Improving mung bean profitability by better understanding the comparative effectiveness of wild and commercial rhizobium

Abstract

Mung beans are a profitable leguminous crop. Expansion of the cultivation of mung beans in many parts of Queensland could benefit growers, by increasing the value and profitability of primary production both through direct sales of seed from this high-value crop, and/or through other crops in the farming system benefiting from residual fixed nitrogen. Grower profits could be improved, by purchasing less, increasingly costly, nitrogen fertilizer. It could also benefit the environment. By maximising nitrogen fixation by legumes, the application of nitrogen for sugarcane, banana and other non-legume crops could be reduced, potentially leading to less run-off and better outcomes for Queensland’s rivers and reef.
Simplifying the operations involved in growing mung beans may encourage more farmers to incorporate it into their farming system.
Anecdotal reports have suggested that wild rhizobia are as effective as commercial inoculum for mung bean in the Burdekin region of far north Queensland. To test this hypothesis, mung bean nodules were collected from mung bean plants from two sites in the Burdekin region. One site had been treated with commercial inoculum CB1015, the other that had never been treated with commercial inoculum. Analysis using Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (MALDI-TOF MS) showed that up to 8 different wild strains of Bradyrhizobium, distinctly different to CB1015, were present in the nodules of these plants. A different wild strain dominated at each collection site. Mung bean plants grown in soils from Millmerran, on the Darling Downs, on the other hand, hosted only CB1015 when inoculated, and did not nodulate when not inoculated. Mung beans inoculated with the two dominant wild rhizobia strains in a controlled glasshouse experiment produced similar biomass compared with those inoculated with CB1015. The plant inoculated with the wild Bradyrhizobium strains however fixed significantly more nitrogen than those with CB1015. The two mung bean varieties and the black gram, responded similarly to the three Bradyrhizobium strains.
These results support the belief that wild rhizobia are as effective at promoting growth and may be superior in fixing nitrogen in mung beans as the commercial strain in some parts of Queensland. However, the application of commercial inoculum is necessary for a healthy crop in other parts of Queensland, and may still be considered a useful “insurance policy” to ensure healthy nodulation of mung bean crops in the Burdekin catchment area