The effect of mechanical impedance on root growth in pea (Pisum sativum). I. Rates of cell flux, mitosis, and strain during recovery

We studied the effect of mechanical impedance on cell flux and meristematic activity in pea roots. Pea seedlings ( Pisum sativum L. cv. Helka) were grown in cores of sand packed to dry bulk densities of either; 1.4 Mg m⁻³ with an additional 2.4 kg uniaxial load applied to the surface to increase the mechanical resistance to growth (penetration resistance of 1.5 MPa); or 1.0 Mg m⁻³ (penetration resistance of 0.05 MPa). A water content of 0.06 g g⁻¹ was chosen for optimum root growth. After 3 days, the seedlings were transferred to hydroponics, colchicine was added and the rate of cell doubling, mitotic index and length of the cell cycle was assessed. Cell flux in the third cortical layer was calculated for roots immediately removed from sand.Mechanical impedance slowed root extension to about 20% of the unimpeded rate, and final cell length was reduced to 50% of the unimpeded length. The rate of cell doubling was 3.4 times slower for roots recovering from mechanical impedance mostly as a result of a longer period spent in interphase. Cell flux in impeded roots was approximately half that of unimpeded roots (5 cells h⁻¹), and contributed to a shorter cell file and elongation zone, and a slower rate of root elongation.