Christopher Chen, Johann Martinez-Luscher, Luca Brillante, and Kaan Kurtural*
*University of California Davis, 1 Shields Avenue, Davis CA 95616 (skkurtural@ucdavis.edu)

Increased daytime temperatures and intermittent heat spikes during engustment negatively affect the biosynthesis, retention, and degradation of flavonoids in grape berries. There is little information on how to mitigate radiation damage and degradation that may occur under these conditions. A field experiment was conducted with Cabernet Sauvignon/110R using one-meter, black, polyethylene shade nets with 40% light transmissivity to determine the effects of partial solar radiation exclusion and applied water amounts on the productivity, primary, and secondary metabolites of grapevine berry. One netting treatment of 40% black net and one untreated control with no net were applied shortly after fruit set at E-L 29 and retained until harvest. Two applied water amounts of 0.65 crop evapotranspiration (ETc, control) and 1.3 ETc, were applied. At harvest, there was no effect of treatments on pH or titratable acidity. However, individual berry mass was greater in controls than in shade net treatments. Diurnal cluster temperature shifts on both sides of the canopy were recorded. During peak daytime temperatures, cluster temperatures were 3.9°C greater in the control, although no effect was attributed to applied water amounts. Cluster damage attributed to solar radiation exposure was quantified. Although yield was unchanged, damaged cluster count and weight were significantly greater in treatments without shade netting applied. Anthocyanins and flavonols of berry skins were measured using reversed-phase HPLC. At harvest, treatments without shade nets had higher flavonol concentrations, but a lower tri/di-hydroxylation ratio, than shaded treatments. However, anthocyanin concentration was highest in shaded berries. Our results provide evidence that, regardless of applied water amounts in a vineyard, effective shade netting may alleviate berry stress from excessive solar radiation exposure. It may do so without greatly affecting yield at harvest, and can mitigate the amount of visible radiation damage on the fruit.

Funding Support: Oakville Experimental Vineyard