This research focuses on the adaptation and drought responses in yield and fruit and wine quality of seventeen, red wine grape cultivars. The project exploits an established variety trial in which the cultivars were selected for potential adaptation to San Joaquin Valley conditions and by doing so extend the information derived from the previous investment to establish this experimental vineyard – used by Dr. Jim Wolpert from 2006 to 2010. Reducing the plant-available water (by restricting irrigation) can be expected to reduce yield, but to also increase water use efficiency and fruit quality of red wine grapes. The timing of water deficits affects many of the vine responses to stress and the resulting wine sensory characteristics. For example, early(preveraison) deficits have a greater effect on tannins than late deficits, whereas for anthocyanins(color) it is the reverse. The studies that have established these phenomena were conducted in moderate (North Coast) climates and with common cultivars such as Merlot, Cabernet Sauvignon and Cabernet franc. This study will test whether those observations hold in the warmer San Joaquin Valley across numerous cultivars.
Estimated ETc from budbreak to average date of harvest across cultivars and irrigation treatments (end of August) was 619 mm (24.4 in). Applied water to vines irrigated at full ET from budbreak to veraison after which the irrigation was terminated (I ? Ni treatment), applied water at 50%of estimated ETc season long (0.5 ETc treatment) and no applied water up to veraison and then applications thereafter at 50%of ETc (Ni ? 0.5) were 397, 347 – 453 and 173 – 293 mm, respectively. The effect of irrigation treatment on vine water status (midday leaf water potential) similarly affected all cultivars.
Early water deficits (no applied water up to veraison; Ni ? 0.5 treatment) greatly reduced berry weight at veraison and harvest compared to the other two irrigation treatments across cultivars. The no applied water after veraison treatment (I ? Ni) reduced berry weight of 13 cultivars at harvest compared to their veraison berry weight. Titratable acidity (TA) in the berries of 15 of the cultivars at harvest was greater for the I ? Ni treatment compared to the other two irrigation treatments (the exceptions were Tinta Amarella and Tinta Madeira). The greatest TA values across irrigation treatments were for Durif, Cabernet Sauvignon and Tannat. Berries and the wines made last year are currently being analyzed for color, phenols and tannins.