Establishing hazelnut stem water potential baseline to improve water management.

Introduction: Irrigation management in modern filbert ( Material and Methods: Vapor pressure deficit (VPD) was used as the environmental driver defining SWP variability under well-watered conditions. A large dataset of SWP-VPD pairs was collected during the 2024 growing season in two major hazelnut-producing countries, Italy (Piacenza) and Oregon (USA), and supplemented with measurements from previous trials. Results and discussion: The resulting baseline, derived from the upper 0.07 fraction of SWP values within VPD classes, revealed a stable and linear relationship between SWP and VPD. This baseline functions as an operational reference: SWP values close to the baseline indicate non-limiting soil water conditions, negative deviations reflect increasing water stress, whereas values exceeding the baseline may signal excessive soil moisture. Overall, the proposed SWP baseline provides a plant-based threshold that can be directly applied in field irrigation management to optimize timing and amount of water application and can be integrated with other plant-based indicators, such as baseline deviations or the Crop Water Stress Index (CWSI), within threshold-based or advanced irrigation decision-support frameworks, supporting more sustainable water use under increasingly variable climatic conditions.