Dissecting Seed Proanthocyandin Composition and Accumulation under Different Berry Ripening Process in Wine Grapes

Grape berry proanthocyandin (PA) mainly exists in the skin and seeds. Its content and composition determine the intensity of bitterness and astringency. Affected by global warming, the world's wine-producing regions, in particular in dry-hot regions such as western China, are facing the problem of unsynchronized berry ripening and seed ripening. Therefore, it is urgent to understand the influence of berry ripening progression on the composition and accumulation of seed PA, ultimately providing strategies for grape harvest decisions. In this paper, Vitis vinfera L. cv. Cabernet sauvignon and Marselan grapes from four sub-regions with different maturation processes were used as experimental materials to study the changes of soluble and insoluble PA contents as well as differences in their composition and mean degree of polymers (mDP) in seeds. The results showed that compared with 'Cabernet sauvignon' seeds, the mDP of soluble and insoluble PA were higher in 'Marselan' seeds. Both varieties showed that the grape berry, with the fastest sugar accumulation, had relatively high soluble PA content in seeds and a high content of (-)-epigallocatechin-3-gallate and (-)-epicatechin in the seed PA composition units. In contrast, the 'Cabernet sauvignon' grapes from the YQ vineyard exhibited the slowest sugar accumulation speed among the four studied vineyards, and their seed PA had the highest mDP and the lowest proportion of (-)-epigallocatechin-3-gallate in the composition units when commercially harvested. According to the results, it is suggested that a faster maturation process would bring about higher levels of bitterness composition, such as (-)-epigallocatechin-3-gallate in seed PA, which is not conducive to the formation of good-tasting tannins.