Field evaluation of water transport in grape berries during water deficits

Abstract

The net flow in vascular and transpirational components of the grape berry water budget was evaluated during water deficits imposed at different stages of fruit development. Diurnal fluctuations in berry diameter were measured on field‐grown grapevines (Vitis vinifera L. cv. Cabernet Sauvignon) by using electronic displacement transducers. Water deficits were imposed by withholding irrigation, and water potentials of mid‐shoot leaves, basal stem xylem and clusters were determined with a pressure chamber. The relative net flows through pedicel xylem and phloem and through berry transpiration were estimated pre‐veraison and post‐veraison. The xylem functioned nearly exclusively in providing net inflow pre‐veraison, while the phloem was clearly dominant post‐veraison. Accordingly, the amplitude of diurnal contraction was markedly smaller post‐veraison than pre‐veraison. The amplitude of diurnal contraction increased dramatically with decreasing plant water status pre‐veraison, yet exhibited little sensitivity to low vine water status post‐veraison. Measurements of the difference in water potential between clusters and source stems did not provide evidence of a gradient that would elicit significant water movement from the cluster to the stem at any time of the day. This was true for both irrigated and non‐irrigated vines, although the non‐irrigated vines exhibited a smaller gradient favoring inflow throughout much of the day. The gradient for xylem water transport to the cluster was considerably smaller post‐veraison than pre‐veraison. The results showed that berry transpiration functioned as the primary pathway for water loss both pre‐ and post‐veraison.