Water Uptake and Hydraulics of the Root Hair Rhizosphere

Plant roots consist of various functional zones; the root cap and beyond the lateral root formation region have very low water permeability due to immature water conduit and root suberization, respectively. The root hair zone, which is located between these regions, is the most permeable zone, where both radial and axial conductivities are high, and has been suggested to play a role in water uptake enhancement. The conventional understanding of root hair function is that root hairs increase root surface area, thereby enhancing water and nutrient uptake. Yet, modeling the soil water status between root hairs shows that the soil water potential there reaches a value close to that of the root in a very short time. The corresponding low water content values within the inter‐root‐hair domain indicates limited mass water flow and ion diffusion rate toward the root. Consequently, we conclude that when the plant transpires (daytime), root hairs do not increase water and nutrient uptake by increasing root surface area. Instead we used both magnetic resonance imaging technology, for measurements and analysis of spatial and dynamic changes in water content in the rhizosphere, and numerical modeling to show that: (i) root hairs function mostly by water uptake through the root hair tip plane; (ii) the growth of root hairs, perpendicular to the root surface, expands the apparent diameter of the cylinder that is characterized by the root water potential, thereby increasing the effective surface area of the root for water uptake; and (iii) the growth of needle‐shaped root hairs requires minimal investment in biomass with less mechanical resistance compared with alternative strategies that require larger root diameter or root length.