Carl Bogdanoff,* Pat Bowen, Brad Estergaard, and Steve Marsh
*Summerland Research & Development Centre, PO Box 5000, 4200 Highway 97, Summerland/BC/V0H 1Z0, Canada (Carl.Bogdanoff@agr.gc.ca)

The effects of irrigation deficit timing on the winter hardiness of grapevine buds were determined in a Merlot vineyard in the Okanagan Valley of British Columbia. Treatments were full irrigation (FI) applied throughout the growing season using two emitters per vine, reduced-to-veraison (RV) irrigation, and reduced-to-harvest (RH) irrigation. The latter two were applied from fruitset to veraison or to harvest, respectively, by eliminating one emitter per vine to provide 50% less water. The final treatment, partial root zone drying (PRD), was applied from fruit set to harvest by eliminating one emitter per vine, alternating every three weeks between the two emitter positions at each vine. The frequency and duration of irrigations were the same for all treatments and were based on maintaining soil moisture at 30 to 60 cm deep in the FI treatment at above 4% (v/v). From July to mid-September, leaf gas exchange and stomatal conductance were greater in vines receiving the higher irrigation rate (i.e., FI vines and RV vines postveraison). Higher yields were produced by vines under FI than RV. Compared with FI, all reduced irrigation treatments (RV, RH and PRD) reduced berry weight and increased juice pH but did not affect juice soluble solids or titratable acidity. Bud hardiness measured during the acclimation and maximum-hardiness periods of vine dormancy was lower in response to PRD than all other irrigation treatments and higher for vines that were fully irrigated after veraison than those under the reduced irrigation to harvest regimes. Across treatments, faster rates of photosynthesis, stomatal conductance, and transpiration were associated with enhanced bud hardiness. These results indicate that water stress leading to reduced leaf gas exchange, especially from veraison to harvest, reduces bud hardiness in Merlot.

Funding Support: BC Wine Grape Council, Agriculture Agri-Food Canada