Quantifying Root Water Uptake Locally Using the Soil Water Profiler

Several shoot and root traits and their response to water availability determine water use of a plant. This requires systematic investigation of root and soil water fluxes under various drought scenarios, which has proven difficult because they occur simultaneously. We developed a highly precise soil water sensor that can quantify soil water depletion in a column with one cm spatial resolution (equivalent to 51.5 mL) and a precision of 3 μL (=6x10^-5 cm³/cm³). Because of this high sensitivity short-term modulations, caused by transpiration, in the root water uptake (RWU) can be followed. Since macroscopic redistributive soil water flow (rSWF) is normally a much slower process compared to RWU, the two phenomena can be separated based on time scale, which is easy to confirm with forward simulations of the Richards’ equation. When we used our Soil Water Profiler (SWaP) on maize plants growing under fluctuating light conditions we were able to quantify the depth profile of root water uptake for several plants over several days, while the soil dried out slowly. This also yielded effective rooting depth, below which plants do not take up water. Comparing the RWU profile to the root length density, measured with MRI, our data suggests that for a young maize plant the axial conductance (proto-xylem) of the main root may restrict deep water uptake.

The SWaP is a user-friendly, low-cost, system that can be used to quantify local soil water content with extremely high precision. Because of this it is now possible to quantify and follow the changes in the depth profiles of root water uptake over different plant growth time points and correlate these to the soil water content. Thereby, we are offering a new way to investigate the response of plants to varying soil water conditions.