Water Productivity, Yield, and Berry Composition in Sustained versus Regulated Deficit Irrigation of Merlot Grapevines

The winegrape cultivar Merlot (Vitis vinifera L.) was irrigated over eight growing seasons at three severities of sustained deficit irrigation (SDI): a standard (STD) amount that provided 90% of estimated crop evapotranspiration (ETc) and 70 or 35% of the standard amount (STD70, STD35) or by a regulated deficit irrigation (RDI) regime that provided the STD35 amount preveraison and the STD70 from veraison until harvest. This work discusses data collected during the final three seasons. Vines under STD irrigation had the greatest proportion of large berries (similar to 1.2 g size category), and large berries, regardless of irrigation regime, had lower concentrations of soluble solids and anthocyanins and higher anthocyanin content per berry than small berries (similar to 0.6 g size category). In berries of similar size, the concentration of soluble solids and the anthocyanin content per berry were higher under RDI than under STD or STD35 and were similar under RDI and STD70. As irrigation amount under SDI decreased 62% from STD to STD35, yield, berry weight, and pruning weight decreased (44, 24, and 55%, respectively) and water productivity increased (24%). The STD70 treatment produced the greatest quantity of fruit with desirable berry composition. Seasonal water use was 15% less under RDI than under STD70; however, the water productivity of both treatments was similar due to reduced berry size and yield under RDI relative to STD70. The RDI regime provides an irrigation option for producing fruit of desirable composition when water conservation necessitates acceptance of a reduction in yield.