Staying hydrated – not easy when it’s hot!

Abstract

Context and purpose of the study – Heat and drought episodes during the growing season are becoming more frequent and more severe in many of the world’s grape‐growing regions. However, the responses of grapevines to a combination of these stress factors are incompletely understood, which hampers the implementation of deficit irrigation and heat mitigation strategies. Our team is investigating impacts of water deficit and temperature alone or in combination on physiology, growth, fruit production and composition of different grape cultivars. In addition, we are also testing different deficit irrigation strategies and novel approaches to canopy heat mitigation.

Material and methods ‒ Experiments are conducted with both field‐ and pot‐grown cultivars of own‐ rooted wine grapes in an arid climate. Drydown and rewatering cycles were applied to 18 cultivars in a vineyard, and changes in soil moisture, leaf water potential, and stomatal conductance were monitored during 4 growing seasons. In another experiment, pot‐grown Cabernet Sauvignon and Riesling vines were exposed in environmentally controlled growth chambers to episodes of water stress, heat stress, and combined water and heat stress, followed by recovery periods. Changes in growth, leaf physiology, and fruit composition were compared with non‐stressed control vines. Finally, a novel mist‐type evaporative cooling system was installed in a Cabernet Sauvignon vineyard and is currently being tested for its ability to mitigate heat stress while maintaining fruit quality.

Results ‒All cultivars tested in the vineyard decreased their leaf water potential as the soil dried. Some cultivars behaved in classic anisohydric fashion, while others only decreased their leaf water status once soil moisture had declined below a threshold, and yet others showed highly variable responses. Irrespective of their hydraulic behavior, all cultivars also responded to soil drying by decreasing stomatal conductance. In the growth chambers, water stress dominated the responses of shoot growth and leaf physiology in both cultivars, but heat stress exacerbated the adverse impact of water stress. By contrast, heat stress dominated the responses of fruit composition, reducing titratable acidity and increasing the pH and total soluble solids. The evaporative cooling system effectively controlled canopy temperatures during heat waves with a minimum supply of water and without adverse effects on disease incidence, fruit yield, and composition.