Explicit 3D modelling of the rhizosphere processes at plant scale demonstrates the impact of soil texture on root water uptake

The relation between plant transpiration rate (E) and leaf water potential (LWP) is a function of both soil and plant hydraulics and can be affected by local rhizosphere processes. Measuring these very localized processes remains a huge challenge, while observing their impact on the E-LWP relationship is easy. Therefore, the underlying mechanisms of how these processes impact root water uptake (RWU) and whether it is soil texture specific remain unknown. In this study we used a 3-D detailed functional-structural root-soil model to investigate how root and rhizosphere hydraulics control the E-LWP relationship for two maize genotypes (with and without root hairs) grown in two soil types (loam and sand) during soil drying. We assumed that the rhizosphere hydraulic resistance can be taken into account via two processes: (1) a drop in soil water potential between the bulk soil and the soil-root interface and (2) a partial soil-root contact. The simulations revealed that the key process controlling the uptake was soil-dependent. In loam, a drop in soil water potential between the bulk soil and the soil-root interface affected the uptake and RWU started to be limited below soil water potential of -610 hPa. In sand, however, the poor soil-root contact was the main constraint, and the rhizosphere conductance limited RWU at much higher soil water potential (around -90 hPa). In contrast to effective models, our explicit three-dimensional simulations provide exact location and the main driver (root or rhizosphere) of the water RWU distribution patterns as well as the quantification of the active root surface ratio for RWU.