Sucrose accumulation in developing peach fruit

Uptake of C-14-sugars and activities of sucrose metabolizing enzymes were determined in order to study the mechanism(s) of sucrose accumulation in developing peach fruit. Mesocarp of young peach fruit contained glucose and fructose but little sucrose. Starting 88 days after anthesis (DAA) the sucrose concentration increased greatly. The mechanism of sucrose accumulation was studied by measuring C-14-sucrose and C-14-glucose uptake rates at three different stages of fruit development, and by assaying weekly the activity of enzymes involved in the hydrolysis and/or synthesis of the soluble sugars. Uptake of 0.5-100 mM C-14-sucrose and C-14-glucose by mesocarp tissue slices showed a complex pattern at the first stage of fruit development (62 DAA). During the subsequent growth stages the pattern of sugar uptake changed and was approximately monophasic at the third stage of fruit development. At 10 mM, glucose was taken up more rapidly than sucrose at the first and second stage of fruit development. Uptake was partially inhibited by the uncoupler carbonyl-cyanide m-chlorophenylhydrazone (CCCP) at 25 mu M. These results, together with the presence of a putative extracellular invertase, suggest an apoplastic route for sucrose uptake which is dependent, at least in part, on energy supply. Activities of sucrose hydrolyzing enzymes (insoluble acid invertase, soluble acid invertase, neutral invertase, sucrose synthase) were high in young fruits and declined sharply with fruit development concomitantly with accumulation of sucrose. The storage of the sugar was not accompanied by a rise in synthetic activities (sucrose synthase, sucrose phosphate synthase), suggesting that sucrose could, at least in part enter the carbohydrate pool directly.