On the importance of rhizosphere conductance and soil-root contact in drying soils

Root water uptake (RWU) is influenced by rhizosphere conductance and soil-root contact, which vary with soil texture and root structure, including root hairs. Current simplified models often fail to capture the spatial complexity of these interactions in drying soils. The aim of this study was to examine how rhizosphere conductance, soil-root contact and root hairs affect RWU.

We used an explicit three-dimensional functional-structural model to investigate how root and rhizosphere hydraulics influence the transpiration rate-leaf water potential relationship of two maize (Zea mays) genotypes (with and without root hairs) grown in two contrasting soil textures (loam and sand) during soil drying. The model incorporated rhizosphere resistance in series with radial root resistance, with the latter being influenced by maturation (development of apoplastic barriers with age). It considered two critical processes: (1) the decrease in soil water potential between bulk soil and the soil-root interface; and (2) the extent of soil-root contact.

The simulations revealed that RWU was highly soil texture specific. In loam, the non-linearity in the transpiration rate-leaf water potential relationship was attributable primarily to localized uptake fluxes and high rhizosphere resistance as soil dried. In sand, however, where soil-root contact was less effective, rhizosphere conductance became a significant limiting factor for RWU, even at relatively higher soil water potential in comparison to loam. Root hairs did not make a significant contribution to rhizosphere conductance, probably owing to the dominant effect of soil-root interaction. Additionally, variations in root hydraulic conductance and its change with root tissue age impacted the accuracy of the model.

The explicit three-dimensional model provides a more precise representation of RWU dynamics by pinpointing exact uptake locations and primary limiting factors and by quantifying the proportion of root surface actively engaged in RWU. This approach offers notable improvements over conventional models for understanding the spatial dynamics of water uptake in different soil environments.