4-06: Increasing biomass production sustainably using natural endosymbionts of poplar

The interior of poplar trees provides habitat for a diversity of microbes, termed endophytes, that benefit the plant host by increasing nutrient acquisition, stress tolerance, and pollutant degradation.  Nitrogen fixed biologically by plant-symbiotic bacteria is ecologically friendly, reducing the need for chemical fertilizers that cause environmental damage.  We have isolated a variety of microorganisms able to grow under nitrogen-limitation from the stems of native poplar (Populus trichocarpa) and willow (Salix sitchensis) in riparian zones in the Pacific Northwest.  The presence of these diazotrophic microorganisms may help explain the ability of these pioneering tree species to grow under nitrogen-limiting conditions.  In lab and greenhouse studies, inoculated poplar plants exhibited increases in biomass, height, carbon content, nitrogen content, and leaf area compared to uninoculated control plants.  ¹⁵N dilution studies indicated that nearly half the nitrogen content in the poplar plants was due to biological nitrogen fixation.  This is the first report of nitrogen fixation in this non-leguminous tree species.  Utilization of native plant growth-promoting, nitrogen-fixing endophytes may enable plantations of these important bioenergy crops to be grown more sustainably and at less cost.