Geraldine Diverres, Danielle Fox, James Harbertson, Manoj Karkee, and Markus Keller*
*Washington State University, 24106 N Bunn Road, Prosser, WA, 99350 (mkeller@wsu.edu)

Regulated deficit irrigation (RDI) and partial rootzone drying (PRD) have produced diverse viticultural and enological outcomes when implemented in vineyard settings around the world. Most research on deficit irrigation strategies has focused on red wine grape cultivars, optimizing attributes important for red wine production, which differ from white winemaking. A three-year field trial was conducted in southeastern Washington with Riesling winegrapes to compare the effect of RDI and PRD with a no-stress control and test their suitability for premium white wine grape production in arid climates. Irrigation scheduling was based on soil moisture thresholds according to vine phenological stage, rather than adhering to specific time intervals. Irrigation water supply, soil and plant water status, canopy size, yield components, and fruit and wine composition data were collected. Midday leaf water potential (Ψleaf) remained stable near -0.7 MPa until the extractable soil water content declined to ~35% (field capacity is at 100%). Beyond this point, Ψleaf declined as soil moisture decreased further, dropping to values as low as -1.5 MPa. Both RDI and PRD conserved irrigation water compared to the control, but both deficit practices also reduced yield. Seasonal variation had a more pronounced effect on basic fruit composition than irrigation practices. Preveraison water deficit, even if slight, reduced canopy size and yield while significantly affecting the volatile composition of the resulting wines. The wine phenolic composition was affected to a lesser extent by irrigation. This research confirms the power of small differences in preveraison water status to manipulate wine style in the vineyard and explores the relationship between the available soil water content and vine water status, using Riesling as a model.

Funding Support: USDA-NIFA Cyber-Physical Systems Program, Washington State Grape and, Wine Research Program, Chateau Ste. Michelle Distinguished Professorship.