Non-destructive Method for Root Elongation Measurement in Soil Using Acoustic Emission Sensors: II. Spatial measurement of single root elongation

A non-destructive method for measurements of spatial root elongation in soil, using acoustic emission (AE) sensors, was developed. Growing roots passing in close proximity to soil particles generate AE pulses, which are detectable as counts by AE sensors. Previously, vertical maize (Zea mays L.) root elongation in soil was successfully measured using AE sensors. The method was expanded to measure spatial maize root elongation in a rectangular stainless steel container. Three AE sensors were placed vertically at 15 mm intervals on each of the four side walls of the container. Spatial root tip position was expressed by the three axes (x, y, z). Relative AE counts on the x axis and y axis (Rxp and RyP) were obtained from three sensors on the same and opposite planes as a fraction of total AE counts on both sides of this plane. RxP and RyP were linearly related to root tip positions on the x axis and y axis, respectively. The relationship was expressed by the equation : Y = 0.740 - 0.0420X (r = 0.861**) where X is the actual distance from the sensor to root tip position and Y is RxP or RyP. Spatial root tip positions can be estimated by the relative AE counts using this regressing equation. Thus, spatial root tip positions were expressed as a function of time. The trajectory of root tip position estimated by AE sensors was consistent with that obtained from actual measurements after excavation of the root from soil. This method can be used to give continuous and three-dimensional information of root tip movements, the rate and direction of root elongation.