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.

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