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 were conducted on two tomato lines with either high or low fruit invertase activities. Experiments were based on determination of the degree to which 3H label from [3H]-(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 synthase activity and sucrose uptake in both low and high invertase lines. In contrast, no correlation was observed between acid or neutral invertase activities and sucrose uptake. Within the putative apoplasmic sap collected from fruit, label in [3H]-(fructosyl)-sucrose was randomized between the free hexoses and sucrose hexose moieties. Label asymmetry was retained in sucrose on arrival within the tissues. Randomization patterns were similar in both the low and high acid invertase lines. These data support the view that sucrose imported into the fruit was not exposed to extracellular hydrolysis. This suggests that movement from the phloem is likely to occur predominantly through a symplastic pathway. About 25% of the sucrose taken up by the fruit was converted into starch regardless of fruit age, suggesting that starch turnover remains constant throughout fruit development and that starch synthesis was dependent on sucrose supply.