The role of root hairs in root water uptake - Insights from an image-based 3D model

Root hairs, tubular protrusions of epidermal root cells, are considered a key rhizosphere feature: by substantially increasing the contact area between roots and soil, they enhance the ability of plants to capture soil resources. Hence, they are considered a breeding target for improving drought tolerance and yield stability of crops. While their pivotal role in the uptake of immobile nutrients such as phosphorus is well accepted, their effect on root water uptake remains controversial as it varies across plant species. 
By means of image-based modelling, our objective was to identify environmental conditions (e.g. soil water content) and hair traits (e.g. root hair length and density) that determine the effectiveness of root hairs in root water uptake. Furthermore, we investigated the effect of drought stress-induced root hair shrinkage on root water uptake.

We scanned root compartments of 8 days old maize seedlings (Zea Mays L.) grown in a loamy soil using synchrotron radiation X-ray CT. Based on the collected image-data, we implemented a 3D root water uptake model. By solving Richards equation numerically, we computed the propagation of water potential gradients across the root-soil continuum which allowed to quantify root water uptake. The high spatial resolution of the acquired images enabled us to explicitly take rhizosphere features, such as root hairs and root-soil matrix contact into account. We determined the key parameters governing the effectiveness of root hairs in water uptake by comparing a set of six maize root compartments before and after digitally removing their hairs. The quantification of root hair turgor-loss in response to progressive soil drying allowed us to implement hair shrinkage within our model.

We found that the effect of root hairs in root water uptake is governed by 1) the root hair induced increase in root soil contact and 2) root hair length. Furthermore, our results suggest that root hairs potentially facilitate root water uptake under dry soil conditions (< -0.1MPa). However, in the dry range, root hair shrinkage severely reduces the effect of hairs. Depending on their turgor-loss curve, root hairs may still provide a positive effect on root water uptake in a narrow range of soil matric potential. 

In summary, the effect of root hairs on root water uptake depends on soil water content, root-soil contact, root hair length and the turgor-loss point of hairs.