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 to separate soil from atmospheric water stress when predicting physiological thresholds using stem water potential.