ROLE OF MUCILAGE ON UPTAKE AND RELEASE OF WATER BY PLANT ROOTS

Key soil resources of terrestrial plants (water, nutrients and air) are frequently scarce and patchy. Therefore, plants often rely on adaptive traits of their root systems to optimally exploit the scare resources. One such trait is the modification of the rhizosphere—a small volume of soil that surrounds each individual root. Here we describe the alteration of flow and transport properties of the rhizosphere by mucilaginous material exuded by root tips. We present experimental observation of soil modification by synthetic mucilage as well as mathematical model of alteration of soil hydrodynamic properties. The adaptive function of mucilage is primarily a result of its elevated water retention potential. We will present results of modeling studies that elucidate two important services of mucilage. First, we demonstrate that exudates play important role in facilitating root water uptake by providing built-in water potential gradient within the rhizosphere. This phenomenon also results in fairly wet environment near the roots, which is important for nutrient diffusion, microbial activity, and nutrient cycling. Secondly, we show that exudates facilitate the release of water from roots to very dry rhizosphere at night, when transpiration is shut down. The latter is a widely documented phenomenon known as hydraulic lift/redistribution. In many dry regions, the hydraulic lift water is likely to be the only source of moisture to drive microbial activity and nutrient diffusion; thereby allowing plants to remain active during long dry spells by acquiring nutrients from the dry near surface soils while relying on deep soil moisture reserves for transpiration. The results of this modeling study suggest rhizosphere modification is a necessary prerequisite for hydraulic lift. In summary, both modeling studies suggest that rhizosphere alteration by mucilage is an actively controlled adaptation mechanism that permits plants to live in otherwise inhabitable conditions.