Johann Martínez-Lüscher,* Luca Brillante, and Sahap Kaan Kurtural
*University of California, Davis, Oakville Experimental Vineyard, 1380 Oakville Grade, Oakville, CA 94562 (jdmar@ucdavis.edu)

Cultural practices such as leaf removal or shoot thinning are used to expose grapes to solar radiation. Thus, there are many studies suggesting that solar radiation may promote flavonoid biosynthesis through transcriptomic regulation. However, as exposure increases there is a growing need to cope with direct damage from incident radiation and elevated berry temperature, which can result in flavonoid synthesis inhibition and degradation. Flavonoids are very effective at absorbing harmful wavelengths when located in the epidermis, but they are also effective free radical scavengers, protecting sensitive cell components such as genetic material, photosynthetic apparatus, or enzymatic machinery. We used a fish-eye lens from the perspective of the grapes to assess canopy porosity at the berry scale. In addition, samples were collected to characterize the effects of exposure at the cluster, berry, and half berry scale. Each of the four flavonoid groups measured by HPLC, anthocyanins, flavonols, flavan-3-ols, and proanthocyanidins showed a different dose-response relationship to berry exposure. This approach allowed us to see clearly three phases in their response: I) increased concentration, resulting from a predominant increase in biosynthesis; II) an optimal peak, resulting from a compensation point between synthesis and degradation; and III) decreased concentration, resulting from a higher rate of degradation compared to their synthesis biosynthesis. For instance, anthocyanins, which are highly desirable in red full-bodied wines, showed a very low compensation point between synthesis and degradation (<20% of canopy porosity). On the other hand, flavonols, which have a predominant role in mitigating excess radiation, had a much higher compensation point. These results suggest a need to revise the relationships between solar radiation and flavonoid synthesis induction, as this could make winegrape producers reconsider cultural practices in locations were light is not limiting.

Funding Support: Oakville Experimental Vineyard