Visualization of root water uptake: Quantification of deuterated water transport in roots using neutron radiography and numerical modeling

Our understanding of soil and plant water relations is limited by the lack of experimental methods to measure water fluxes in soil and plants. Here, we describe a new method to noninvasively quantify water fluxes in roots. To this end, neutron radiography was used to trace the transport of deuterated water (D₂O) into roots. The results showed that (1) the radial transport of D₂O from soil to the roots depended similarly on diffusive and convective transport and (2) the axial transport of D₂O along the root xylem was largely dominated by convection. To quantify the convective fluxes from the radiographs, we introduced a convection diffusion model to simulate the D₂O transport in roots. The model takes into account different pathways of water across the root tissue, the endodermis as a layer with distinct transport properties, and the axial transport of D₂O in the xylem. The diffusion coefficients of the root tissues were inversely estimated by simulating the experiments at night under the assumption that the convective fluxes were negligible. Inverse modeling of the experiment at day gave the profile of water fluxes into the roots. For a 24-d-old lupine (Lupinus albus) grown in a soil with uniform water content, root water uptake was higher in the proximal parts of lateral roots and decreased toward the distal parts. The method allows the quantification of the root properties and the regions of root water uptake along the root systems.