Effect of auxin treatment on delaying maturation of grape cultivars in the Valpolicella viticultural area

Abstract

The temperature increase related to ongoing climate changes is causing a progressive anticipation of the ripening time, negatively affecting grape quality at harvest. Various short-term adaptation strategies have been proposed to mitigate these effects. In this study, we collected data from multi-year auxin treatment trials aimed at delaying the ripening of some of the most important grape varieties cultivated in Valpolicella (Verona, Italy). We evaluated cultivar-specific responses in terms of ripening dynamics and berry composition. Additionally, we analyzed the effects of auxin on berry transcriptome rearrangements. The trials were conducted over several years in Guyot-trained vineyards located in Valpolicella. Clusters of cvs. Corvina, Corvinone and Rondinella were treated with 1-naphthaleneacetic acid (NAA) before veraison.

The dynamics of the main technological parameters were monitored from the time of treatment until harvest. A genome-wide gene expression analysis was performed on berry samples collected throughout the ripening period, and the genes modulated by the treatment at different time points were identified. The treatment of grape clusters with auxin strongly impacted the ripening dynamics, postponing sugar and anthocyanin accumulation and slowing the decrease in acidity, albeit with marked differences related to the grape variety.  In some years, a positive re-balancing between sugars and anthocyanins was observed for cv. Corvina. However, for cv. Corvinone, the delay in ripening was consistently excessive, resulting in treated grapes failing to achieve acceptable maturity levels. Molecular analyses showed that the entire ripening program was postponed, and the longer stay of NAA-treated clusters on the vine appears to have had some effects on berry secondary metabolism. Moreover, we observed a strong modulation of numerous auxin-related genes, as well as genes related to gibberellin, ABA, ethylene, and brassinosteroids. This modulation began a few days after the treatment, confirming previous indications that crosstalk of multiple hormone signals controls grape ripening initiation. These results provide valuable insights for vineyard management strategies aimed at adapting viticulture to climate change in those areas where the grape varieties are part of the terroir paradigm.