Spatially resolved root water uptake determination using a precise soil water sensor.

To answer long-standing questions about how plants use and regulate water, an affordable, non-invasive way to determine local root water uptake is required. Here, we present a sensor, the Soil Water Profiler (SWaP), which can determine local soil water content (θ) with a precision of 6.10-5 cm3cm-3, an accuracy of 0.002 cm3cm-3, a temporal resolution of 24 minutes and a one-dimensional spatial resolution of 1 cm. The sensor comprises two copper sheets, integrated into a sleeve and connected to a coil, thereby forming a resonant circuit. A vector network analyzer, inductively coupled to the resonant circuit, measures the resonance frequency, against which θ was calibrated. The sensors were integrated into a positioning system, which measures θ along the depth of cylindrical tubes. When combined with modulating light (4-hour period) and resultant modulating plant transpiration, the SWaP enables quantification of the component of root water uptake distribution that varies proportionally with total plant water uptake, and distinguishes it from soil water redistribution via soil pores and roots. Additionally, as a young, growing maize (Zea mays) plant progressively tapped its soil environment dry, we observed clear changes in plant-driven root water uptake and soil water redistribution profiles. Our SWaP setup can measure the root water uptake and redistribution of sandy soil water content with unprecedented precision. The SWaP is therefore a promising device offering new insights into soil-plant hydrology, with applications for functional root phenotyping in non-saline, temperature controlled conditions, at low cost.