SUCROSE METABOLISM AND CELL ELONGATION IN DEVELOPING SUNFLOWER HYPOCOTYLS

The relationships between cell elongation and changes in specific acti vities of enzymes of sucrose metabolism were investigated in the growi ng 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 inhibit ion of cell elongation was observed. In etiolated stems, changes in en zymes of sucrose breakdown (acid invertase, sucrose synthase) were clo sely correlated with the rate of cell elongation. Irradiation with WL induced a large drop in acid invertase and a significant decrease in s ucrose synthase. The changes in concentration of sucrose were inversel y correlated with the activities of the sucrose breakdown enzymes, A s hort-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-phos phate 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 l arger than those of sucrose biosynthesis, indicating that the green (d e-etiolated) stem remains a sink for sucrose. We suggest that the rela tive maintenance of cell osmotic pressure and turgor during rapid cell elongation in darkness is due to enhanced hydrolysis of imported sucr ose, which is cleaved by two enzymes (invertase, sucrose synthase). Th is process is regulated by light and hence is under environmental cont rol.