Root shrinkage and its mechanisms: why context matter

Modeling plant responses to drought over short-to long-term is crucial under rising global warming threats. Roots form the critical gateway between a plant and its water sources in the soil, yet their connection with the soil is still poorly understood. As the soil dries, roots shrink gradually disconnecting from the surrounding soil. This progressive reduction of root-soil contact interrupts the liquid-phase continuity and limits the water movement. The importance of the loss of contact between soil and roots depends on the water potential at which this occurs. If roots lose contact at potential close or beyond the wilting point, when the low soil hydraulic conductivity is already limiting, the loss of contact might not be as important. But if this occurs in still relatively wet conditions, it might trigger an earlier limitation of root water uptake.

Currently, it is known at what water potential roots lose contact with the soil. Furthermore, we expect that this critical water potential is not unique, but it depends on soil properties, soil particle size and porosity, and root properties, such as root hair density and mucilage production.

Here we present an analysis to identify and quantify the forces that bind the soil to the root for different soil textures. We estimate the adhesive forces that hold roots in contact with the soil and that counteract root shrinkage caused by decreasing water potential and cells losing turgor. The ingredients of our analysis are: root hairs, capillary forces and mucilage elastic properties. Thresholds of gap formation at the root-soil interface are identified for varying soil particle size and porosity and for varying root hair density and mucilage elastic properties.

This analysis shows that root-soil contact dynamics do not depend only on the root cell turgor loss point, but also on soil properties, and helps to identify the mechanisms impacting the hydraulic continuity across the root-soil interface.