EFFECTS OF SOIL MECHANICAL IMPEDANCE ON ROOT AND SHOOT GROWTH OF LOLIUM-PERENNE L, AGROSTIS-CAPILLARIS AND TRIFOLIUM-REPENS L

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 wer e packed with sand to give the following mechanical impedances: 0.25 M Pa penetration resistance (negligible impedance), 1.40 MPa (moderate i mpedance) and 2.30 MPa (severe impedance), Seedlings of Lolium perenne L., Trifolium repens L. and Agrostis capillaris were grown for 23 d i n each impedance treatment and effects on both roots and shoots were s tudied, Severe mechanical impedance affected both root and shoot growt h rates for all three species resulting in smaller leaves and shorter roots, For the grasses, the root-to-shoot ratio at harvest was the sam e 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 pe netrating the impeding treatments may have caused signalling that kept shoot and root growth in synchrony, Differences observed in the respo nse to mechanical impedance of T. repens compared to the other two spe cies may be a function of differences in the physiology between mono- and dicotyledonous species.