Effects of Mechanical Impedance on Root Growth in Barley (Hordeum vulgare L.): III. OBSERVATIONS ON THE MECHANISM OF RESPONSE

The effects of mechanical stress on whole root systems was investigatedusing beds of solid glass spheres (ballotini) continuously suppliedwith aerated nutrient solution. As noted in earlier experiments,increased mechanical impedance slowed root extension and alteredcell size and number; it also caused distortion of the rootapex, stimulated growth of lateral shoot meristems, and inducedthe formation of nodal roots. The development of lateral branchroots was enhanced and where root axes curved around ballotinilateral roots formed preferentially on the outer (convex) sidewhereas root hairs developed on the inner (concave) side. After roots were relieved from mechanical stress at least 3d elapsed before the rate of extension growth equalled thatof unimpeded plants. When intact Zea mays root apices first made contact with ballotinitheir elongation was slowed by 70% for about 10 min; where rootcaps were removed before the roots made contact, no such effectswere seen. We discuss the general nature of the mechanism of response tomechanical stress.