) and soya (Glycine max L.) plants were grown to a comparative stage of phosphate deficiency as measured by leaf Pi levels. These species showed a uniform response to phosphate deficiency by increasing starch synthesis relative to sucrose but the accompanying limitation on photosynthetic capacity varied considerably among the species. Interspecific differences in assimilate partitioning between starch and sucrose were maintained over a wide range of Pi supply.