Quantitative Visualization of Root Water Uptake: Approaching the Cellular Scale

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. In former studies, we developed a new non-destructive method to measure the local fluxes of water into roots of growing plants in soil. The technique consisted of combining neutron radiography and injection of deuterated water (D₂O) near the roots in soils. This technique was successfully employed to quantify the location and rate of water into the roots at the scale of root system. However, due to composite and dynamic structure of root tissue, the radial flow of water across the root system is complex and may occur across different pathways (apoplastic and cell to cell pathway). Up to date, the relative importance of different pathways in root water uptake is still matter of debate. Here, an on the fly neutron tomography combined with the injection of D₂O was used to monitor the transport of D₂O across the root tissue with a high temporal and spatial resolution. A full neutron tomography with 181 projections was performed at 30 seconds. After reconstruction of 3D data, the profile of D₂O concentration across the roots was determined at varying times after D₂O injection. The results showed that the radial transport of D₂O across the root tissue was slower during night-time (when plant was not transpiring) than the day-time. Additionally, during night-time a steep gradient in concentration of D₂O within different compartments of the root tissue was observed while this gradient was less steep during day-time.

To determine the convective fluxes of water across different pathways, the transport of D₂O was numerically simulated by solving a convection-diffusion equation. The flow domain across the root tissue was constructed based on the root cross section obtained from microscopic study of root tissue. This allowed us to include a realistic geometry for the apoplastic and symplastic pathways, endodermis and the xylem.

The results of inverse modelling gave the diffusional permeability and the hydraulic conductivity of cortical cells and endodermis. Additionally, it was shown that the transport of water across the root tissue till endodermis was mainly through the apoplastic pathways and the endodermis was the main resistance to the transport of water.

This study proves that on the fly tomography can be successfully used to study the flow of water across the root tissue in 3D at sufficiently high spatial and temporal resolution. The success in our 3D visualization of D₂O transport across the root tissue opens new avenues to study the relative importance of the apoplastic and cell-to-cell pathways in root water uptake under varying soil condition. Such information is urgently needed for understanding the physical mechanisms controlling water, solute and hormone transport in the roots.