Spatial distribution of rhizodeposits provides built-in water potential gradient in the rhizosphere

Abstract

Plant roots alter soil properties at an expensive physiological cost by releasing large quantities of organic carbon (rhizodeposition). The role of rhizodeposits in enhancing beneficial microbial activity and biogeochemical nutrient mobilization is widely appreciated. But the role of rhizodeposits in water uptake has started gaining modest attention only recently. In this study we present a single root model, which demonstrates the possibility for rhizodeposits to create built-in water potential gradient. The conceptual basis for this model rests on three premises: (a) rhizodeposits are distributed in declining profile with distance from the root surface, (b) considerable fraction of rhizodeposits are strongly adhered to soil particles, and (c) rhizodeposits have the ability to retain water. Thus, variable concentration of affixed rhizodeposits results in a gradient of water potential without commensurate decline in water content with proximity to root surface. To corroborate premises (b) and (c), we conducted experiments using synthetic analog of rhizodeposits (Polygalacturonic Acid, PGA) and glass-bead and sand media. Environmental scanning electron microscopy was utilized to show affixation of PGA on glass beads during drying as well as pore-scale enhanced water retention. Macroscopic enhancement of water retention was characterized by dew-point potentiametry. We simulated water uptake by a root at constant potential transpiration rates representing high atmospheric demand and considered three distinct spatial distribution patterns of rhizodeposits as well as a control (without rhizodeposition). The model simulations indicate that the benefit of such variable distribution of exudates is more pronounced when (a) the potential water uptake rate is high or (b) the rhizodeposits are constrained to a narrow volume of rhizosphere soil.