The legume tree Pongamia pinnata (also called Millettia pinnata) is a non-food crop that can grow on marginal land not destined for the cultivation of food crops. It is an important candidate for the production of biofuels (such as biodiesel and aviation fuel) from its oil-rich seeds. Pongamia is resilient against abiotic stresses such as drought, salinity and acidity. An important trait that establishes Pongamia as a superior biofuel feedstock is its ability to grow in marginal, nitrogen-limited soils. This growth ability is attributed to the nitrogen-fixation activity of root nodules developed by symbiosis with bacterial microsymbionts broadly known as rhizobia.
Pongamia-Rhizobia symbiosis was improved by the inoculation of Rhizobium-related strain PR-UQ-05 to pongamia, which naturally occurs in soils across Queensland. Pongamia pinnata produced active and efficient nodules using this strain when cultivated with low levels of nitrate and even under saline conditions. Nodules started to form within three weeks of inoculation and became mature (nitrogen fixing) between 8-12 weeks. Pongamia may also utilize ureides such as allantoin, aside from amides, to transport fixed nitrogen in the plant, as determined by ureide analysis. Lastly, the assessment of the process of nitrogen fixation through – (i) acetylene reduction assay, (ii) N-difference method and (iii) isotope dilution, proved that Pongamia fixed sufficient amounts of nitrogen for growth and reproduction. Therefore, Pongamia can now be commercially grown without the addition of potentially environmentally harmful and expensive chemical fertilizers, since its symbiotic nitrogen fixation has been established and improved with the use of an efficient rhizobia strain and grown under favourable nodulation conditions.
(Photographs courtesy of Phoebe Nemenzo-Calica, former PhD student at the University of Queensland, Australia and currently a faculty member of Ateneo de Davao University, Roxas Avenue, Davao City, Philippines 8000)
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