THE RATE OF DEVELOPMENT OF MYCORRHIZAS AFFECTS THE ONSET OF SPORULATION AND PRODUCTION OF EXTERNAL HYPHAE BY 2 SPECIES OF ACAULOSPORA

Subterranean clover was inoculated with Acaulospora laevis, [WUM 11(4)] or an undescribed Acaulospora sp. [WUM 18(1)] at three inoculum quantities and the rate of development of mycorrhizas and the production of spores and external hyphae were monitored. A critical length of mycorrhizal root was found to be associated with the onset of sporulation for both fungi. This was less for Acaulospora sp. than it was for A. laevis and was first reached for plants inoculated with the highest quantity of inoculum for each fungus. Root length colonized and number of spores produced were well correlated at early sampling times once sporulation had begun, but were not well correlated at the end of the experiment. Acaulospora sp. produced six times as many spores as did A. laevis by the final harvest, but this only represented half the volume of spores produced by the latter. After sporulation had commenced the percentage of root length colonized by A. laevis declined while the percentage of root length colonized by Acaulospora sp. remained high, perhaps because of differences in the length of external hyphae produced by each fungus. The length of external hyphae per gram of soil increased with Acaulospora sp. but not for A. laevis, throughout the experiment. However for each fungus there was not a dose relationship between the length of external hyphae and the number of spores produced.