Analysis of the Potential for Diazotrophic Endophytes to Increase Efficiency of Bioenergy Crop Production Growth promotion effects of the endophytes isolated from Populus trichocarpa and Salix sitchensis

Sustainable production of biomass for bioenergy relies on low input crop production. Inoculation of bioenergy crops with plant growth promoting endophytes has the potential to reduce the fertilizer inputs through the enhancement of biological nitrogen fixation. While nitrogen fixing (diazotrophic) endophytes colonize many wild plants, these natural relationships may be disrupted in cultivated crop species where breeding and genotype selection often occur under conditions of intensive fertilization and irrigation. A selection of diazotrophic endophytes isolated from willow (Salix sitchensis, Sitka willow) and poplar (Populus trichocarpa, black cottonwood) growing in nutrient poor river sides were used as inoculum in a series of greenhouse and field trials designed to test the overall hypothesis that naturally occurring diazotrophic endophytes impart plant growth promotion to their host plants. Endophyte inoculations contributed to increased biomass over uninoculated control plants. Biological nitrogen fixation calculated from 15N isotope dilution ranged from 18 -- 65%. No significant effect on leaf physiology was observed for plants inoculated with endophytes previously isolated from similar host plant species; however, consistently higher rates of net CO2 assimilation was observed as a result of cross species endophyte inoculations. Additionally, phenotypic plasticity in biomass allocation and branch production as a result of endophyte inoculations were observed and may be useful in bioenergy crop breeding and engineering programs.