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Two methods for monitoring stem water content in the arborescent palm, Sabal palmetto by determining its dielectric constantwere compared. The first approach used an oscillating circuit whose frequency (40 to 70 kHz) was determined by a parallel-plate capacitor that sandwiched a portion of the stem. The second technique was based on measurement of the velocity of an electromagnetic pulse (frequency range of 500 kHz and 1 GHz) propagating within a wave-guide embedded in the stem (Time-Domain Reflectometry, TDR). There was basic agreement in the apparent dielectric constant as determined by the two techniques; both resulted in values of approximately 90 when the plant was fully hydrated, falling to values near 50 when water was withheld for one week. The capacitance technique was non-invasive, but was influenced by temperature fluctuations, and we were unable to calibrate it accurately against stem volumetric water content. Insertion of TDR probes did not lead to tissue damage and determination of an empirical relationship to volumetric water content allowed quantitative estimates of stem water content. Sensitivity of TDR to small changes in stem water content was restricted by the fact that attenuation of the pulse within the stem necessitated the use of short (0·125 m) wave guides. Despite this, during periods of high transpiration (>10kg plant−1 d−1) bi-hourly changes in stemmoisture content were detectable.
Holbrook et al. (Wed,) studied this question.