Pyruvate phosphate dikinase-modulated fructose 2,6-bisphosphate content in sorghum leaves in relation to varying light intensity

Abstract

The light activation of pyruvate phosphate dikinase was examined in sorghum (Sorghum bicolor L., var. CSH-5) leaves in relation to the photosynthetic rates under varying light intensities. Significantly high correlation was found between extractable activities of pyruvate phosphate dikinase and photosynthetic rates under different light intensities suggesting the rate-limitation of photosynthesis by pyruvate dikinase in sorghum leaves. The levels of phosphoenolpyruvate increased in parallel with CO2 uptake at light intensities between 400 and 1600 μmol m-2 s-1. A sharp decrease in the levels of fructose 2,6-bisphosphate was coincident with the lower activities of fructose 6-p-2, kinase with increasing irradiance. A strong negative correlation was observed between the activities of pyruvate, Pi dikinase and fructose-6-phosphate-2, kinase. PEP (2 mM) inhibited the activities of fructose-6-phosphate-2, kinase while it has no effect on fructose 2,6-bisphosphatase. Leaf fructose 2,6-bisphosphate concentration was negatively correlated with net photosynthetic rate and leaf PEP concentration. These results indicated that PEP could be an important regulator of the synthesis of fructose 2,6-bisphosphate in C4 plants.