Final Report: January 2023
Principal Investigator: Markus Keller, Washington State University-Irrigated Agriculture Research and Extension Center, Prosser
Co-PIs: Esther Hernandez-Montes, WSU-Irrigated Agriculture Research and Extension Center, Prosser
Jim Harbertson, WSU Wine Science Center, Richland
Summary: The main objective of this project is to study how plant water status and fruit temperature affect grape berry acidity, pH, and potassium (K+). Two white wine grape varieties in consultation with industry members were selected. Their different juice/wine acidity profiles made Riesling and Chardonnay interesting varieties to study the relationship between acidity, K+, and pH in Washington. During 2019-2021, field experiments were carried out in the WSU research vineyard at the Irrigated Agriculture Research and Extension Center in Prosser. Two irrigation regimes and three light exposure treatments were tested. Moderate water deficit was applied from late July through harvest and compared with a no-stress control. Sun-exposed clusters were generated at the pea-size stage by removing all leaves in the fruiting zone on the east side of the canopy. A portion of these clusters was changed to shaded clusters at veraison by repositioning shoots to create an extra leaf curtain on the east side of the canopy. For control clusters the canopy was left untreated. Temperature sensors in the fruiting zone were installed to monitor simulated cluster temperatures. Sun-exposed clusters remained at higher temperatures longer than control clusters in both Riesling and Chardonnay and irrespective of the irrigation regime. Cluster shading after veraison alleviated the high-temperature stress on the clusters only in the vines that were fully irrigated. The changes in fruit zone temperature did not correlate well with any changes in fruit composition. Berries of fully irrigated vines retained higher malate and titratable acidity than those of water-stressed vines. Irrigation treatments did not negatively impact other quality parameters such as total soluble solids, pH, or potassium concentrations. Consequently, irrigation may be more important than canopy management to manipulate acidity in ripening grape berries.
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