Quantification of Root-Soil and Root-Insect Interactions Using X-Ray Microtomography.
Peter J. Gregory1, Scott N. Johnson1, Derek B. Read2, Caroline E. Hargreaves2, Dimitry V. Grinev3, and Iain M. Young3. (1) SCRI, Invergowrie, Dundee, DD2 5DA, United Kingdom, (2) Department of Soil Science, University of Reading, Whiteknights, Reading, RG6 6DW, United Kingdom, (3) SIMBIOS Centre, University of Abertay, Dundee, DD1 1HG, United Kingdom
X-ray microtomography is rapidly developing as a technique to investigate both the structure of soils and rhizosphere interactions (Gregory et al., 2003). Specialist benchtop instruments have been developed that allow spatial resolution of about 0.1 mm for sample sizes of up to 50 mm diameter. In this paper we report studies on the growth of wheat and rape roots in response to a band of phosphate and on the movement of insects (Sitona lepidus and Agrioties spp.) in the rhizosphere. Pre-germinated wheat (Triticum aestivum L.) seeds were grown in pots of 210 mm diameter acrylic tubing, 250 mm long, lined with a polythene bag. The pots were filled in three layers: the upper and lower layers consisted of acid washed sand (air-dried, sieved to <2 mm), pre-wet to a gravimetric water content of 17.5% with de-ionised water. The middle layer consisted of Bullionfield soil (air-dried, sieved to <2 mm), also pre-wet to a gravimetric water content of 17.5%, but with nutrient solution (0.5 g l-1 KH2PO4). The soil layer was 2 cm deep and the upper sand layer 3cm deep. The main root axes contacted the soil band within two days of germination. Lateral roots were visible 4 days later, and 8 days after germination, an extensive array of lateral roots was visible on the radicle, within the soil band (Fig.1). The movement of Sitona lepidus larvae towards nodules of white clover (Trifolium repens L.) roots growing in re-packed soil was studied non-invasively (Johnson et al., 2004). The larvae travelled between 9 and 27 mm in 9h at a mean speed of 1.8 mm h-1. Burrowing patterns were convoluted rather than linear with changes in trajectory evident from this study that would be masked in more commonly used “slant boards”. These studies demonstrated that the larvae responded to a chemical signal from the nodule/root, possibly an isoflavonoid (Johnson et al., 2005). More recently studies with Agriotes spp. (Fig. 2) have demonstrated the preferential movement to some genotypes of potato but not to others, again suggesting response to semiochemicals. Fig. 1. Growth of a wheat seedling through a soil band enriched with P after 8 days. The horizontal lateral roots show the boundaries of the soil band. Fig. 2. A mature wireworm (Agriotes spp.) about 20 mm long moving through re-packed soil.