THE APPARENT WIDTH OF THE RHIZOSPHERE OF TRIFOLIUM SUBTERRANEUM L. FOR VESICULAR-ARBUSCULAR MYCORRHIZAL NFECTION:...

We use the mathematical model developed by Ferriss (1981) for the study of plant pathogens to calculate the apparent width of the rhizosphere (W) of Trifolium subterraneum L. for infection by propagules of vesicular-arbuscular mycorrhizal fungi in soil, examining the effects of wetting the soil for different times before planting and of harvesting at different times. The reasons for choosing this model rather than that of Gilligan (1979) are given. Propagule densities were determined by the most probable number (MPN) method at 28 d using the same host species as the test plant, thus including a factor in the calculations which is the product of the efficiency of the propagules to infect and the susceptibility of the roots. Values for W at 10 to 12 d were between 2.5 and 6.5 mm for mixed soil inoculum and between 8.9 and 13.2 mm for Glomus mosseae (Nicol & Gerd.) Gerdeman & Trappe. W increased with time, associated with an increase in the number of infections formed. Factors contributing to this include progressive germination of propagules following wetting the soil, growth of hyphae from greater distances to the root and formation of secondary infections. Wetting the soil before planting had the greatest effect on W in 4 d old plants. By 10 d, there was no effect, supporting the idea that most propagules are likely to have germinated by that time. The contribution of secondary infections is harder to determine, but is likely to be important from about 10 d onwards, resulting in overestimation of W. By 12 to 16 d, in experiments in which six plants were grown in 210 or 222 cm³ soil, the apparent volume of the rhizosphere plus root exceeded the volume of soil. Values of the apparent width of the rhizosphere in our experiments are therefore most reliable before the plants are 10 d old. If the MPN method is used to determine propagule densities at the same harvest times and under the same conditions as those used to collect other data, then the equations can be used to compare different fungi and environmental conditions with respect to infection of young plants.