Stem water potential measurements obtained using standard methodologies diverge under extreme drought in tomato and grapevine

Stem water potential (ψstem) is one of the most common metrics used to define plant water status. Accurate measurement of ψstem is therefore essential to quantify critical plant physiological processes in response to water availability. In theory, leaf and stem water potential equilibrate when leaf transpiration is prevented, and leaves and stems are hydraulically connected. Therefore, many studies quantify ψstem by measuring equilibrated leaves with a pressure chamber. However, leaf tissue damage occurring during dehydration due to xylem cavitation events may impact the accuracy of this indirect measurement of ψstem. Here, we present two case-studies in which ψstem close to the lethal threshold for leaf mesophyll tissue led to high discrepancies between pressure chamber measurements of equilibrated leaves and direct psychrometric measurements. We dehydrated (i) whole plants of an herbaceous species, tomato, under typical diurnal cycles in a glasshouse, and (ii) branches of a woody species, grapevine, under laboratory conditions. Dehydration beyond the point of xylem cavitation is expected to lead to leaf water potential falling below ψstem, but rather we observed that once ψstem declined to values expected to cause considerable loss of leaf xylem function, indirect ψstem values from leaves (ψleaf-eq) remained higher than direct ψstem measured with psychrometers (ψstem-PSY). A decline in the osmotic potential of the leaf xylem sap (ψπ-sap) was consistent in both species, possibly indicating that the contents of disrupted leaf cells contributed to that decline. These results demonstrate that, at least for tomato and grapevine species, caution should be exercised when using the pressure chamber at water potential levels that may induce leaf tissue damage and support a method for approximating these levels by leaf xylem sap extraction.