Mitigating Water-Deficit Stress in Mediterranean Grapevines: Field Responses of Agiorgitiko and Assyrtiko to a Proline-Rich Yeast Derivative.

Rising temperatures and increasing drought frequency threaten grapevine physiological performance, yield, and berry quality in Mediterranean vineyards. This study evaluated the responses of two field-grown Greek Vitis vinifera L. cultivars, the red Agiorgitiko and the white Assyrtiko, to contrasting water regimes and foliar application of a proline-rich yeast derivative over two consecutive growing seasons. Five field treatments were included: a fully irrigated untreated reference (C), deficit-irrigated grapevines with or without yeast application (DI-Y and DI-NY), and rainfed grapevines with or without yeast application (NI-Y and NI-NY). The four water-limited treatments constituted a 2 × 2 factorial subset for formal testing of water condition, yeast application, and their interaction. Vegetative growth and physiological parameters were assessed in 2022, whereas yield and berry composition were evaluated in 2022 and 2023. In Agiorgitiko, significant water × yeast interactions were detected for primary leaf area and shoot length and, during morning measurements, for leaf assimilation rate, transpiration, stomatal conductance, and intrinsic water use efficiency, indicating water condition-dependent responses to the yeast derivative for these traits. In Assyrtiko, such interactions were less evident for vegetative and physiological traits but were pronounced for several berry traits and for total soluble solids. Yield did not show a significant water × yeast interaction in either cultivar, despite numerical increases in NI-Y relative to NI-NY. Leaf temperature responses were mainly associated with yeast and date-dependent effects rather than a consistent water × yeast interaction. Overall, the response to the proline-rich yeast derivative was cultivar- and trait-dependent, supporting its potential as a complementary tool under water limitation while emphasizing the importance of stress level and seasonal conditions.