Selection of Strains of Root Nodule Bacteria to Improve Inoculant Performance and Increase Legume Productivity in Stressful Environments

Pulse, pasture and grain legumes are important for maintaining productivity in many agricultural systems. The formation of effective nitrogen-fixing symbioses between legumes and root nodule bacteria is essential for many legumes in agriculture. In many situations, soil inoculation with effective strains of root nodule bacteria is required for maximising legume yields. A key strategy to enhance the performance of inoculants to improve productivity of legumes is the selection of elite strains with improved characteristics such as greater nitrogen fixing ability, ability to survive stressful edaphic conditions, and greater competitive ability. The approach used with success at the Centre for Rhizobium Studies to identify, characterise and select strains of root nodule bacteria for use in Australian agriculture includes a four-phase glasshouse and field-based program. LEGUMES are well recognised for the important roles they have in maintaining productivity in agricultural systems (Giller and Wilson 1991; O’Hara 1998; Graham and Vance 2000). Pulse, pasture and grain legumes are grown because they can form nitrogenfixing symbioses with soil inhabiting root nodule bacteria (Vance 1997), and because their use in rotation systems can help control pests, disease and weeds (Howieson et al. 2000b). As agriculture continues to develop there are new roles emerging for legumes in the new farming systems such as the continued expansion of pulse crops onto infertile more stressful soils, and the exploration for new genera and species of pasture and forage legumes with deeprooting habits for control of soil moisture to combat salinity (Howieson et al. 2000b). The successful use of legumes in these new roles in agriculture will be dependent upon appropriate attention to the formation of effective symbioses with root nodule bacteria. An essential component for increasing the use of legumes is the integration of plant breeding and cultivar development with appropriate research leading to the selection of elite strains of root nodule bacteria. In addition the benefits of this research for farmers will be increased through advances in the application of appropriate inoculation technologies to deliver the new strains of bacteria to the soil with enhanced survival (Smith 1992; Stephens and Rask 2000). This paper outlines the approach used at the Centre for Rhizobium Studies (CRS) to select, from a genepool of natural isolates, improved inoculant quality strains of root nodule bacteria for increasing productivity of pulse, pasture and forage legumes in Mediterranean environments in Australian agriculture. The approaches, techniques and procedures outlined here have been successfully used to select a range of new inoculant strains for both established and new legumes being grown in Australia (Howieson et al. 2000a). The principle theme we address here is that in order to maximise productivity in legumebased agriculture there is an essential requirement for matching of root nodule bacteria to both host legumes and soil conditions. The paper also discusses some of the present and future challenges for strain selection in root nodule bacteria and introduces some novel uses for this technology in agriculture. Is There a Need for Inoculation? Before embarking on a lengthy program to select inoculant strains of root nodule bacteria it is essential 1 Centre for Rhizobium Studies, Division of Science and Engineering, Murdoch University, Murdoch, WA, 6150 Australia