Factors controlling rhizosphere extension: the critical role of root growth and rhizosphere water content

The spatial distribution of root exudates in the rhizosphere is controlled by several processes, including diffusion, exudation rate and root growth. We tested the effect of soil water content on these processes and on exudate distribution. Maize plants were grown in rhizoboxes at 6 (dry) or 20% (wet) soil water content. Root exudates were measured in-situ using 14C-imaging. Rhizosphere water content was visualized using neutron radiography. A numerical model of exudate diffusion was employed to predict the distribution of root exudates. Root growth decreased by 30-48% in dry soil. Similarly, the exudation rate decreased by 31-32% compared to wet soil. The radial profiles of root exudates were independent on soil moisture because of two factors: 1) the exudation rate per root length remained constant; 2) the diffusion coefficients estimated by the model were equal in the two treatments (D=4.63E-09-1.2x10-E8). This unexpected result was explained by the neutron radiographs, which showed that the water content close to the root surface was nearly identical (35%) in both wet and dry soil, most likely because of the high water retention by mucilage. This study showed that root growth and rhizosphere hydraulic properties contribute to shaping the profiles of root exudates in the rhizosphere.

Figure 1: 14C images of a plant grown in wet soil (left site) and a plant grown in dry soil (right site). Bright color indicates high 14C activity; dark color indicates low 14C activity. The size of the imaged part of the sample is 20*30 cm.