Modulation of sucrose content by fructose 2,6-bisphosphate during photosynthesis in rice leaves growing at different light intensities

Abstract

The relationship between the rate of CO₂ fixation and sucrose concentration in the leaves of rice (Oryza sativa L.) grown at different light intensities was investigated. Maximum sucrose content coincided with maximum rates of CO₂ fixation, achieved at a photon flux density of 1600 μ mol m^-2 s^-1. The levels of sucrose and fructose 2,6-bisphosphate were also compared in the leaves under different light intensities. Fructose 2,6-Msphosphate accumulated during growth at low light. The activity of fructose-6-phosphate 2-kinase was high in the leaves grown at low light while that of fructose-2,6-bisphosphatase was low. The activities of phosphoglucose isomerase and phospho-glucomutase were slightly increased by growth at low light The activities of UDP glucose pyrophosphorylase were adversely affected in vitro with increased concentrations of fructose 2,6-bisphosphate while those of sucrose phosphate synthase were moderately affected. Phosphoglucose isomerase and phosphoglucomutase were activated by fructose 2,6-bisphosphate (8-0 mmol m^-3) by 12-15%. The results suggested that low light intensities during growth result in an accumulation of fructose 2,6-bisphosphate which modulates the key enzymes of sucrose biosynthesis thus regulating carbon flow under conditions of limited photosynthesis.