Root–soil friction: quantification provides evidence for measurable benefits for manipulation of root‐tip traits

To penetrate soil, a root requires pressure both to expand the cavity it is to occupy, σ_n, and to overcome root–soil friction, σ_f. Difficulties in estimating these two pressures independently have limited our ability to estimate the coefficient of soil–root friction, μ_sr. We used a rotated penetrometer probe, of similar dimensions to a root, and for the first time entering the soil at a similar rate to a root tip, to estimate σ_n. Separately we measured root penetration resistance (PR) Q_r. Root PR was between two to four times σ_n. We estimated that the coefficient of root–soil friction (μ_sr) was 0.21–0.26, based on the geometry of the root tip. This is slightly larger than the 0.05–0.15 characteristic of boundary lubricants. Scanning electron microscopy showed that turgid border cells lined the root channel, supporting our hypothesis that the lubricant consisted of mucilage sandwiched between border cells and the surface of the root cap and epidermis. This cell–cell lubrication greatly decreased the friction that would otherwise be experienced had the surface of the root proper slid directly past unlubricated soil particles. Because root–soil friction can be a substantial component of root PR, successful manipulation of friction represents a promising opportunity for improving plant performance.