Starch synthesis in tomato remains constant throughout fruit development and is dependent on sucrose supply and sucrose synthase activity

Studies designed to investigate the cellular pathway of phloem unloading we re conducted on two tomato lines with either high or low fruit invertase ac tivities. Experiments were based on determination of the degree to which H- 3 label from [H-3]-(fructosyl)-sucrose was randomized between fructose and glucose following exposure of excised fruit to a pulse of labelled sucrose delivered through the pedicels. Fruit from the low invertase line harvested 10, 20 and 40 d after anthesis had similar sucrose uptake kinetics to the high invertase line. A positive correlation was found between sucrose synth ase activity and sucrose uptake in both low and high invertase lines. In co ntrast, no correlation was observed between acid or neutral invertase activ ities and sucrose uptake. Within the putative apoplasmic sap collected from fruit, label in [H-3]-(fructosyl)-sucrose was randomized between the free hexoses and sucrose hexose moieties. Label asymmetry was retained in sucros e on arrival within the tissues. Randomization patterns were similar in bot h the low and high acid invertase lines. These data support the view that s ucrose imported into the fruit was not exposed to extracellular hydrolysis, This suggests that movement from the phloem is likely to occur predominant ly through a symplastic pathway. About 25% of the sucrose taken up by the f ruit was converted into starch regardless of fruit age, suggesting that sta rch turnover remains constant throughout fruit development and that starch synthesis was dependent on sucrose supply.