Root exudation and the biophysics of the rhizosphere

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During the next decades crop production has to increase to meet the food demand of the growing world population. One strategy to secure food production in a sustainable way is to grow plants with higher nutrient and water use efficiency. The rhizosphere, the region of soil near the roots, plays a crucial role in water and nutrient uptake: in the rhizosphere, root exudates enhance microbial activity and uptake of nutrients, favoring plant nutrient efficiency. The underlying hypothesis of this project is that the degree to which exudates can positively interact with plants depends on their distribution around the root. Our objective is to understand what factors affect the spatial and temporal distribution of root exudates in the rhizosphere. We plan to investigate the effect of root exuded mucilage on the diffusion of low molecular weight exudates across the rhizosphere. Mucilage has been shown to change the water related properties of the rhizosphere compared to the bulk soil. It is likely that mucilage strongly affects root exudates distribution in the rhizosphere. We hypothesize that: 1) in wet soils mucilage decreases the diffusion of exudates and it helps to maintain them near the root surface and to increase interaction with plants; 2) in dry soils mucilage increases the rhizosphere water content and, consequently, it enhances the diffusion of root exudates, enhancing rhizosphere extension. Over time, mucilage undergoes physiochemical alterations and it is transformed into a substance called mucigel. We assume that diffusivity of root exudates is reduced with increasing age of mucigel. Furthermore, we will test whether root water uptake affects exudate distribution and whether water content influences the amount of exudation. We plan to perform experiments in rhizoboxes combining labeling of maize, 14C imaging and neutron radiography. In the first work package (WP1) we will test the effect of water convection (i.e. root water uptake) and soil water content on root exudate distribution we will quantify rhizodepostion depending on water content. In WP2 we will visualize and quantify the distribution of root exudates, water content and mucilage along the root system (two-dimensional approach) using 14C imaging and neutron radiography. In WP3 we will study how mucilage affects the distribution of root exudates in the rhizosphere in a simplified system. In WP4 we will measure the degradation of root exudates and mucilage. Finally, a numerical model will be implemented to simulate the effects of water flow, soil properties, mucilage and degradation on the spatial distribution of exudates (WP5). The combination of new experimental and computational methods will enable us to disentangle the interactions of root exudation and mucilage release in soil. This will lead to better understanding of biochemical and physical processes at the soil-root interface which in turn affect plant water and nutrient use efficiency.