Quantitative imaging of water transport in soil and roots using neutron radiography, D2O and a new numerical model

Abstract:

Our understanding of soil and plant water relations is currently limited by the lack of experimental methods to measure the water fluxes in soil and plants. Our study aimed to develop a new non-destructive method to measure the local fluxes of water into roots of plants growing in soils. We injected deuterated water (D₂O) near the roots of lupines growing in sandy soils, and we used neutron radiography to image the transport of D₂O through the root system. The experiments were performed during day, when plants were transpiring, and at night, when transpiration was reduced. The radiographs showed that: 1) the radial transport of D₂O from soil and roots depended similarly to diffusion and convection; and 2) the axial transport of D₂O along the root xylem was largely dominated by convection. To determine the convective fluxes from the radiographs, we simulated the D₂O transport in soils and roots. A dual porosity model was used to describe the apoplastic and symplastic pathways of water across the root tissue. Other features such as the endodermis and the xylem were also included in the model. The D₂O transport was modelled solving a convection-diffusion numerical model in soil and plants. The diffusion coefficients of the root tissues were inversely estimated by simulating the experiments at night under the assumption that at night the convective fluxes were negligible. Inverse modelling of the experiment at day gave the profile of water fluxes into the roots. For 24 day-old lupine grown in a sandy soil with uniform water content, our modelling results showed that root water uptake was higher at the proximal parts of the roots near soil surface and it decreased toward the distal parts. The method allows the quantification of the root properties and the regions of root water uptake along root systems growing in soils. Future applications of this method include the characterization of varying root systems, the radial and axial hydraulic conductivity of different root types, the changes in root permeability with root age and soil conditions, and the effect of aquaporines, endodermis and number and radius of xylem vessels on water transport.