Deficit Irrigation Strategies and Wine Grape Cold Hardiness

An increase in frequency of extreme and variable weather events, predicted by climate change models, poses a threat to wine grape production at northern latitudes where winter survival depends upon the ability of ecodormant tissue to withstand low temperature exposure and dynamically acclimate to winter temperature fluctuations. Deficit irrigation is used extensively in arid wine grape production regions to manage growth for improved grape quality, yet the influence of deficit irrigation strategies, such as sustained or regulated deficit irrigation, on the resilience of grape tissue to winter injury has received little research attention. The cold hardiness of field-grown vines of the cultivar Merlot was evaluated after a minimum of six sequential growing seasons where the severity of water deficit was either sustained (SDI) or varied (RDI) during berry development. Replicated trial plots were deficit irrigated from fruit set to harvest at fractional amounts of their estimated crop evapotranspiration (ET_c).  Plots under SDI were irrigated at 90, 70, or 35% ET_c and plots under RDI were irrigated at 35% ET_c pre-veraison and 70% ET_c from veraison to harvest. Cold hardiness was evaluated by ratings of visible injury in the spring, bud-forcing bioassays with single node cane sections and Differential Thermal Analysis. Spring injury was significantly greater in vines under SDI at 35% ET_c than under RDI or SDI at 70 or 90% ET_c. The low temperature exotherm of xylem tissue in 1-yr old canes occurred at a warmer temperature in vines under SDI at 35% ET_c than vines under RDI or SDI at 90% ET_c. Single node, dormant cane sections de-acclimated under bud-forcing conditions significantly faster in vines under SDI at 35% ET_c than vines under RDI or SDI at 90% ET_c. The total fresh and dry weight of clusters that emerged from single node cane sections was lowest in vines under SDI at 35% ET_c and RDI and highest under SDI at 90% ET_c. Results from this research show that irrigation practices during the growing season, particularly vine water deficit severity and phenological timing of deficit severity, influence subsequent cold hardiness of vine tissue. The alleviation of water deficit severity between veraison and harvest appeared to beneficially influence cold hardiness. Development of irrigation strategies to enhance grapevine cold hardiness requires an understanding of the underlying mechanisms by which vine water status influences subsequent cold hardiness in grapevine. Results from this research demonstrate that this topic warrants further investigation.