A new portable electrical impedance tomography system for measuring two-dimensional stem water content distribution

Abstract

Stem water content (StWC) is an important parameter for characterizing plant–water relations. Most sensors and methods can measure only one-dimensional StWC, and those measuring two-dimensional (2D) StWC distributions are expensive. In this study, we developed a low-cost and portable electrical impedance tomography (EIT) system to measure 2D StWC distributions in vivo. The performance of the EIT system was tested in a laboratory setting. An analog stem made of gypsum and two true stems cut from apple trees were used to calibrate the relationship between the average StWC (θv¯) and average change in electrical conductivity (Δσ¯) determined by the EIT system during the stepwise drying procedure. Evaporation experiments were conducted in the laboratory with hollow analog and cut stems to detect variations in StWC (Δθv) and its 2D distribution. Finally, the EIT system was installed in an apple tree for the in vivo monitoring of the diurnal variation of the 2D StWC distribution. The results indicated that the optimal measurement frequency of the EIT system was 47 kHz. After correcting for the effect of temperature, the average relative error of the measurement was reduced (approximately 80.21%). The effective height of the volume sensitivity of the EIT system was approximately 6 cm. The relationship between θv¯ and Δσ¯ showed a strong positive correlation, with all R2 values higher than 0.9808 for the analog stem and two cut stems. Moreover, the actual and EIT-estimatedθv¯ of the analog and cut stems during the evaporation experiments agreed well (R2 = 0.9367 and 0.9610, respectively), which validated the feasibility of the EIT system for StWC measurements. Furthermore, the in vivo EIT-estimated Δθv distribution of the apple tree was highly consistent with the regulation of diurnal variation in StWC in a greenhouse under irrigation and drought stress conditions. The new portable EIT system was able to measure the 2D StWC distribution and has great potential for application in drought monitoring of plant in the near future.