Root responses to soil physical conditions and the role of root-particle contact
This thesis considers the elongation o f root and shoots in relation to matric potential and soil strength in contrasting species. The role of root-particle contact for root and shoot elongation in relation to particle/aggregate size and bulk density at various matric potentials is discussed. Root and shoot elongation o f maize and lupin in soil and vermiculite at matric potentials ranging from -0.03 MPa to -1.6 MPa were investigated. Both root and shoot elongation rate of maize and lupin were significantly slower in vermiculite than in soil (p <0.001). As vermiculite has very different particle size distribution and hydraulic properties from soil, the degree of contact between root and vermiculite was thought to provide a possible explanation for the slower elongation rates. A new method was developed to quantify root-particle contact using X-ray microtomography and verified using ‘phantoms’ (model systems o f known dimensions). Root-particle contact was approximately 25 % greater in soil than in vermiculite. The greater root-particle contact in soil was thought to provide better growth conditions than in vermiculite. Root and shoot elongation were examined when plants were place in humid air above an osmotic solution (KC1) to evaluate the degree to which root elongation could occur in the absence o f solution contact. No significant shoot elongation occurred and root elongation was more reduced than in vermiculite. Hairless maize and barley mutants and their wildtypes were used to investigate further the effects o f root-particle contact when water availability is limiting in both soil and vermiculite systems. Root elongation rates o f the hairless mutants were slower than those of the wildtypes, when the growth medium was wetter than -1.6 MPa. However the reduction in root elongation o f hairless maize may have been due to pleiotropic effects slowing the elongation (the elongation rate relative to the maximum elongation rate was not significantly different). The combined effects o f mechanical impedance and decreasing matric potentials on root and shoot elongation were tested. Maize and lupin were grown in soil packed to five bulk densities (bulk densities 1.1, 1.2, 1.3, 1.4 and 1.5 g cm'3) and wetted to three matric potentials ranging from -0.01 MPa to -1.2 MPa (Chapter 6). Root elongation rate decreased with increasing penetrometer resistance and maize was considerably more sensitive than lupin towards increase in these soil physical stresses. The effects of soil mechanical impedance dominated any improvement in root-soil contact. The averaged length of the root elongation zone (estimated from the distance between root hair zone and root tip) was linearly related to elongation rates. This is a possible method for estimating root elongation rates in situ. To manipulate root-soil contact in loosely packed aggregates, seedlings were grown at various matric potentials in soils o f different aggregate sizes (4—2 mm, 2-1 mm, 1- 0.5 mm and <0.5 mm). The finer the aggregates were the faster the roots elongated, they also had better root-soil contact (72-79 % at <0.5 mm and 23-25 % at 4 -2 mm). A method was developed to investigate the role o f liquid and solid contact with roots. Roots were exposed in different portions to the mist produced in an aeroponic system. In another experiment roots were placed above a water surface and supplied at different parts with water through cotton wool. Different portions of the total surface of the root were in contact with liquid or solid phase. Root and shoot elongation were not significantly affected by the portion of root surface in contact with water or solid phase. A good root-particle contact can can improve plant growth when water availability is limiting growth but the effects of greater contact area are dependant on matric potential, plant species, and soil strength.
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