Detecting Water Stress in Trees Using Stem Electrical Conductivity Measurements.

Using time domain reflectometry (TDR), we studied stem water content (θ_stem), stem electrical conductivity (σ_stem), and their ratio for 220 d in stressed, installation-cured, living trees of four species. Lysimeter-grown mango (Mangifëra indica L.), banana (Musa acuminata Colla), date (Phoenix dactylifera L.), and olive (Olea europaea L.) were subjected to several types of mild (intensity and duration) water stresses simulating horticultural orchard irrigation practices. This study of living trees was triggered by our previous study accomplished in uncured, thawed, native, cut stem segments. We have confirmed in living trees our earlier findings that θ_stem reacts sensitively and within minutes to water stress. This response is the main driver of θ_stem changes, by far exceeding the salinity effect on σ_stem. Known irrigation rates, half-hourly tree weights from load cells, and frequent sampling of drainage solution for volume and salinity independently confirmed our findings. Relative to θ_stem, resistivity measurements have lower scatter because θ_stem-dielectric constant (e) relationships are exponential and θ_stem-resistivity relationships are linear. With resistivity, there is no need to match impedances among meter, cable, and probe, implying a larger flexibility in probe geometry, longer cables, and higher accuracy with shorter rods. There is a clear economic advantage in resistivity over C measurements. The linkage between stem resistivity and water status (designated as the linkage factor) for lysimeter plants, orchard trees, and cut stem segments demonstrates the potential in scheduling irrigation according to plant water needs with an inexpensive, direct, and simple resistivity measurement. [ABSTRACT FROM AUTHOR]