Maintaining root–soil contact in drying soils: the role of mucilage and root hairs

The rhizosphere plays a key role in regulating plant water uptake during soil drying, yet it is often represented in soil–plant models as hydraulically and mechanically equivalent to bulk soil. While the influence of root mucilage and extracellular polymeric substances (EPS) on rhizosphere water retention is well recognized, their mechanical role—together with that of root hairs—in controlling root–soil contact and soil structural dynamics remains insufficiently explored.

Recent biomechanical insights into drying liquid bridges reveal that polymer-rich solutions behave fundamentally differently from water. Whereas capillary water bridges weaken and fail during drying—particularly in coarse-textured soils such as sand—mucilage can form viscoelastic filaments that persist during drying and generate increasing tensile forces as the polymer network is stretched. As a result, the mechanical contribution of mucilage on maintaining root-soil contact is negligible in fine-textured soils where capillary forces are already strong but is particularly relevant in sandy soils where water bridges alone provide little mechanical adhesion.

These biomechanical properties have important consequences for root–soil contact dynamics and rhizosphere structure. Elastic polymer bridges, in combination with root hairs that increase contact area and provide additional anchoring points, offer a mechanism by which plants can maintain physical contact with the surrounding soil as roots shrink during drying. This mechanical reinforcement may delay both hydraulic disconnection and associated mechanical loss of contact of roots from the soil, preserving water uptake at water potentials where capillary connectivity alone would already be limiting.

At the same time, tensile forces generated by drying polymeric gels promote aggregation of soil particles, contributing to the formation of a mechanically coherent rhizosphere with altered pore geometry and connectivity. Such aggregation reinforces the distinction between rhizosphere and bulk soil properties and may further modulate local water distribution and hydraulic conductivity near the root surface.

This perspective highlights the need to move beyond purely hydraulic descriptions of the rhizosphere and to incorporate the mechanical effects of mucilage, EPS, and root hairs into conceptual and numerical models of root water uptake, particularly under drought conditions.