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Grape ripening is affected by a number of environmental factors, and of these light and nitrogen (N) are of particular importance. Using pot-grown Cabernet Sauvignon (Vitis vinifera L.) vines, we studied the effects of N availability at bloom (0.34, 1.7, or 3.4 g N per plant as NH4NO3) and light intensity at veraison (3 weeks at 100%, 20%, or 2% sunlight, using shade cloth) on the accumulation of total phenols, anthocyanins and flavonols in ripening grapes. Total soluble solids in the pulp were closely correlated with total phenols (r2 = 0.91***), total anthocyanins (r2 = 0.90***) and total flavonols (r2 = 0.52***) in the skin in all treatments. High rates of N supply delayed the accumulation of phenolic compounds, particularly flavonols, in the grape skin at veraison. However, the differences between N treatments generally decreased towards fruit maturity. Low light intensity at veraison restricted phenolics accumulation even more than did heavy N supply and the effects of low light were most severe in vines with high N status. However, ripening was not stopped entirely by light intensities as low as the light compensation point of photosynthesis. Moreover, removal of the shade cloth partly restored the rate of phenolics accumulation in the berry skin. All five anthocyanins started accumulating concurrently at the inception of ripening. However, while peonidin-, malvidin-, and cyanidin-3-glucosides were the major pigments at veraison, malvidin- and delphinidin-3-glucosides became most abundant towards maturity. The accumulation of cyanidin-3-glucoside was most strongly influenced by prevailing environmental conditions, while malvidin-3-glucoside was the least affected. Thus, the proportions of individual anthocyanins were most equal at low N status and high light intensity, whereas the percentage of malvidin-3-glucoside increased with high rates of N fertilization and became predominant under low light intensity. This is important for red winemaking, because the grape skins9 anthocyanin profile determines the color potential of the resulting wine. Excessive N fertilization can decrease wine quality, particularly when light conditions during grape ripening are poor.
Keller et al. (Thu,) studied this question.