Where and how fast do roots take up water from the soil? To address this question, we introduced and tested a new technique based on neutron radiography to localize and quantify water uptake along the roots of a living plant. This technique will help in a better understanding of root functioning and provide a database to evaluate and improve existing models.

Knowledge of local water fluxes across the soil–root interface is essential to understand and model root water uptake. Despite its importance, there is a lack of direct methods to measure the location of water uptake along roots. The aim of this study was to develop a technique to quantify local flux of water from soil to the roots of living plants. To this end, we used neutron radiography to trace the transport of deuterated water (D₂O) into individual roots. We grew lupin (Lupinus albus L.) in 30- by 25- by 1-cm containers filled with a sandy soil, which was partitioned into different compartments using 1-cm-thick layers of coarse sand. We locally injected D₂O into a selected soil compartment near the roots of 18-d-old lupin during the day (transpiring) and night (not transpiring). The transport of D₂O into roots was then monitored using time-series neutron radiography. The results show that: (i) the transport of D₂O into roots was faster during the day than at night; and (ii) during the day, D₂O was quickly transported along the roots toward the shoots, while at night this transport was insignificant. The differences between day and night measurements were explained by convective transport of D₂O into the roots driven by transpiration. To quantify the local transport of D₂O into roots, we developed a simple convection–diffusion model that assumed the endodermis as the main resistance to water transport. The D₂O uptake predicted by the model was in agreement with the axial flow within the roots as derived from D₂O transport behind the capillary barrier. This new method allows quantification of local water uptake in different parts of the root system.