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Root caps provide a protective layer in front of the meristem that protects the meristem from abrasion by soil particles. The continuous production and sloughing of the root cap cells may be an adaptation to decrease the friction at the soil-root interface by acting as a low-friction lining to the channel formed by the root. Experiments were performed which provide the first direct evidence that such cell sloughing decreases frictional resistance to root penetration. The penetration resistance (force per unit crosssectional area) to maize roots, which were pushed mechanically into the soil, was compared with the penetration resistance to growing roots and to 1 mm diameter metal probes (cone semi-angles of 7.5° or 30°). The pushed roots experienced only about 40% of the penetration resistance experienced by the 7.5° metal probe that was pushed into the soil at the same rate. Thus, the friction between the soil and the pushed root was much smaller than between the soil and the metal probe. The penetration resistance to the growing root was between 50% and 100% of that to the pushed root, indicating that the relief of friction and slower rate of soil compression were more efficient around the growing root. SEM examination of the surface of roots pushed or grown into the soil showed that numerous root cap cells had detached from the cap and slid for several millimetres relative to the root. The low friction properties of roots may be due largely to the low coefficient of friction between sloughing root cap cells, and may be decreased further by intracellular mucilage secretions.
Bengough et al. (Wed,) studied this question.