Temporal extension of ripening beyond its physiological limits imposes physical and osmotic challenges perturbing metabolism in grape (Vitis vinifera L.) berries

Abstract

Fruit ripening can be delayed or hastened by chemical means to time harvest at desired dates; however, it is not clear if the ripening window can be extended merely by delaying harvest in nonclimacteric fruits such as grape to optimize fruit quality. In this study, we investigated the consequences of extending ripening known as “hang time” in viticultural parlance on primary and secondary metabolites typically sought for making wine. Field-grown grapevine cultivar Merlot was harvested at three different times to simulate extended ripening; the first date coincided with the commercial harvest and the latter two harvests 25 and 34days after the first harvest constituted extended ripening. Berry growth measured at weekly intervals showed a double sigmoidal growth trajectory interrupted by a lag phase. Physiologically possible highest total soluble solids (24 Brix) was observed at first harvest and coincided with osmotic potential of −3MPa, thereafter Brix increased during the extended ripening due to dehydration manifested as reductions in berry weight, volume, and a very negative osmotic potential (−6 to −7MPa). The osmotic stress resulting from solute concentration increased the pH whereas the titratable acidity and malic acid first declined and then increased as the berries dehydrated during the extended ripening. While the tartaric and oxalic acids declined, citric acid increased throughout the whole ripening period. Among the secondary metabolites, the skin and seed phenolics and seed tannins remained unaffected whereas the skin tannins, small polymeric pigments (SPP) and large polymeric pigments (LPP) increased throughout the extended ripening period. The peduncle underwent secondary growth as well as periderm formation with extensive accumulation of callose in secondary phloem sieve tubes and starch in the xylem parenchyma cells. The mesocarp showed loss of cell integrity especially in berries with extended ripening. The study clearly demonstrated that vine holding of ripe fruit beyond the attainment of maximum sugar levels does not extend ripening rather instill negative osmotic challenges.