Leaf seasonal osmotic adjustment is not driven by temperature or water deficit

Osmotic adjustment (OA), the active accumulation of osmolytes in plant cells, plays a crucial role in plant-drought response. OA occurs throughout the growing season, but the seasonal effects of temperature and water status on OA remain unclear. We investigated grapevine (Vitis vinifera) leaves' regulation of osmotic potential over time, across different temperatures and water deficit conditions and the contributions of sugars and cations driving these changes. Potted grapevines were grown under greenhouse-controlled conditions at 20, 25 and 30°C, with half of the vines being exposed to mild water deficit (ca. -1 MPa) for 35 days. Leaf osmotic potential at full turgor (π₁₀₀) was measured via osmometer, while sugar and cation concentrations were quantified by HPIC. Furthermore, π₁₀₀ was also monitored in field-grown vines. Our findings confirmed seasonal OA in grapevine, with a progressive decrease in π₁₀₀ due to solute accumulation. However, the adjustment rate was not significantly influenced by temperature or mild drought, although both factors affected absolute π₁₀₀. Sugars and cations accounted for 54.5% of the π₁₀₀, with little variation over the season, suggesting OA is not driven by a single osmolyte. These findings support the idea that plants from Mediterranean/temperate climates follow a pre-programmed seasonal OA strategy, largely independent of environmental variability.