The rapidly changing climate is creating challenges for the selection and management of woody perennial crops. For North American (NA) cultivars of hazelnut (Corylus avellana), there is insufficient information on water stress management to maintain physiological performance and optimize productivity under limited soil water availability. Current plantings of NA hazelnuts are predominantly comprised of cultivars resistant to biotic stress (e.g., Eastern Filbert Blight) such as ‘Jefferson’ and ‘Yamhill’ cultivars, but their responses to abiotic stressors exacerbated by climate change is unknown. Our research objectives were to: 1. identify cultivar-specific physiological thresholds in response to water stress such as negligible leaf gas exchange (i.e., stomatal closure) and onset of leaf wilting (i.e., cell turgor loss) for phenotyping in greenhouse conditions; and 2. relate vapor pressure deficit to plant water status in order to generate a water-potential baseline capable of differentiating between atmospheric and soil moisture impacts on water stress in field conditions. Using the water potential (Ψ) curve (WPC) method, stomatal closure was initiated at less negative Ψ in ‘Jefferson’ (-0.85 MPa) compared to ‘Yamhill’ (-1.1 MPa). Similarly, turgor loss was found to occur at less negative Ψ in ‘Jefferson’ (-1.26 MPa) compared to ‘Yamhill’ (-1.48 MPa). These cultivar-specific differences were confirmed with direct measurements of stomatal conductance using a porometer and an evaluation of turgor loss point using the pressure-volume curve method. In the field, we established a water potential baseline to distinguish between the effects of soil moisture and vapor pressure deficit on Ψ. Our field results found a deviation from baseline of -1.0 MPa resulted in stomatal closure in Yamhill, which was consistent with our prediction from the WPC. ‘Yamhill’ trees that had Ψ on average -0.68 MPa below baseline over the growing season were also observed to have 34% smaller nuts, 46% higher shell-to-kernel ratio, and an estimated 50% of total in-shell yield. Upcoming research will seek to replicate results experimentally with both cultivars. In summary, our results indicate that the WPC is a valid tool for physiological phenotyping and preliminary results suggest that thresholds from the WPC provide viable cultivar-specific targets for improving irrigation management in hazelnuts. These results highlight methods to help determine sustainable irrigation management targets that can help conserve water resources strained by climate change while also maintaining plant productivity.
