Seasonal dynamics of water and sugar compartmentalization in grape clusters under deficit irrigation

Abstract

Water stress triggers functional compartmentalization in grapevines, influencing how resources are allocated to different plant organs. However, comprehensive insights on how water availability affects xylem functionality in each specific grape berry organ remain scarce. This study aims to explore the impact of irrigation regimes (different water stress levels) on hydraulic adjustments in berry clusters of ‘Touriga Nacional’ (TN) and ‘Syrah’ (SY) varieties, at key seasonal growth stages. Plants from a commercial vineyard were exposed to three long-term irrigation treatments over five consecutive years: full irrigation (FI, 100% ETc); deficit irrigation (DI, 50% FI); and no irrigation (NI, rain-fed).

Measurements of stomatal conductance (gs), predawn leaf water potential (ψpd), and hydraulic conductance of berry cluster (Khrachis+pedicel and Khrachis)) were taken at four phenological phases during 2022 growing season: peppercorn-size (PCS), pea-size (PS), veraison (VER), and full maturation (FM). Additionally, the gene expression of ten aquaporins (AQPs) and five sugar transporters was analyzed in the rachis, pedicel, berry pulp and skin.

The results indicate that Kh varies depending on the variety, the specific grapevine organ and phenology. Following PCS, Khcluster dropped sharply in the three irrigation treatments and in both varieties. In TN, after VER, Khcluster significantly rose across all treatments, despite the significant differences in ψpd. In SY, on the contrary, after PCS, Khcluster maintained relatively stable values across all phenological stages, and irrigation treatments, despite the significant differences in ψpd, except in FI plants at PS, AQPs were regulated differently depending on variety, treatment, organ, and phenological stage. In general, low gene expression level was observed in the rachis of both varieties. The most pronounced treatment-related differences were seen at VER and FM. Irrigation influenced sugar transporter activity in both leaves and berry clusters. Notably, the sugar transporter VviSWEET14 was up-regulated in both DI and NI conditions from PCS onward in leaves and berries, while other hexose transporters were mainly down-regulated. The influence of hydraulic and molecular mechanisms in the ecophysiology of both varieties will be thoroughly explored, along with its impact on irrigation management.