How mucilage alter the biophysical properties of the rhizosphere: Potential impacts on root water and nutrient uptake

Given the limited water and nutrient resources, an important strategy to meet the increasing demand for food production must focus on selecting plant varieties with better performance and higher water and nutrient use efficiency under drought and nutrient-limited conditions. The success of such programs requires a deep mechanistic understanding of the functioning of the roots in water and nutrient uptake and also the factors affecting the transport of water and nutrients within the soil-plant-atmosphere continuum, particularly under limited conditions. Among different potential candidates affecting plant access to water and nutrients, the rhizosphere has recently received significant attention as a new avenue to maximize efficient and sustainable use of limited water and soil nutrient resources (Gregory, ۲۰۰۶; Dotaniya & Meena, ۲۰۱۵).Despite the consistent interest in rhizosphere studies, it is surprising that we are only now starting to understand the biophysical aspects of the rhizosphere, particularly the effect of mucilage on the transport of water and nutrients, while its distinct biochemical properties and their impacts have been intensively investigated. Mucilage is a gel-like substance that consists of polysaccharides, lipids, and root border cells. The focus of the majority of the current studies interested in the physical and chemical properties of the rhizosphere has been so far on the impact of rhizodeposition on increasing the volume/quantity of water and nutrients in the rhizosphere. However, the missing information is still the impact of mucilage on the transport of water and nutrients across the soil and towards the roots, which becomes critical under drying soil conditions.This contribution presents conceptual and experimental evidence showing how mucilage may favor water and nutrient uptake under soil drying conditions. The underlying hypothesis is that, besides maintaining the rhizosphere wetter during soil drying, mucilage also alters the spatial distribution of water within the pore space maintaining the connectivity of the liquid phase in drying soil(Fig. ۱). This positive effect is originated from three main intrinsic properties of mucilage (high viscosity, low surface tension, and high water retention), which are shared among all the mucilage secreted by plant roots(Naveed et al., ۲۰۱۹). These modifications enhance water retention and liquid connectivity of soil during a drying cycle, facilitating both i) the flow of water and ii) the transport of nutrients across the rhizosphere and therefore favors water and nutrient uptake by plant roots.