Effects of variable irrigation practices during extreme heat events on grapevine physiology and berry chemistry Dr. Elisabeth Forrestel University of California at Davis, USA

Future climate will expose vineyards to extreme heat events of greater frequency, intensity and duration. These extreme events will be coupled with higher evaporative demand under warming climates and increased drought potential across many wine growing regions of the world, including California. In grapevine and other speciality crops, these higher temperatures, which occur during critical developmental stages, result in reduced photosynthetic capacity, delayed ripening, as well as reduced yield and quality. As growers primarily respond to extreme heat via irrigation, understanding cultivar-specific water demands will better inform grower decisions and increase water use efficiency. However, little information exists on current irrigation and cultivar-specific responses to extreme heat. To evaluate the use of irrigation applied prior to and during heat waves (HWs), and its effect on grapevine physiology and berry composition, we exposed Vitis vinifera cv. Cabernet Sauvignon vines to three differential irrigation treatments across two seasons in an established vineyard in the Lodi AVA of the Central Valley of California. The baseline treatment was under water deficit (60% ET), while the 2x baseline ET and 3x baseline ET treatments had double and triple the irrigation of the baseline, respectively. Throughout HWs in the 2019 season there was a significant reduction in gas exchange, an increase in leaf temperature, and lower evaporative cooling in the baseline treatment, while no differences were observed between 2x and 3x treatments. However, after HWs the baseline treatment showed signs of recovery from physiological stress. Skin tannin and anthocyanin content, the onset of anthocyanin synthesis, pH, and acidity were affected negatively by underwatering (60% ET) or overwatering (3x). Additionally, the baseline treatment had the highest total soluble solids (TSS), and the lowest yield. Furthermore, significant changes in anthocyanin hydroxylation and profile were found due to differential irrigation, where the 2x treatment had a higher proportion of dihydroxylated anthocyanins and malvidins. HW1, which took place pre-veraison, had a significant effect on the changes seen among treatments on berry composition and yield, while HW2 accentuated the differences found in berry composition. This study highlights the detrimental effects of insufficient or excess water applications during heat waves on grapevine physiology and berry composition. Our preliminary work moves towards generating better irrigation management guidelines during heatwaves, which will be critical under increasing heat and drought in many agricultural regions around the world.