Reduced root water uptake after drying and rewetting

The ability of plants to extract water from soil is controlled by the water-potential gradient between root and soil, by the hydraulic conductivity of roots, and, as the soil dries, by that of the soil near the roots (rhizosphere). Recent experiments showed that the rhizosphere turned hydrophobic after drying and it remained temporarily dry after rewetting. Our objective was to investigate whether rhizosphere hydrophobicity is associated with a reduction in root water uptake after drying and rewetting. We used neutron radiography to trace the transport of deuterated water (D2O) in the roots of lupines growing in a sandy soil. The plants were grown in aluminum containers (28 x 28 x 1cm(3)) filled with a sandy soil. The soil was initially partitioned into different compartments using a 1-cm layer of coarse sand (three vertical x three horizontal compartments). We grew plants in relatively moist conditions (0.1 < < 0.2). Three weeks after planting, we let the upper left compartment of soil to dry for 2-3 d while we irrigated the rest of the soil. Then, we injected D2O in this compartment and in the upper right compartment that was kept wet. We monitored D2O transport in soil and roots with time-series neutron radiography. From the changes of D2O concentration inside roots, we estimated the root water uptake. We found that root water uptake in the soil region that was let dry and rewetted was 4-8 times smaller than that in the region that was kept moist. The reduced uptake persisted for > 1-0.5 h. We conclude that a reduction in hydraulic conductivity occurred during drying and persisted after rewetting. This reduction in conductivity could have occurred in roots, in the rhizosphere, or more likely in both of them.