Water uptake by plant roots: I - Formation and propagation of a water extraction front in mature root systems as evidenced by 2D light transmission imaging

Soil water extraction by plant roots results from plant and soil transport processes interacting at different space and time scales. At the single root scale, local soil hydraulic status and plant physiology strongly control water uptake. At the whole root system level, these local, spatially interacting processes, are integrated and modulated depending on the root system hydraulics and plant transpiration. Most often, architectural and physiological characteristics of the root system are poorly taken into account in water uptake studies. This work aims at (i) studying root water extraction by mature root systems from the single root to the whole root system scale and (ii) providing experimental data for the assessment of a detailed model of water transport in the soil-root system presented in a companion paper (Doussan et al., Plant Soil 2006, this issue). Based on the dynamic imaging of soil water depletion around roots, we examined the influence of root system architecture and soil hydraulic properties on water uptake. We worked with narrow-leaf lupin plants whose root system architecture ranged from taprooted to fasciculate. Plants were grown in large thin containers (rhizotron) filled with a translucent sand/clay mix growing medium. Water transfer in the soil, together with root water uptake, were monitored in laboratory experiments by means of 2D light transmission imaging. This technique enables the mapping of the soil water content at high spatial and temporal resolutions. Throughout water uptake events, we clearly observed and quantified the formation and movement of a water extraction front and of high gradients of soil water content next to the roots. The data obtained also demonstrate that water uptake is never restricted to a specific portion of a root and that the contribution of a specific portion of a root to the overall uptake varies with time and with the position of the root within the root system. Finally, we found that different root system architectures induced different water uptake patterns.