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

Abstract

Background and aims: 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. This study aims to examine how rhizosphere conductance, soil-root contact, and root hairs affect RWU.

Methods: We utilized an explicit 3D functional-structural model to investigate how root and rhizosphere hydraulics influence the transpiration rate and leaf water potential (T-LWP) 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.

Key results: The simulations revealed that RWU was highly soil texture-specific. In loam, the non-linearity in the T-LWP relationship was primarily due 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 compared to loam. Root hairs did not show a significant contribution to rhizosphere conductance, likely due to the dominant effect of soil-root interaction. Additionally, variations in root hydraulic conductance and its change with root tissue age impacted the model's accuracy.

Conclusions: The explicit 3D model provides a more precise representation of RWU dynamics by pinpointing exact uptake locations, primary limiting factors, and quantifying the proportion of root surface actively engaged in RWU. This approach offers notable improvements over conventional models in understanding the spatial dynamics of water uptake in different soil environments.