Irrigation as a tool for heatwave mitigation: the effect of irrigation intensity and timing in Cabernet Sauvignon

Abstract

Context and purpose of the study – Heatwave events, defined as 2 or more days reaching ≥ 38 °C, are an increasingly frequent phenomenon threatening grape production worldwide. Heat stress has been shown to have negative consequences on grapevine physiology, leading to increased evaporative demand and intensified water stress. Due to heatwaves overlapping with important stages of grapevine reproductive development, spanning from berry set to the ripening stage, severe heat can potentially compromise yield and grape quality. The physiological response of grapevine to heat stress suggests a potential use of irrigation to mitigate heatwaves, however there is limited information regarding the irrigation amounts and timings needed for this purpose. Following up on a pivotal trial conducted between 2019 and 2022, in this study irrigation treatments with varying intensity and timing of application were refined to determine their potential mitigation of heat-associated damage to yield and fruit composition. 

Material and methods – The experiment was conducted in 2022 in two commercial Cabernet Sauvignon vineyards located in Lodi and Sonoma AVAs, CA. Vines were submitted to six experimental treatments which differed to the control (80% evapotranspiration [ET]) by the irrigation amount (120 % and 160 % ET) and/or the timing (0, 1 or 2 days) of the application of supplemental irrigations prior to the anticipated beginning of the heatwave event. Each treatment was replicated four times. Diurnal changes in grape water status and gas exchange were monitored during the heatwave, and berry samples collected before and after the heatwave were used to characterize treatment effects on berry weight and grape chemical composition. At harvest, yield components were measured and grape chemical composition analyzed. Wines from each treatment were also produced.

Results – Increased irrigation amounts and earlier initiation of additional irrigation were able to improve the water status of Cabernet Sauvignon vines during heatwave events (highest temperatures recorded: Lodi, 44.5 °C; Sonoma, 47 °C). Diurnal measurements of gas exchange revealed that additional irrigation mitigated reductions in photosynthesis and stomatal conductance caused by extreme heat events. Similarly, increasing amounts of water demonstrated to be generally more effective at reducing berry dehydration, limiting yield losses at harvest. Irrigation regimes affected fruit composition, most importantly C6 compounds, IBMP, anthocyanins and tannins, which was partly explained by various levels of berry dehydration. Non-linear relationships were observed between irrigation levels and several yield and fruit composition parameters, and treatment efficacy was not always stable across the two sites. These results indicate the importance of calibrating water amounts based on singular growing conditions to maximize yield and preserve fruit quality with respect to desired production targets.