The reduction of plant sink/source does not systematically improve the metabolic composition of Vitis vinifera white fruit.

The quality of wine grapes depends on the balance between primary and secondary metabolites. Unlike many perennial crops that accumulate starch in the fruits before ripening, the non-climacteric grapes ripe with no previous carbon reserves. Based on the assumption that fruit carbon sink is limiting metabolite accumulation in grapes, bunch thinning is performed to limit plant Sink/Source (S/S). We studied the effects of severe bunch thinning on the accumulation of primary metabolites and on four families of glycosylated aroma precursors (GAPs) at the arrest of fruit phloem unloading of two white grape Vitis vinifera cvs. At plant level, crop reduction resulted in significant losses of metabolites to be accumulated in the fruits: i.e. up to 72% for sugars, 75% for organic acids and GAPs. Nevertheless, S/S manipulation could not modify the balance between GAPs and primary metabolites or increase the concentration in GAPs in the physiologically ripe grape.