Biophysical rhizosphere processes affecting root water uptake

Abstract

Despite detailed images of water content, water fluxes and soil structure in the rhizosphere, a general understanding of how the rhizosphere affects root water uptake is still lacking. The missing elements of the puzzle are the gradient in water potential around roots. Measurements of the xylem water potential at varying soil water potentials and transpiration rates supported by numerical models of root water uptake would allow the estimation of the water potential across the rhizosphere. Such measurements are crucial to comprehend how water enters the roots.

Spatial Heterogeneity Enables Higher Root Water Uptake in Dry Soil but Protracts Water Stress After Transpiration Decline: A Numerical Study

“…Important advances in imaging techniques make it possible to obtain high resolution images for the much necessary quantification of the physical soil structure around the roots (Carminati et al, 2016;Hinsinger et al, 2009). These images have shown that compared to the bulk soil, the rhizosphere exerts a considerable heterogeneity driven by the propagation of mucilage within the pore space, changes in soil structure around roots (e.g., by soil swelling and shrinkage), the emergence of water repellent zones, the formation of rhizosheath and physical interactions with microorganisms (Carminati et al, 2016). Spatial variability in root-soil contact (partial contact) may further add to this complexity (Carminati et al, 2013;de Willigen et al, 2018;Herkelrath et al, 1977;North & Nobel, 1997;Veen et al, 1992).…”

“…However, root water uptake can be well approximated by a single pathway and by using an average root hydraulic conductivity (Landsberg & Fowkes, ; Zarebanadkouki et al, ). Water flow in the root can thus be described by the following equation (Carminati et al, ; Couvreur et al, ; Javaux et al, ) where (cm/s) is the root water uptake rate, (cm/s) is the overall hydraulic conductivity of the root tissue between the root surface and the xylem, (cm) is the pressure at the root surface, and (cm) is the xylem pressure. Since the fundamental work of Gardner (), an extensive range of studies simulated the water flow from the bulk soil to the root in terms of a cylindrical geometry.…”

“…However, root water uptake can be well approximated by a single pathway and by using an average root hydraulic conductivity (Landsberg & Fowkes, 1978;Zarebanadkouki et al, 2014). Water flow in the root can thus be described by the following equation (Carminati et al, 2016;Couvreur et al, 2014;Javaux et al, 2013)…”

Preferential flow systems amended with biogeochemical components: imaging of a two-dimensional study

“…However, even with such an acceleration of research, the specific distribution and paths that the fluid takes from the soil matrix into and through the plant itself remain unknown . As stated by Carminati et al (2016), "Coupling these measurements with imaging techniques that allow visualization of the root architecture and the structure and moisture of the rhizosphere could be the ultimate experiment to measure the rhizosphere properties." The analysis presented here clearly indicates the effectiveness of combining 2-D tank systems with imaging techniques such as LTM to furthering understanding of high spatial and temporal fluid distribution and moisture gradients in the rhizosphere directly under the influence of plant roots.…”

“…This is a crucial knowledge gap in terms of the biogeochemical influences on the hydrologic processes occurring in the rhizosphere, as stated by Bengough (2012), particularly the effect that root exudates have on the nonequilibrium dynamics of interfaces and flow in porous media. Carminati et al (2016) recently analyzed the rhizosphere processes to more thoroughly elucidate the system, with a particular focus on determining the role of mucilage in root water uptake. They found that wet mucilage enhances fluid transport, but when dry, the mucilage causes hydrophobic tendencies in the rhizosphere .…”