SUCROSE METABOLISM AND LIPID MOBILIZATION DURING LIGHT-INDUCED EXPANSION OF SUNFLOWER COTYLEDONS

Sunflower seedlings (Helianthus annuus L.) were grown in darkness or t ransferred to continuous white light (WL) at day 3 after sowing. Cotyl edons were harvested daily and analysed. In WL a rapid stimulation of fresh mass increase (growth) was induced, but the osmotic concentratio n of the tissue sap was as large as in the slowly expanding dark-grown control. Specific catalytic activities of enzymes of sucrose breakdow n (acid invertase, sucrose synthase) and sucrose biosynthesis (sucrose -phosphate synthase, ribulose 1,5-bisphosphate carboxylase/oxygenase) were enhanced by WL. The concentration of sucrose decreased after irra diation whereas that of hexoses (glucose, fructose) showed a large inc rease. Light stimulated the rate of lipid breakdown. The starch conten t of the etiolated cotyledons was very low and increased upon exposure to WL. The results indicate that the relative maintenance of cell osm otic pressure during light-mediated expansion of the cotyledons is att ributable to an enhanced lipid breakdown, followed by sucrolysis via a cid invertase and sucrose synthase. In addition, our data show that th e greening cotyledons synthesize starch and are capable of producing s ucrose.