Sucrose metabolism and cell elongation in developing sunflower hypocotyls

Abstract

The relationships between cell elongation and changes in specific activities of enzymes of sucrose metabolism were investigated in the growing region of hypocotyls of sunflower seedlings (Helianthus annuus L.) that were grown either in darkness or irradiated with continuous white light (WL). After transfer of dark-grown seedlings into WL an inhibition of cell elongation was observed. In etiolated stems, changes in enzymes of sucrose breakdown (acid invertase, sucrose synthase) were closely correlated with the rate of cell elongation. Irradiation with WL induced a large drop in acid invertase and a significant decrease in sucrose synthase. The changes in concentration of sucrose were inversely correlated with the activities of the sucrose breakdown enzymes. A short-term experiment revealed that the effect of WL on growth was more rapid than the inhibitory effect on invertase activity. In dark-grown stems the activities of enzymes of sucrose biosynthesis (sucrose-phosphate synthase, ribulose 1,5 bisphosphate carboxylase/oxygenase) were very low. After WL irradiation significant enhancements were measured. However, activities of enzymes of sucrose breakdown were still much larger than those of sucrose biosynthesis, indicating that the green (de-etiolated) stem remains a sink for sucrose, We suggest that the relative maintenance of cell osmotic pressure and turgor during rapid cell elongation in darkness is due to enhanced hydrolysis of imported sucrose, which is cleaved by two enzymes (invertase, sucrose synthase). This process is regulated by light and hence is under environmental control