Water flux of Vitis vinifera L. cv. Shiraz bunches throughout development and in relation to late-season weight loss

Vitis vinifera (L.) cv. Shiraz commonly loses a significant amount of bunch water during late ripening, with attendant yield losses. The objective of the study was to quantitatively assess the hypothesis that this sustained weight loss was due to decreased vascular flow and increased transpiration. Transpiration, fresh and dry weights, and water content of whole attached grape bunches were measured from flowering to maturity on vines grown in controlled environments and in the vineyard. Seasonal changes in net water fluxes into and out of bunches were determined. A simple technique of weighing bunches in situ also provided independent measurement of daily rates of water gain and loss. Bunch transpiration rates were high just after flowering but declined to 0.2 g g (dry wt)-1 d-1 at about harvest date. Bunch net water import rates also showed a 90% decrease with development from 1.0 to 0.1 g g (dry wt)-1 d-1. Comparisons of these rates revealed net water import exceeded transpiration throughout early and midbunch development. However, at 60 to 80 days after flowering, import rates had declined to an extent that transpiration now exceeded import and an overall loss of water occurred. Quantitatively comparable rates of water gain and loss determined on Shiraz bunches over four growing seasons on vineyard-grown vines conformed closely with those rates determined as above. Changes in diurnal bunch water fluxes supported the conclusion that net water import exceeded transpiration losses throughout bunch development until the late stage of ripening. The hypothesis that sustained weight loss in late-ripening Shiraz grape bunches occurred because bunch water fluxes shifted from a net import to a net loss by transpiration was confirmed