Drip irrigation and precision cooling reduce impact of extreme heat events during berry ripening

Abstract

Context and purpose of the study. Heatwaves have become more frequent and intense in several winegrowing regions. Supra-optimal temperatures (≥ 35 °C) reduce vine physiological activity and increase water stress. Yield is negatively affected by extreme heat, with average losses of 20% or more. Grape quality is also impacted, as color, mouthfeel and aroma compounds become more prone to degradation over 35 °C. Drip irrigation is a useful tool for heatwave mitigation. Berry dehydration and quality losses were reduced by increasing irrigation during heatwaves, even more so when irrigation was increased prior to the event. Advanced cooling techniques may also provide added benefits by directly improving vine microclimate. In the present study, drip irrigation and precision cooling systems for heatwave mitigation were evaluated in California.

Material and methods. Experimental treatments were applied to Cabernet Sauvignon vines grown commercially in the Sonoma AVA, CA in 2024. Six drip irrigation and four canopy cooling treatments were compared to control vines drip-irrigated at 80 % of crop evaporanspiration (ETc), with four replicates each. Drip irrigation treatments were a combination of amount (120 % or 160 % ETc) and timing (0, 1 or 2 days prior) of supplemental irrigation. Precision cooling treatments varied by the emitter location (above, in or below the canopy), flow rate and distribution. Canopy cooling was applied automatically at Tambient ≥ 35 °C. Fruit zone temperature and vine water status were monitored during heatwaves, yield components and fruit composition were assessed before and after each heatwave and at harvest.

Results. The 2024 season was characterized by unprecedented levels of heat, mostly concentrated pre-veraison. All cooling treatments were able to reduce fruit zone temperature, ranging -1.5 to -6 °C. The effect of drip irrigation on fruit zone temperature was negligible. Both drip irrigation and precision cooling led to higher yield at harvest, up to +50% with the highest volume of irrigation. Grape composition at harvest was improved by means of drip irrigation and canopy cooling, with a prevailing effect of cooling techniques. Cooler fruit zone temperature was linked to greater berry anthocyanins, moisture content and lower off-note volatiles, namely 1-octen-3-ol and acetic acid. These results highlight how different irrigation techniques may be used to prevent yield and quality losses caused by extreme heat events.