Water absorption into stems affects the measurement of vulnerability curves as a function of plant water status

Abstract

Xylem hydraulic properties are of great significance for plant growth and performance under drought. The ability of plants to avoid drought-induced cavitation and loss of hydraulic conductivity (K) can be characterized with vulnerability curves (VC). A VC describes the sigmoidal relationship between percentage loss of K (PLC) and xylem water potential (ψxyl). The ψ at 50% loss of conductance indicates a commonly used threshold for detrimental embolism (P50). The slope (b) represents cavitation resistance. The standard hydraulic method to determine VC's requires the measurement of water flow rate (WFR) per pressure gradient through stem segments, either by measuring outflow from the stem gravimetrically or inflow using a flow meter. In a comparative study using both measurements of inflow and outflow in asparagus stems, we found considerable disparities in the resulting shapes of VCs (P50 and b). We hypothesized that water uptake of stem tissue occurs during the pressure-driven water transit, particularly at low water potential and that differences in the initial K might result from measurements of inflow or outflow. To determine whether water uptake of stem tissue occurs during K measurements of asparagus plants, we tested for effects of ψxyl on the initial inflow and outflow K at different pressure gradients and investigated if passive water uptake can be estimated by extrapolation from the linear regression between WFR and pressure gradient based on K at two pressures. Initial K differed significantly between inlet and outlet measurements at low ψxyl, whereas maximum K did not, providing evidence of water uptake during transit through droughted stems. The resulting parameters P50 and b, and thus the shape of VCs, differed as well. The extrapolation resulted in the first estimate of passive water uptake, leading to a convergence of the VC at inlet and at outlet. We conclude that differences between in- and outflow may play a major role in K measurements.