Consecutive Moderate and Severe Drought Stresses Affect Chlorophyll Fluorescence and Non-structural Carbohydrates Dynamics in Grapevine Leaves

Drought events represent a growing challenge for agriculture in the Mediterranean region, particularly for Vitis vinifera, a species with economic and cultural significance. This study evaluates the effects of two subsequent drought stress events on grapevine cv. Sauvignon blanc, combining physiological and biochemical approaches. The trial was conducted in a semi-controlled tunnel from mid-June to mid-July 2024 on potted vines. Plants were divided into well-watered and drought groups. Drought-stressed vines underwent two drought cycles: the first reaching -1.3 MPa stem water potential, followed by rewatering, and the second with either moderate (-1.3 MPa) or severe (-2.5 MPa) drought before another rewatering. Stomatal conductance, chlorophyll fluorescence, gas exchange, and nonstructural carbohydrate levels were measured to assess physiological responses. Grape bunches were analyzed for C13/C12 isotope ratios at harvest. Significant physiological and biochemical differences were observed between the first moderate drought and both the second moderate and severe drought, highlighting distinct plant responses to water stress and rewatering. In the second cycle, vines subjected to severe drought showed significantly reduced photosynthetic efficiency compared to those under moderate drought, suggesting a decline in resilience. Chlorophyll fluorescence data indicated sustained photoinhibition after severe drought, while differences in nonstructural carbohydrate levels between treatments and across different times of the day revealed shifts in carbon metabolism. Carbon isotope composition confirmed the effect of double water stress. These results highlight the grapevines' capacity for physiological adaptation to repeated drought while also indicating the potential accumulation of negative effects if stress becomes excessive or prolonged.