Research progress of ground penetrating radar and electrical capacitance for in-situ non-destructive measurement of crop roots

Abstract: Improving root structure of crops is an effective way to solve the conflict between food production and environment. However, traditional root research methods (such as excavation) are time-consuming, labor-intensive, destroying the in-situ state of roots and point based, which make them difficult to meet the requirements to obtain root information for cultivation and breeding. Therefore, it is urgent to develop in situ non-destructive method for detecting the root. While, some progress has been made in the non-invasive detection of roots in developed countries, the related research is limited in China. Hence, this paper summarized the research status of in situ root non-destructive detection technology and then took the ground penetrating radar and electric capacitance method as examples, which can detect crop root in field non-invasively, to systematically review the principles, applications, unsolved questions and future research focus. Both ground penetrating radar and electrical capacitance can use linear models to predict root size (such as root length, diameter and biomass etc). Besides that, ground penetrating radar can also measure root parameters by nonlinear model and forward model. However, ground penetrating radar was extensively used to measure the coarse root not in fine root detection. So far, there is limited literature to summarize the model for ground penetrating radar to predict crop root parameters. As contrast, electrical capacitance method was widely used to measure crop roots. But in recent years, it was questioned that the feasibility of measuring root, which mainly consists of two aspects: as the leakage of electric current from the near root base, root system can't be detected; the stem and root system form a series circuit with the growth medium as the boundary, In the series circuit, the value of the capacitor is mainly determined by the value of the smaller capacitor, because the capacitance of the stem is less than that of the root, so the value measured by the capacitance method reflects the information of the interface between the stem and the substrate. Therefore, this paper focuses on: (1) the principle of ground penetrating radar and electrical capacitance for phenotyping roots and the summary as well as comparison of models for predicting root parameters; (2) the applications of ground penetrating radar and electric capacitance for measuring root system, analysis of main influencing factors for measuring root system and particular discussion about the electric capacitance feasibility in detecting roots; (3) the comparison between ground penetrating radar and electrical capacitance. The main focus of future research is to improve the accuracy of ground penetrating radar measurement of crop roots and to verify the feasibility of electrical capacitance method for root measurement. Hence, four suggestions were made for the future development: (1) improving the accuracy of ground penetrating radar in detecting crop roots; (2) clarifying the principle of electrical capacitance to detect root system; (3) measuring more root traits; (4) considering the knowledge of physics, mathematics, plant physiology, breeding and other related subjects in application. Furthermore, as the spatial variation of soil water moisture is consistent with the distribution of root system, rapid monitoring the spatial changes in soil moisture by ground penetrating radar and electrical resistivity tomography may provide information of root distribution. Therefore, this paper also briefly reviews the application of these two techniques in soil moisture detection. Overall, this review aims to provide important references for domestic researchers.