Annual medics (primarily Medicago truncatula, M. littoralis, M. polymorpha, and several other less important species) form the basis of the majority of pastures grown on neutral to alkaline soils in the low to medium rainfall zone (250 - 500 mm annual rainfall) of the Australian wheatbelt. The general practice on these soils is to sow uninoculated seed and rely on naturalised R. meliloti strains for nitrogen fixation. In Western Australia (WA), annual medics (primarily M. polymorpha) are also grown on slightly acid soils. An overview of current research on the Rhizobium meliloti - annual medic symbiosis is outlined.

Recent surveys (Slattery et al. 1998) have confirmed the importance of soil pH on the population size of R. meliloti. Populations are generally high (>1000 per g soil) on neutral to alkaline soils (pH Ca > 6.5). This relationship leads to the situation of generally large naturalised populations in South Australia (SA) and Victoria (Vic), whereas WA soils generally support low populations. The effectiveness (ability to fix nitrogen) of naturalised populations is strongly dependent on the host legume, but for most of the dominant commercial species, effectiveness is usually adequate, although wide variation exists.

Annual medics vary in their ability to form effective symbioses with different R. meliloti strains. Some species (eg. M. littoralis), are highly specific in their strain requirement, whereas other species (eg. M. truncatula) are able to form effective symbioses with a wide range of strains. The Australian inoculant industry accommodates species requirements by providing two inoculant subgroups. Strain WSM688 (marketed as ‘AM’) services all annual medics other than M. littoralis and M. tornata. Strain WSM826 (marketed as ‘AL’) services the latter two medics as well as lucerne. Improved strains are being sought to replace both incumbent strains.

In SA and Vic, one of the intractable problems has been the introduction of inoculant strains into often large naturalised populations that vary in their effectiveness with the sown host. This problem may be averted by;

1. selecting host genotypes on the basis of their ability to nodulate effectively with the naturalised rhizobia population,

2. selecting host species/cultivars, which are adapted to the edaphic environment, but do not form a symbiosis with the naturalised rhizobia.

In contrast, naturalised populations in WA are often low, and the development of acid tolerance in the medic - rhizobia symbiosis (Howieson and Ewing, 1989) has resulted in the expansion of annual medics in WA. New methods for strain identification (Richardson et al. 1995), and other molecular genetics methods, are being employed at a number of locations and promise to greatly improve our understanding of rhizobial ecology.

Howieson, J.G. and Ewing, M.A. 1989. Aust. J. Agric. Res. 40, 843-850

Richardson, A.E., et al. 1995. Soil Biol. and Biochem. 27, 515-524.

Slattery, J.F., et al. 1998. Plant and Soil. In press.