Light and Temperature Effects On Phenolics in Dark- Skinned Grapes

During the past decade we refined our understanding of the effects of solar radiation and temperature on the accumulation of phenolic compounds in grapes in the field, particularly dark-skinned cultivars used for red wine. The work was precipitated by nearly universal prescriptive advice in the previous decade for growers to increase the exposure of the grapevine's fruiting zone to solar radiation, especially during ripening. However, in regions with hot summers and clear skies, growers and wine makers reported deleterious consequences of these practices on fruit quality. Our experimental approach included a field system to control dynamically the temperature of individual berry clusters on mature vines. In three separate studies we deployed up to 10 combinations of berry temperature and exposure to solar radiation, then assessed by HPLC methods the outcome on the classes of phenolic compounds that are most of interest to the grape and wine industry: flavonol-glycosides, anthocyanins, and proanthocyanidins in the berry skins. Concentrations of flavonol-glycosides increased with exposure to solar radiation, but these compounds were insensitive to field temperatures. High berry temperatures led to lower total concentrations of anthocyanins, except that malvidin-based anthocyanins, the predominant form, did not vary in concentration. Thus malvidin-based anthocyanins comprised a larger proportion of the total anthocyanin pool at higher berry temperature. A combination of low light and high berry temperature also decreased the total concentration of anthocyanins. At equal temperature, shaded and sunlit berries differed: exposure to solar radiation decreased the proportion of anthocyanins comprised of acylated derivatives and increased the proportion formed along one branch of the biosynthetic pathway (dihydroxylated anthocyanins). The consequence for wine quality from these compositional changes is not yet understood. Proanthocyanidins accumulate in the berry skin before the onset of ripening, with compositional changes occurring during ripening. Proanthocyanidins are otherwise known as "condensed tannins" and impart color stability and astringency to red wine. When the diurnal fluctuation in berry temperature was reduced ("damped") by cooling the berries during the day and heating them at night, the onset of ripening was accelerated. Higher berry temperature was positively associated with higher concentrations of skin proanthocyanidins at the onset of ripening.