Effects of Mechanical Impedance on Root Growth in Barley (Hordeum vulgareL.)

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Abstract
The effects of mechanical stress on whole root systems was investigated using beds of solid glass spheres (ballotini) continuously supplied with aerated nutrient solution. As noted in earlier experiments, increased mechanical impedance slowed root extension and altered cell size and number; it also caused distortion of the root apex, stimulated growth of lateral shoot meristems, and induced the formation of nodal roots. The development of lateral branch roots was enhanced and where root axes curved around ballotini lateral roots formed preferentially on the outer (convex) side whereas root hairs developed on the inner (concave) side. After roots were relieved from mechanical stress at least 3 d elapsed before the rate of extension growth equalled that of unimpeded plants. When intact Zea mays root apices first made contact with ballotini their elongation was slowed by 70% for about 10 min; where root caps were removed before the roots made contact, no such effects were seen. We discuss the general nature of the mechanism of response to mechanical stress.