Physiological Responses Consistent with Near-Isohydric and Anisohydric Behaviour in Grapevine Cultivars Șarba and Fetească Neagră (Vitis vinifera L.) Under Semi-Arid Conditions

Climate change strongly impacts the physiological processes in the grapevine. Two Romanian autochthonous cultivars, Șarba and Fetească neagră (Vitis vinifera L.), grown under semi-arid conditions in 2024 (De Martonne Index: 23.6; Hydrothermal Coefficient: 0.75), showed different physiological behaviours. Leaf gas exchange parameters (A, gs, E, Ci) and water use efficiency (WUE) were assessed at three phenophases (flowering, berry growth, and véraison), while photosynthetic pigment indices (Chl a/b, Chl/C+X) and leaf dry matter content (dm) were additionally determined at harvest maturity. Multivariate analysis consistently separated the two cultivars into distinct physiological groups: responses consistent with near-isohydric behaviour in Șarba and anisohydric behaviour in Fetească neagră, based on gas-exchange parameters interpreted within the established isohydric/anisohydric framework, as direct water potential measurements were not performed. Șarba exhibited a water-conserving strategy at véraison—characterised by early stomatal closure, high WUE, and maintained chlorophyll—protecting vine water status at the cost of reduced leaf carbon assimilation. Fetească neagră, by contrast, kept its stomata progressively open, sustaining high gas exchange rates in a pattern consistent with progressively declining shoot water potential. Berry sugars are expected to concentrate passively late in ripening, while severe deficit risks berry shrivelling and, under prolonged drought, premature senescence of basal leaves, to our knowledge, a phenomenon not previously reported for this cultivar. These findings support cultivar-specific management, with implications for irrigation scheduling and varietal selection under climate change.