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

Highlights

• A dielectric sensor with interdigitated-electrodes (IE) is developed to measure StWC.

• Assess E-field distributions of the IE sensor and an existing dielectric sensor.

• The IE sensor has higher sensitivity to StWC measurement than the existing sensor.

• The IE sensor is less susceptible to stem diameter variation than the existing sensor.

• The IE sensor provides accurate, in-situ, measurement under water deficit condition.

Abstract

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.

Introduction

Plant stems function in water storage as well as in providing the pathway for transport of water and solutes between roots and shoots. Stem water content (StWC) is an important horticultural and ecophysiological indicator of plant water status (Brough et al., 1986; Tyree and Ewers., 1991; Holbrook, 1995; Link et al., 1998; Steppe et al., 2006; Dzikiti et al., 2007; Fernández and Cuevas, 2010; Zhou et al., 2015; Zhou et al., 2018; Sun et al., 2019). Water stored within the living tissues (newly differentiating xylem at the cambium and phloem) is responsible for the majority of stem diameter variation (Schepper et al., 2012). The hydroactive xylem tissue located deeper within the trunk is responsible for the majority of plant water content transport and storage, which does not necessarily have such a large impact on stem diameter fluctuations due to the empty (dead) cell nature of xylem vessels (Wullschleger et al., 1996; Hao et al., 2013; Matheny et al., 2017; Matheny et al., 2015). The variations of StWC can reflect irreversible radial growth of stem which is divided into elastic tension-driven and elastic osmotically driven changes (Mencuccini et al., 2017). The StWC can be well correlated with water potential of plant (Kursar et al., 2009), and the potential value of using water content for prediction of tree morality risk was discussed by Martinez-Vilalta et al. (2019).

Stem water content has been determined using a variety of sensing techniques (e.g. Zhou et al., 2015). Stem diameter sensors (Klepper et al., 1971; Link et al., 1998; Fernández and Cuevas, 2010) have been widely used to provide information about stem water storage because stems swell or shrink in relation to diurnal variation of stem water content. However, stem diameter data is only indirectly related to stem water content Fernández and Cuevas (2010). Dielectric sensors, such as TDR are regarded as invasive methods and signal perturbation caused by installation and release of cell contents could last for several weeks (Holbrook et al., 1992; Wullschleger et al., 1998; Lu et al., 2002; Nadler et al., 2003; Madurapperuma et al., 2009). A prominent advantage of external frequency-domain (FD) sensors is that their geometry is very adaptable, accelerating the development of various configurations for specific purposes (Sun et al., 2005). However, while the previously described inner fringing FD sensor (Zhou et al., 2015; Zhou et al., 2018) successfully measured water content of both herbaceous and woody plants, its output was highly susceptible to change in stem diameter when the StWC is constant (Zhou et al., 2015).

In the present study, we improve upon the external FD design to eliminate sensitivity to stem diameter. We characterize the volume of sensitivity (VOS) and accuracy of a novel interdigitated-electrodes (IE) dielectric sensor relative to an existing pair-strap-ring-electrodes (2RE) sensor, and directly contrast the two, along with concurrent measurement of stem diameter and stem water content during an irrigation cycle in three greenhouse-grown apple trees.