Is weight loss in ripening grape berries cv. Shiraz caused by impeded phloem transport?

Berry shrinkage in ripening grapes cv. Shiraz is systemic within a given grapevine and coincident between grapevines in any given season. In this present study on weight loss in ripening berries, ripening curves of non‐solutes per berry (largely water) were similar to curves for berry weight (as a function of time). Both sets of curves were equivalent with respect to timing of maximum weights and subsequent rates of weight loss. However, curves of solutes per berry (largely sugar) increased steeply up to the time of maximum berry weight, then slowed and plateau‐ed. We suggest that phloem sap is the sole source for water and solutes that enter grape berries subsequent to veraison, and accumulate until maximum berry weight. We further suggest that phloem flow becomes impeded at maximum berry weight. As berry ripening proceeds, continuation of berry transpiration leads to berry shrinkage and a concentration of solutes; i.e. any increase in juice Brix depends on shrinkage. One implication is that assimilates enter a berry up to the onset of shrinkage, whereafter accumulation of non‐anthocyanin glycosides (including glycosides of flavour compounds) depends upon their synthesis in situ.