Variable effects of maize mucilage on rhizosphere rewetting – a new method to collect mucilage

Abstract:

Recent experiments suggested that the mucilaginous fraction of root exudates may cause water repellency of the rhizosphere. Our objectives were to: 1) investigate whether maize rhizosphere turns hydrophobic; 2) measure the contact angle of mucilage collected from plants growing in wet and dry soils; and 3) find a quantitative relation between rhizosphere rewetting, particle size, soil matric potential and mucilage concentration.

Maize plants were grown in sandy soil for five weeks. The soil was then allowed to dry and it was irrigated. The soil water content during irrigation was imaged using neutron radiography. In a parallel experiment, mucilage was collected from brace roots. The contact angle was measured for varying mucilage concentration. Additionally, capillary rise experiments were performed in soils of different particle size and mucilage concentration. We then used a pore-network model in which mucilage was randomly distributed in a cubic lattice. The general idea was that the rewetting of a pore is impeded when the concentration of mucilage on the pore surface [g cm^-2] is higher than a given threshold value. The threshold value depended on soil matric potential, pore radius and contact angle. Then, we randomly distributed mucilage in the pore network and we calculated the percolation of water across a cubic lattice for varying soil particle size, mucilage concentration and matric potential.

Our results showed that: 1) the rhizosphere stayed temporarily dry after irrigation; 2) in both plants growing in wet and dry soils, mucilage became hydrophobic after drying. Mucilage contact angle increased with mucilage concentration. Interestingly, the contact angle of mucilage from plants growing in dry soil was higher than the one from plants growing in wet soils; 3) water could easily cross the rhizosphere when the mucilage concentration was below a given threshold. In contrast, above a critical mucilage concentration water could not flow through the rhizosphere. The critical mucilage concentration decreased with increasing particle size and decreasing matric potential.

These results show the importance of mucilage for water fluxes across the root-soil interface. Our percolation model predicts at what mucilage concentration the rhizosphere turns hydrophobic depending on soil texture and matric potential.