Where do roots take up water? Neutron radiography of waterflow into the roots of transpiring plants growing in soil

摘要

Where and how fast does water flow from soil into roots? The answer to this questionrequires direct and in situ measurement of local flow of water into roots of transpiring plantsgrowing in soil. We used neutron radiography to trace the transport of deuterated water (D_2O) in lupin(Lupinus albus) roots. Lupins were grown in aluminum containers (309 25 9 1 cm) filled withsandy soil. D_2O was injected in different soil regions and its transport in soil and roots wasmonitored by neutron radiography. The transport of water into roots was then quantifiedusing a convection–diffusion model of D_2O transport into roots. The results showed that water uptake was not uniform along roots. Water uptake washigher in the upper soil layers than in the lower ones. Along an individual root, the radial fluxwas higher in the proximal segments than in the distal segments. In lupins, most of the water uptake occurred in lateral roots. The function of the taprootwas to collect water from laterals and transport it to the shoot. This function is ensured by alow radial conductivity and a high axial conductivity. Lupin root architecture seems welldesigned to take up water from deep soil layers.