In-situ and Non-Invasive Measurement of Stem Water Content of Trees Using an Innovative Interdigitated-Electrodes Dielectric Sensor Less Susceptible to Stem Diameter Variation

Stem water content (StWC) of plants is an important parameter for assessing plant response to drought stress in arid and semi-arid areas and for irrigation scheduling. The measurement accuracies of in-situ non-invasive approaches are degraded by plant growth and by diurnal changes in stem diameter associated with water content. Here we develop a frequency-domain (FD) dielectric sensor operating at 100 MHz with an innovative interdigitated-electrodes (IE) probe design for measuring StWC. We characterize the performance of the IE sensor and compare it with a previously described pair-strap-ring-electrodes (2RE) FD sensor. Simulations and experimental measurements were conducted to assess the electric field distribution and volume of sensitivity (VOS) of each probe, the sensitivity of each probe to stem diameter and the accuracy of each sensor for determining StWC of three apple trees in a greenhouse environment. The simulation analysis and the measurement showed that the IE probe has a smaller but denser VOS than the 2RE probe. The sensitivity test showed that the new IE probe (0.85 mV mm(-1), R-2 = 0.7108) was less susceptible to stem diameter variation in comparison with the 2RE probe (32.83 mV mm(-1), R-2 = 0.9977). The observations in the greenhouse showed that the three apple trees (AT-1, AT-2 and AT-3) experienced a daily dehydration-rehydration cycle and the averaged midnight-to-predawn StWC gradually decreased without irrigation. According to the reference values of the maximum daily trunk shrinkage (MDS) reported by a previous study, both AT-2 and AT-3 may experience water deficit before the irrigation. Besides, the stem diameter variation decreased the measurement accuracy of 2RE sensor to be similar to 0.0410 cm(3) cm(-3) mm(-1). The stronger electric field intensity of the IE probe and its less susceptible to stem diameter variation make the new IE dielectric sensor an improved method for accurate in-situ measurement of diurnal stem water content.