Sloughing of root cap cells and exudation of mucilage plays an important role in the penetration of compacted soils by roots. For the first time we have quantified the rate of sloughing of root cap cells in an abrasive growth medium that was compacted to create mechanical impedance to root growth. The number of maize (Zea mays) root cap cells sloughed into sand increased as a result of compaction, from 1930 to 3220 d −" per primary root. This represented a 12-fold increase in the number of cells sloughed per mm root extension (from 60 to 700). We estimated that the whole of the cap surface area was covered with detached cells in compacted sand, compared with c. 7% of the surface area in loose sand. This lubricating layer of sloughed cells and mucilage probably decreases frictional resistance to soil penetration. The total carbon deposited by the root was estimated at c. 110 µg g −" sand d −" . Sloughed cells accounted for 10% of the total carbon, the vast majority of carbon being contained in mucilage exudates.

The root cap assists the passage of the root through soil by means of its slimy mucilage secretion and by the sloughing of its outer cells. The root penetration resistance of decapped primary roots of maize (Zea mays L. cv. Mephisto) was compared with that of intact roots in loose (dry bulk density 1.0 g cm-3; penetration resistance 0.06 MPa) and compact soil (1.4 g cm-3; penetration resistance 1.0 MPa), to evaluate the contribution of the cap to decreasing the impedance to root growth. Root elongation rate and diameter were the same for decapped and intact roots when the plants were grown in loose soil. In compacted soil, however, the elongation rate of decapped roots was only about half that of intact roots, whilst the diameter was 30% larger. Root penetration resistances of intact and decapped seminal axis were 0.31 and 0.52 MPa, respectively, when the roots were grown in compacted soil. These results indicated that the presence of a root cap alleviates much of the mechanical impedance to root penetration, and enables roots to grow faster in compacted soils.

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