Summary Sunflower seedlings (Helianthus annuus L.) were grown in darkness or transferred to continuous white light (WL) at day 3 after sowing. Cotyledons were harvested daily and analysed. In WL a rapid stimulation of fresh mass increase (growth) was induced, but the osmotic concentration of the tissue sap was as large as in the slowly expanding dark-grown control. Specific catalytic activities of enzymes of sucrose breakdown (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 irradiation whereas that of hexoses (glucose, fructose) showed a large increase. Light stimulated the rate of lipid breakdown. The starch content of the etiolated cotyledons was very low and increased upon exposure to WL. The results indicate that the relative maintenance of cell osmotic pressure during light-mediated expansion of the cotyledons is attributable to an enhanced lipid breakdown, followed by sucrolysis via acid invertase and sucrose synthase. In addition, our data show that the greening cotyledons synthesize starch and are capable of producing sucrose.