Runze Yu and Kaan Kurtural* 
*University of California, Davis, 1380 Oakville Grade Rd, Oakville, CA 94562 (skkurtural@ucdavis.edu)

The relationships between variations in grapevine (Vitis vinifera L. cv. Merlot), fruit zone light exposure, and water deficits and the resulting berry flavonoid composition were investigated in a hot climate. The experimental design involved application of mechanical leaf removal (control, prebloom, post-fruit set) and differing water deficits (sustained deficit irrigation and regulated deficit irrigation). Flavonol and anthocyanin concentration increased with prebloom leaf removal in 2013, but with post-fruit set leaf removal in 2014. Post-fruit set leaf removal increased total proanthocyanidin concentration in both years, while no effect of applied water amounts was observed. Mean degree of polymerization of skin proanthocyanidins increased with post-fruit set leaf removal compared to prebloom, while water deficit had no effect. Conversion yield was greater with post-fruit set leaf removal. Seed proanthocyanidin concentration was rarely affected by applied treatments. The application of post-fruit set leaf removal, regardless of water deficit, increased the proportion of proanthocyanidins derived from the skin, while no leaf removal or prebloom leaf removal, regardless of water deficits, increased the proportion of seed-derived proanthocyanidins. The study provides fundamental information to viticulturists and winemakers on how to manage red wine grape low molecular weight phenolics and polymeric proanthocyanidin composition in a hot climate.

Funding Support: American Vineyard Foundation