Ripening grape berries remain hydraulically connected to the shoot

Abstract

Berry diameter was monitored during dry-down and rewatering cycles and pressurization of the root system of Vitis vinifera (cv. Merlot) and Vitis labruscana (cv. Concord) to test changes in xylem functionality during grape ripening. Prior to veraison (onset of ripening), berries maintained their size under declining soil moisture until the plants had used 80% of the transpirable soil water, began to shrink thereafter, and recovered rapidly after rewatering. By contrast, berry diameter declined slowly but steadily during post-veraison water stress and did not recover after rewatering; irrigation merely prevented further shrinking. Preconditioning vines with a period of water stress after flowering did not influence the berries' reaction to subsequent changes in transpirable soil water. Pressurizing the root system led to concomitant changes in berry diameter only prior to veraison, although some post-veraison Concord, but not Merlot, berries cracked under root pressurization. The xylem-mobile dye basic fuchsin, infused via the shoot base, moved throughout the berry vasculature before veraison, but became gradually confined to the brush area during ripening. When the dye was infused through the stylar end of attached berries, it readily moved back to the plant both before and after veraison. Our work demonstrated that berry-xylem conduits retain their capacity for water and solute transport during ripening. It is proposed here that apoplastic phloem unloading coupled with solute accumulation in the berry apoplast may be responsible for the decline in xylem water influx into ripening grape berries. Instead, the xylem may serve to recycle excess phloem water back to the shoot.