Terroir of Winter Hardiness: Bud LT₅₀, Water Metrics, Yield, and Berry Composition in Ontario Riesling

Grapevine winter hardiness is governed by several factors besides extreme weather conditions, such as site-specific soil factors (texture, composition, moisture, drainage), vine water status, and yield. It was hypothesized that winter hardiness would be influenced by specific vineyard terroir-driven factors and that zones in vineyards with low water status (leaf water potential [Ψ]) would likewise be more winter hardy than vines with high water status (less negative leaf Ψ). Six Riesling vineyards were chosen throughout the Niagara region in Ontario. Data were collected at fruit set, lag phase, and veraison (soil water content [SWC], leaf Ψ), at harvest (yield components, berry composition), and three times during the winter (LT₅₀; the temperature at which 50% of buds die) in 2010 to 2012. Interpolation and mapping of variables was completed using kriging, and statistical analyses (linear correlation, k-means clustering, principal components analysis, multilinear regression) were performed. SWC, leaf Ψ, yield components, berry composition, and LT₅₀ were spatially clustered in each vineyard. GIS and multilinear regression revealed that leaf Ψ could predict the LT₅₀ value, with strong correlations observed between LT₅₀ and leaf Ψ values in most of the vineyard blocks in 2010 to 2011 (4/6 and 5/6, respectively). In the extremely dry 2012 season, leaf Ψ (range across sites at veraison 0.9 to 1.4 MPa) was positively correlated to LT₅₀, yield, titratable acidity, pH, and Brix and inversely correlated to SWC and monoterpene concentration. Results suggest that there is a spatial component to winter hardiness, as with other aspects of terroir. This study allows a method to compare winter hardiness to other critical variables to better understand terroir-based factors of the Niagara region.