The effects of sunlight exposure on the berry growth and composition of two red wine grape cultivars grown in the central San Joaquin Valley of California (Region V) were examined. Field grown Cabernet Sauvignon and Grenache grape clusters were grown over a range of sunlight exposures (mid-day PAR <10 µmol m^-2 sec^-1 [shaded] to >600 µmol m^-2 sec^-1 [fully exposed]) from berry set to harvest. Both cultivars were planted in east-west oriented rows, and experimental clusters were evenly distributed between the north (afternoon shaded) and south (afternoon exposed) sides of the canopy. Fruit response to sunlight varied based on cluster location within the canopy, and these results were at least partially due to measured differences in berry temperature. At the same exposure level or PAR (photosynthetically active radiation), mid-day berry temperature was generally 3 to 4°C greater for clusters on the south side of the canopy compared to clusters on the north. Soluble solids initially increased with greater sunlight exposure, then declined when mid-day PAR exceeded 31 to 50 and 51 to 100 µmol m^-2 sec^-1, respectively, for clusters on the north and south sides of the canopy. Titratable acidity generally declined as sunlight exposure increased, with Cabernet Sauvignon clusters on the north side of the canopy maintaining greater acidity at the same exposure level than clusters on the south. Juice pH declined as exposure increased on the north side of the canopy, while sunlight had little effect on juice pH for clusters on the south. Anthocyanins increased linearly as sunlight exposure on the north side of the canopy increased, but declined when cluster exposure on the south exceeded 100µmol m^-2 sec^-1. Total phenolics generally followed a similar pattern. The results suggest that the effects of light on fruit composition are heavily dependent upon the extent to which berry temperature is elevated as a result of increased sunlight exposure. Prolonged exposure of clusters to direct sunlight should be avoided for maximum berry color in the central San Joaquin Valley and other warm regions.

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