Irrigation during heat waves sustains gas exchange, evapotranspirational cooling, and yield in grapevines

Increasing frequency, intensity, and duration of heat waves (HWs) threaten agricultural production globally by constraining physiological function and fruit production. Although supplemental irrigation is widely used to mitigate heat stress, optimal irrigation strategies during transient heat events remain poorly understood under realistic field conditions. We conducted a three-year field experiment in a commercial Cabernet Sauvignon vineyard manipulating irrigation during natural heat waves at baseline (50% ET), moderate (90-120% ET), and high (120-180% ET) application rates. We evaluated leaf-level gas exchange, plant water status, leaf temperature, yield components, berry composition, and water-use efficiency to assess how irrigation timing mediates plant responses to extreme heat. Across heat events, baseline irrigation was associated with reductions in stomatal conductance and net photosynthesis, elevated leaf temperatures, and declines in productivity, reflecting constrained transpirational cooling and carbon assimilation under high atmospheric demand. Moderate supplemental irrigation maintained gas exchange, supported leaf thermal regulation, and preserved yield and crop water-use efficiency. In contrast, irrigation exceeding moderate supplementation provided no additional physiological or yield benefits and reduced berry quality and overall water-use efficiency. Despite transient post-event recovery in physiological function, repeated exposure to heat stress under lower irrigation contributed to cumulative reductions in productivity across seasons. These results demonstrate that episodic, heat-event-targeted irrigation can sustain leaf physiological function and productivity in water-limited vineyards. Our study emphasizes the critical role of irrigation timing, rather than simply total water applied, in supporting perennial crops under extreme heat. Tailoring irrigation to periods of high thermal stress provides an effective strategy to sustain physiological function and productivity while conserving water resources under increasing climatic extremes.