Quantifying rhizosphere particle movement around mutant maize roots using time‐lapse imaging and particle image velocimetry

Abstract

Soil surrounding a growing root must be displaced to accommodate the increased root volume. To ease soil penetration, root caps produce border cells and mucilage that lubricate the root surface, decreasing friction at the root-soil interface. Rhizosphere deformations caused by roots with or without a functional root cap were compared to determine the effects of the root cap on sand displacement and penetration. Intact (KYS wild type) and decapped (agt1 dec mutant) primary maize roots were grown in observation chambers filled with sand. Non-destructive time-lapse micro-imaging combined with particle image velocimetry was used to visualize and quantify sand displacements as small as 0.5 μm caused by growing roots. Decapped (agt1 dec ) roots displayed typical responses of mechanically impeded roots at sand densities that did not affect intact KYS roots. Sand displacement decreased exponentially with distance from the root and extended four to eight root radii into the sand. The calculated mean sand density increase and the compressed sand area were doubled by decapping. Maximum density often occurred in front of the apex of decapped roots whereas it occurred along the sides of intact roots. Periodic variation in sand deformation was observed, probably associated with root circumnutation, which may also facilitate soil penetration. Sand particles moved alongside KYS roots more easily than they did alongside agt1 dec roots. A functional exuding cap was therefore essential for efficient rhizosphere deformation and penetration by roots. Manipulating root tip, and specifically root cap, properties is a possible target for improving root penetration in hard soil.

Mapping soil deformation around plant roots using in vivo 4D X-ray Computed Tomography and Digital Volume Correlation

“…Across all time-steps, the displacement vectors in the immediate vicinity of the root tip were oriented approximately normal to the root surface, consistent with the morphologies of 2D displacement fields previously quantified around spherical penetrometers (Dexter and Tanner, 1972) and roots (Vollsnes et al, 2010) using DIC. This observation agrees with the proposal that the expanding meristem exerts pressure on the soil, redistributing the matrix to generate a cavity that accommodates the new root volume (Bengough and Mullins, 1990).…”

“…The root followed a curved path during the experiment, and as observed by (Vollsnes et al, 2010), the magnitude of soil displacement was generally larger on the convex side of the root (Figure 8). …”

“…Applying the techniques developed in this paper to investigate genotypes that contrast in mucilage exudation (Vollsnes et al, 2010) and root hair traits (Gahoonia et al, 2001) will offer compelling opportunities to probe the fundamentals of rhizosphere development.…”

“…By employing cross-correlation between grey-level values in reference and deformed images, full field components of material deformation can be computed. Vollsnes et al (2010) have demonstrated twodimensional digital image correlation (DIC) to map sand displacements around Zea mays roots in a rhizobox system. Others have combined XCT and digital volume correlation (DVC) to investigate sand and soil deformations under mechanical and hydraulic stresses (Hall et al, 2010;Peth et al, 2010;Schlüter et al, 2016).…”

“…In 2D systems, deformation at the imaging plane is influenced by friction, shear and local compaction at the transparent interface, and quantification is limited to in-plane directions. Being a 3D method, XCT requires no transparent interface, and root tip positions do not need to be inferred from the deformation field (Vollsnes et al, 2010).…”