Markus Keller,* and Yun Zhang
*Washington State University, 24106 N Bunn Rd, Prosser, WA 99350 (mkeller@wsu.edu)

Deficit irrigation is a common management tool in viticulture, especially in arid and semiarid regions. However, recurring heat waves in recent years have posed challenges for grapevines under water stress. The combination of heat stress and soil water deficit may lead to undesirable impacts on grapevine growth and grape ripening. To understand the interactive effects of heat and water stress, two contrasting cultivars, Cabernet Sauvignon and Riesling, were chosen for this study. Treatments included water stress only, heat stress only, the combination of water and heat stress, and a no-stress control. Treatments were imposed in climate-controlled growth rooms twice (before and during veraison), and lasted seven days each time. Each treatment round was followed by a seven-day recovery period. In both cultivars, vines under water stress had reduced shoot growth and leaf gas exchange, regardless of the presence or absence of heat. Heat stress alone had less-pronounced adverse effects on vegetative growth than did water stress. Additive negative effects of the combination of stresses were found on leaf water potential, and sometimes on vegetative growth. Significant interactions between water and heat stress were found for leaf gas exchange in both cultivars. Contrary to expectations, the relationship between leaf water potential and stomatal conductance was similar in these two cultivars. In terms of berry ripening, the two cultivars differed in their responses: water stress advanced the onset of ripening in Cabernet Sauvignon, but heat stress had no effect; however, the opposite occurred in Riesling. These preliminary results indicate that, compared with heat stress, water stress is the dominant factor affecting grapevine growth and gas exchange. The interplay of heat and water stress on grape ripening is more complicated, and requires further investigation.

Funding Support: Specialty Crop Block Grant Program, the Washington State Grape and Wine Research Program