Rhizosheath formation depends on mucilage concentration and water content

Abstract

Aims: Root exudates contain polymers that form crosslinks and can create a jelly like substance known as mucilage, which adheres to soil and thus promotes the formation of rhizosheaths, i.e. soil that remains attached to the roots after gentle shaking. We hypothesized that rhizosheath formation is optimal at an intermediate chia seed mucilage concentration and water content, but that its formation is limited at both a high concentration of chia seed mucilage and under dry conditions as well as at a low concentration of chia seed mucilage and under wet conditions. We used an artificial root soil system in which soil moisture and mucilage concentrations could be varied independently from one another with respect to their effect on rhizosheath formation. Methods: Jute cords were disposed in sandy loam soil and in quartz sand. In a subsequent study, they were also amended to different moisture contents with five different concentrations of mucilage (from 0 to 0.2 g dry mucilage g−1 water), before being isolated from chia and flaxseed mucilage after swelling of the respective seeds in distilled water for 15 min. Results: We found that in dry soil, rhizosheath formation peaked at an intermediate chia seed mucilage concentration. This behavior was supported by our conceptual model of mucilage spreading and rhizosheath formation, which relies on a radial diffusion equation and assumes that at low mucilage concentration, molecule numbers are insufficient to support polymer-like networks that stick soil particles together. In a very concentrated gel, however, mucilage is too sticky to diffuse far into the soil. Increasing soil moisture promotes rhizosheath formation both in a low and a high mucilage concentration range, although only up to an intermediate volumetric water content of 0.15cm3 cm–3. Conclusions: We conclude that both water and chia seed mucilage concentration are important drivers of rhizosheath formation. The effects are not additive but can combine to an optimum range, with a maximum formation of rhizosheaths observed in this study at 0.12 g mucilage g−1 rhizosphere water.