SLOUGHING OF ROOT CAP CELLS DECREASES THE FRICTIONAL RESISTANCE TO MAIZE (ZEA-MAYS L) ROOT-GROWTH

Root caps provide a protective layer in front of the meristem that pro tects the meristem from abrasion by soil particles. The continuous pro duction and sloughing of the root cap cells may be an adaptation to de crease the friction at the soil-root interface by acting as a low-fric tion lining to the channel formed by the root. Experiments were perfor med which provide the first direct evidence that such cell sloughing d ecreases frictional resistance to root penetration. The penetration re sistance (force per unit cross-sectional area) to maize roots, which w ere pushed mechanically into the soil, was compared with the penetrati on resistance to growing roots and to 1 mm diameter metal probes (cone semi-angles of 7.5 degrees or 30 degrees). The pushed roots experienc ed only about 40% of the penetration resistance experienced by the 7.5 degrees metal probe that was pushed into the soil at the same rate. T hus, the friction between the soil and the pushed root was much smalle r than between the soil and the metal probe. The penetration resistanc e to the growing root was between 50% and 100% of that to the pushed r oot, indicating that the relief of friction and slower rate of soil co mpression were more efficient around the growing root. SEM examination of the surface of roots pushed or grown into the soil showed that num erous root cap cells had detached from the cap and slid for several mi llimetres relative to the root. The low friction properties of roots m ay be due largely to the low coefficient of friction between sloughing root cap cells, and may be decreased further by intracellular mucilag e secretions.