Microtensiometers Accurately Measure Stem Water Potential in Woody Perennials

Stem water potential (Ψ_stem) is considered to be the standard measure of plant water status. However, it is measured with the pressure chamber (PC), an equipment that can neither provide continuous information nor be automated, limiting its use. Recent developments of microtensiometers (MT; FloraPulse sensors), which can continuously measure water tension in woody tissue of the trunk of the tree, can potentially highlight the dynamic nature of plant water relations. Thus, this study aimed to validate and assess the usefulness of the MT by comparing the Ψ_stem provided by MT with those same measurements from the PC. Here, two irrigation treatments (a control and a deficit treatment) were applied in a pear (Pyrus communis L.) orchard in Washington State (USA) to capture the full range of water potentials in this environment. Discrete measurements of leaf gas exchange, canopy temperature and Ψ_stem measured with PC and MT were made every two hours for four days from dawn to sunset. There were strong linear relationships between the Ψ_stem-MT and Ψ_stem-PC (R² > 0.8) and with vapor pressure deficit (R² > 0.7). However, Ψ_stem-MT was more variable and lower than Ψ_stem-PC when Ψ_stem-MT was below -1.5 MPa, especially during the evening. Minimum Ψ_stem-MT occurred later in the afternoon compared to Ψ_stem-PC. Ψ_stem showed similar sensitivity and coefficients of variation for both PC and MT acquired data. Overall, the promising results achieved indicated the potential for MT to be used to continuously assess tree water status.