Sucrose metabolism during apical hook opening in sunflower hypocotyls

The relationships between cell elongation and changes in the activities of enzymes of sucrose metabolism were investigated in the hypocotyl hook of sunflower seedlings (Helianthus annuus L.) that were grown either in darkness or irradiated with continuous white light (WL). Hook opening was promoted by white light (WL). In etiolated seedlings, endogenous hook opening occurred between days 5 and 6 after sowing (darkness). This response is due to a large increase in the rate of cell elongation in the inner side and a decrease in cell length on the outer surface of the organ. After WL irradiation and between days 5 and 6 after sowing (darkness) an enhancement in the specific activity of the enzyme soluble acid invertase (EC 3.2.1.26) was measured, whereas that of sucrose synthase (EC 2.4.1.13) was unaffected. The concentration of sucrose decreased in WL, and to a lesser extent, in darkness. The results demonstrate that endogenous and WL-induced apical hook opening is accompanied by sucrolysis via acid invertase. We suggest that apical hook opening promotes invertase activity and thus sucrose catabolism.