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, R² = 0.7108) was less susceptible to stem diameter variation in comparison with the 2RE probe (32.83 mV mm⁻¹, R² = 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 ~0.0410 cm³ 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.

植物干水含量（StWC）是评估干旱和半干旱地区植物对干旱胁迫的响应以及灌溉计划的重要参数。原位非侵入性方法的测量精度会因植物生长以及与水含量相关的茎直径的日变化而降低。在这里，我们开发了一种工作于100 MHz的频域（FD）介电传感器，并采用了创新的叉指电极（IE）探头设计来测量StWC。我们表征IE传感器的性能，并将其与先前描述的成对带状环形电极（2RE）FD传感器进行比较。进行了仿真和实验测量，以评估每个探针的电场分布和灵敏度（VOS）量，每个探针对茎直径的敏感性以及每个传感器在温室环境中确定三棵苹果树的StWC的准确性。仿真分析和测量结果表明，IE探针比2RE探针具有更小但更密集的VOS。灵敏度测试表明，新型IE探头（0.85 mV mm^-1，R ²  = 0.7108）与2RE探针（32.83 mV mm ^-1，R ²  = 0.9977）相比，不易受到茎直径变化的影响。温室中的观察结果表明，三棵苹果树（AT-1，AT-2和AT-3）每天都经历脱水-脱水循环，并且从午夜到黎明前的平均StWC在不灌溉的情况下逐渐降低。根据先前研究报告的最大每日树干收缩量（MDS）的参考值，AT-2和AT-3都可能在灌溉前出现缺水现象。此外，阀杆直径的变化使2RE传感器的测量精度降低至〜0.0410 cm ³  cm ^-3  mm ^-1。IE探头更强的电场强度及其不易受到茎直径变化的影响，使新型IE介电传感器成为改进的方法，可用于精确地日间测量茎干水分含量。