Impact of Regulated Deficit Irrigation Strategies on Grape Quality in the Semi Arid Region of the Okanagan Valley (BC, Canada) Dr. Simone Castellarin Wine Research Centre, The University of British Columbia, Vancouver, BC, Canada

In the semi-arid climate of the Okanagan Valley – one of the major wine regions of Canada – vineyards experience a gap between the grapevine water requirements and the growing season precipitation. Grape growers must accommodate this gap through irrigation supplied from pre-blooming stages to harvest. Moreover, future climate trends for the Okanagan Valley are expected to include increased variability in precipitation events, decreased summer and autumn water supply, and increased temperature. As a result, the development of practical solutions that minimize in-season use of irrigation, while maximizing crop quality, are central to the sustainability of agriculture in this region as well as in many other wine regions.

Regulated deficit irrigation is a viticultural practice known to reduce water inputs and improve grape phenolics and color in red grape varieties. From 2015 to 2019 we have conducted experiments in Viognier and Gewürztraminer vineyards in order to assess the effect of these strategies on grape composition and, particularly, on the accumulation of grape aromas.

Moderate water deficit was applied via regulated deficit irrigation before veraison and throughout the season to Viognier grapevines and was compared in its effects to a well-irrigated control. Similar treatments were applied to Gewürztraminer grapevines, but the application of moderate water deficit after veraison was also tested. The effects of the above treatments on canopy growth, leaf photosynthesis, bud fertility, vine yield, and berry composition during the growing season were assessed. Free and bound terpene levels in the grapes were analysed at harvest and before harvest – as terpenes are the most important aromas in Viognier and Gewürztraminer wines.

The study revealed that, despite determining a significant reduction in photosynthesis, moderate water deficit can improve terpene levels in the grapes at harvest without significantly limiting vine yield. In Viognier, terpene levels were induced when moderate water deficit was applied before veraison; vice versa, in Gewürztraminer, the highest levels of terpenes were detected in grapes exposed to moderate water deficit after veraison. These strategies allowed to reduce water inputs by 30% and potentially improve wine quality.