Water use efficiency and must quality of irrigated grapevines of north-eastern Portugal

This is an experiment to study the effect of water supply on the quality of the must produced by grapevines, and to determine the water use efficiency (WUE) of different irrigation strategies to give farmers a tool to optimize irrigation, reducing water use with the least negative effect on production. WUE was based on the ratio between a given parameter (yield, different must solubes) and the water used by plant transpiration plus evaporation from soil. The experiment was a completely randomized design with six irrigation treatments (non-irrigated, irrigated from flowering to veraison at rates of 4 and 8 mm day (1), irrigated from veraison to commercial maturation at rates of 4 and 8 mm day(-1), and irrigated from flowering to commercial maturation at a rate of 8 mm day(-1)) in three replications over three seasons (2004-2006). Irrigation significantly increased grapevine yield and, because the plants produced larger berries, there was a dilution of colour, aroma and soluble solids that corresponded to a lower quality of the must. The higher WUE was achieved with no irrigation but yield was very low, albeit with an outstanding quality, that might not be profitable. The best strategy is to balance the water stress of the vines that best combines yield and berry quality and this was achieved by irrigating from flowering to veraison at a rate of similar to 50% of the potential evapotranspiration. This strategy might conserve water, an increasingly scarce resource that we are pressed to use with the utmost efficiency.