Electrical capacitance estimates crop root traits best under dry conditions-a case study in cotton (Gossypium hirsutum L.)

Electrical capacitance (EC) is widely used to measure root traits especially in hydroponic and wet soil environments, but its feasibility was recently questioned due to the possible leakage of electrical current before entering the root system. To investigate whether the current can travel deeper in woody roots, EC was evaluated in cotton (Gossypium hirsutum L.) grown under different soil moisture conditions (two rounds of pot experiments, one PVC tube experiment and one field experiment). In these experiments, electrical capacitance of the root (ECroot) was recorded before and after severing roots, and immediately afterward, the roots were collected to measure root dry weight, surface area, length, and volume. The results showed that ECroot was significantly reduced after root cutting especially under dry conditions in the three experiments. The PVC tube experiment further demonstrated that root cutting did not significantly affect the changes in ECroot measured at different soil layers under dry conditions possibly suggesting that the current could flow into the deep roots under these conditions. Moreover, incorporating root tissue density and soil moisture into the prediction model can improve the predictive accuracy of ECroot. In contrast, ECroot was less affected by root cutting under wet soil conditions, indicating that it may not directly measure roots under these conditions. ECroot shows the potential to directly quantify root traits under dry conditions rather than wet conditions.