DIFFERENTIAL RHIZOBIAL COLONIZATION OF THE ROOTS OF SOWN AND VOLUNTEER ANNUAL SPECIES OF MEDICAGO IN AN ACID SOIL

Two annual medics, Medicago murex and M. truncatula, were inoculated with specific strains of Rhizobium meliloti, WSM540 and CC169, respectively, and sown into a mildly acid red earth in a semi-arid environment at Cobar, New South Wales. A third medic, M. laciniata, volunteered abundantly at the experimental site and large populations of its specific strain(s) of R. meliloti were naturalised in the soil. Serial dilution, plant infection tests, using 3 test plants in parallel, were employed to count the populations of each of the 3 types of R. meliloti that colonised the roots (rhizospheres) of the 3 medics. The size of the rhizosphere populations was regarded as an index of nodulation potential. Large populations of the naturalised rhizobia (about 45000/plant) developed in the rhizospheres of M. laciniata; these rhizobia also colonised the root surfaces of the sown medics but in much lower numbers (<80/plant). Strain WSM540 colonised the rhizospheres of M. murex in sufficient numbers (1121 rhizobia/plant) to indicate that an effective nitrogen-fixing symbiosis would have established had seasonal conditions permitted. By contrast, the colonisation of M. truncatula rhizospheres by CC169 was significantly lower (64 rhizobia/plant; P<0.05) and the likelihood of nodulation was uncertain. Both WSM540 and M. murex are known to be acid tolerant whereas both CC169 and M. truncatula are acid sensitive. There was very little colonisation of the rhizospheres of the sown medics by non-specific inoculant strains. Nor was there evidence that the large naturalised population of rhizobia for M. laciniata competed with the specific inoculant strains for colonisation of the roots of the sown medics or interfered with their potential nodulation.