The growing season impacts the accumulation and composition of flavonoids in grape skins in two-crop-a-year viticulture.

The influence of growing season (winter vs. summer) on the flavonoid accumulation and composition was studied in the skins of three grape cultivars for two consecutive years under a two-crop-a-year viticulture practice in Southwest China. The total anthocyanin, flavonol and flavan-3-ol contents in winter berry skins were significantly higher than those in summer berry skins for 'Kyoho' and 'Muscat Hamburg'. Reversely, the content of anthocyanin in 'NW196' winter berry was lower than summer berry. However, the percentage of diglycosylated, trihydroxylated, methylated, and acylated anthocyanins, trihydroxylated and methylated flavonols, and flavan-3-ol polymers were higher in the summer berry skins than the winter berry skins among all the three grape cultivars. Winter climatic conditions were favorable to flavonoid accumulation for the non native grapes 'Kyoho' and 'Muscat Hamburg', while the summer climatic conditions were beneficial to anthocyanin accumulation for 'NW196' that has 50% genetic background from a local wild grape species Vitis quinquangularis. These seasonal variations of flavonoid accumulations and compositions in the grape skins were primarily contributed by different climatic factors, such as temperature, solar radiation, and rainfall.