Extension of the triphasic water potential curve: Accounting for air vapor pressure deficit under soil water stress

For woody plants subjected to soil dehydration, physiological thresholds of drought-induced stomatal closure (i.e. minimum stomatal conductance, gs-min) and turgor loss point (TLP) can be derived from the triphasic relationship of stem water potential (Ψ) at midday and predawn, i.e. the "Ψ-curve". In this study, we provide an extension of the Ψ-curve approach that accounts for vapor pressure deficit (VPD). Experimental data were collected in a greenhouse for potted hazelnut (Corylus avellana) trees (varieties "Jefferson" and "Yamhill")-a species known for its VPD-sensitivity. Consistent with the original Ψ-curve, the "VPD-adjusted" Ψ-curve exhibited a triphasic shape. Predicted thresholds of Θ1' and Θ2' were comparable to independent measures of gs-min (-0.86 "Jefferson"; -1.16 MPa "Yamhill") and TLP (-1.76 MPa "Jefferson"; -2.06 MPa "Yamhill"), respectively. In conclusion, the extended Ψ-curve approach allows the separatation of soil from atmospheric water stress when predicting physiological thresholds using stem water potential.