SUCROSE METABOLISM DURING APICAL HOOK OPENING IN SUNFLOWER HYPOCOTYLS

The relationships between cell elongation and changes in the activitie s of enzymes of sucrose metabolism were investigated in the hypocotyl hook of sunflower seedlings (Helianthus annuus L.) that were grown eit her in darkness or irradiated with continuous white light (WL). Hook o pening was promoted by white light (WL). In etiolated seedlings, endog enous hook opening occurred between days 5 and 6 after sowing (darknes s). This response is due to a large increase in the rate of cell elong ation in the inner side and a decrease in cell length on the outer sur face of the organ. After WL irradiation and between days 5 and 6 after sowing (darkness) an enhancement in the specific activity of the enzy me 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 su crose decreased in WL, and to a lesser extent, in darkness. The result s demonstrate that endogenous and WL-induced apical hook opening is ac companied by sucrolysis via acid invertase. We suggest that apical hoo k opening promotes invertase activity and thus sucrose catabolism. (C) Elsevier, Paris.