Inner fringing-field capacitance sensor for measurement of stem water content

Stem water content and sap flow rate are regarded as two important parameters associated with water transport through plant stem. Gamma-ray instruments, although often highly accurate and noninvasive, carried a potential risk of radiation exposure restricting their applications. The MRI method is noninvasive and safe, but it is costly and impractical for long-term monitoring of plant water status in natural circumstances. Generally TDR and FD sensors are often regarded as invasive methods because both techniques are depended on inserting two or more waveguide lines into the stem, causing some tissue damages in the stem. For this purpose, a novel sensor was presented based on the inner fringing field of capacitor. The sensor comprises a pair of metallic electrodes designed to wrap around stems likes an upper and a lower strap-ring. The strap-rings were connected to a high frequency oscillator operating at 100 MHz. Because the dielectric property of stems is closely correlated with the amount of water in the stem, stem water content can be measured without causing stem tissue damage. To verify its technical performance, firstly, three different physical tests were conducted in the laboratory. The results confirmed that both sensitivity and linearity of the sensor were satisfactory. Then these sensors were tested with three potted apple trees grown in a greenhouse. By using the proposed technique, the actual water recharge and discharge occurred between the stem tissues and the water pipes in the stem were successfully observed. Furthermore, the turgor breakdown and recovering process were observed when these apple tree samples suffered from water deficit. In general, all measured data were clearly interpretable to the known knowledge of plant water relation © 2016, Chinese Society of Agricultural Machinery. All right reserved.