A biophysical analysis of root growth under mechanical stress

The factors controlling root growth in hard soils are reviewed alongside summarised results from our recent studies. The turgor in cells in the elongation zone of roots pushes the apex forward, resisted by the external pressure of the soil and the tension in the cell walls. The external pressure of the soil consists of the pressure required to deform the soil, plus a component of frictional resistance between the root and soil. This frictional component is probably small due to the continuous sloughing of root cap cells forming a low-friction lining surrounding the root. Mechanically impeded roots are not only thicker, but are differently shaped, continuing to increase in diameter for a greater distance behind the root tip than in unimpeded roots. The osmotic potential decreases in mechanically impeded roots, possibly due to accumulation of solutes as a result of the slower root extension rate. This more negative osmotic potential is not always translated into increased turgor pressure, and the reasons for this require further investigation. The persistent effect of mechanical impedance on root growth is associated with both a stiffening of cell walls in the axial direction, and with a slowing of the rate of cell production.