A method was used for applying a uniform mechanical impedance to plant roots using sand packed at different bulk densities with depth within each growth cylinder. In a growth experiment, replicate cylinders were packed with sand to give the following mechanical impedances: 0.25 MPa penetration resistance (negligible impedance), 1.40 MPa (moderate impedance) and 2.30 MPa (severe impedance). Seedlings of Lolium perenne L., Trifolium repens L. and Agrostis capillaris were grown for 23 d in each impedance treatment and effects on both roots and shoots were studied. Severe mechanical impedance affected both root and shoot growth rates for all three species resulting in smaller leaves and shorter roots. For the grasses, the root-to-shoot ratio at harvest was the same for all the treatments, but a delay in the initiation of both shoots and roots was observed in the severe mechanical impedance treatment. The results are discussed in relation to the possibility that roots penetrating the impeding treatments may have caused signalling that kept shoot and root growth in synchrony. Differences observed in the response to mechanical impedance of T. repens compared to the other two species may be a function of differences in the physiology between mono- and dicotyledonous species.
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Cook et al. (1996) studied this question.