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SOIL PHYSICS

Detecting Water Stress in Trees Using Stem Electrical Conductivity Measurements

^a Inst. of Soil, Water, and Environ. Sciences, ARO, Bet Dagan, 50250 Israel
^b Gilat Research Center, Mobile Post Negev, 85280 Israel
^c Institute of Agricultural Engineering, ARO, Bet Dagan, 50250 Israel
^d Environmental Group, HortResearch, Private Bag 11-030, Palmerston North, New Zealand

^* Corresponding author (vwnad{at}volcani.agri.gov.il).

Using time domain reflectometry (TDR), we studied stem water content (_stem), stem electrical conductivity (_stem), and their ratio for 220 d in stressed, installation-cured, living trees of four species. Lysimeter-grown mango (Mangifera 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 _stem reacts sensitively and within minutes to water stress. This response is the main driver of _stem changes, by far exceeding the salinity effect on _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 _stem, resistivity measurements have lower scatter because _stem–dielectric constant () relationships are exponential and _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 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.

Abbreviations: DOY, day of the year • LF, linkage factor • TDR, time domain reflectometry • , volumetric water content • , electrical conductivity • , dielectric constant