Yun Zhang,* Letizia Rocchi, James F. Harbertson, and Markus Keller
*Irrigated Agriculture Research and Extension Center, Washington State University, 24106 N. Bunn Rd., Prosser, WA 99350 (yunzhang@wsu.edu)

Most knowledge about irrigation management (especially deficit irrigation) has been generated using red winegrape cultivars. Little is known about the optimal irrigation practice for white cultivars. Therefore, three deficit irrigation regimes and a full irrigation treatment (as control) were implemented in commercial vineyards of Chardonnay and Riesling. These treatments were (i) moderate water stress from fruit set to harvest (SDI), (ii) moderate water stress from fruit set to veraison and then no stress through harvest (RDI), (iii) partial rootzone drying from fruit set to harvest (PRD), and (iv) full irrigation (FI). For both cultivars and consistently for two growing seasons, FI vines had higher rates of leaf gas exchange, more canopy growth (vigor, leaf layers, and canopy density), higher yield, and larger berries than SDI or RDI. Compared to SDI, after veraison, RDI vines had increased vine water status and gas exchange rates but no difference in canopy size or yield at harvest. Based on soil moisture measurements, irrigation in PRD was alternated between sides every two to three weeks. In both growing seasons, PRD resulted in a more open canopy and smaller berry size than FI and larger berry size than SDI or RDI. In the second growing season only, PRD increased canopy growth relative to SDI or RDI. Overall, PRD vines had similar berry composition and irrigation water use but lower yield or reduced berry size compared to SDI/RDI; therefore, PRD may be beneficial for white winegrape production. Under similar light conditions, no difference in berry skin flavonoids (flavonols and flavanols) was found between FI and SDI in either sun-exposed or shaded berries. Therefore, light conditions had a dominant impact on berry skin flavonoids compared with irrigation treatments. Irrigation may have indirectly affected skin flavonoids by altering the canopy microclimate.

Funding Support: WSDA Specialty Crop Block Grant, Washington State Grape & Wine Research Program, University of Milan, COST Action, Innovine European Project