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UBC Theses and Dissertations

Evaluating the comprehensive role of endophytic nitrogen-fixing bacteria in sustaining the growth of boreal forest trees on nutrient-poor soils Puri, Akshit


West Chilcotin region located in the Sub-Boreal Pine-Spruce xeric-cold biogeoclimatic zone of British Columbia is characterized by cold climate, low annual precipitation and weakly developed soils. Lodgepole pine and hybrid white spruce comprise the most common tree stands in this region. Soils examined from pine and spruce stands revealed that they have poor physico-chemical health and limited plant-available nutrients, particularly nitrogen. Despite the severely nitrogen-limited soils, the continued, vigorous growth of pine and spruce trees is a major conundrum. A rarely evaluated but possible nitrogen-input could be through endophytic diazotrophic bacteria – nitrogen-fixing bacteria living inside plant tissues. Examining this possibility, 55 and 48 bacterial strains were isolated on nitrogen-free media from internal tissues of pine and spruce, respectively, of which 23 and 18 strains showed positive nitrogenase activity. Six strains each from pine and spruce that showed highest nitrogenase activity were evaluated for their effects on their original host as well as the other tree species (i.e., spruce bacteria tested with pine and pine bacteria tested with spruce) in greenhouse trials. All strains colonized both pine and spruce and performed similarly well in both hosts, contributing 15-56% of host’s foliar nitrogen from the atmosphere after one year and significantly enhancing seedling length and biomass. These strains also possess several other plant-growth-promoting abilities such as inorganic and organic phosphate solubilization, IAA production, ACC deaminase activity, siderophore production and lytic enzyme activity. Notably, four strains closely related to the genus Burkholderia – Caballeronia sordidicola LS-S2r and HP-S1r, Caballeronia udeis LP-R2r and Paraburkholderia phytofirmans LP-R1r – provided up to 5.5mg nitrogen per kg tissue to their host after one year, enhanced seedling biomass by up to 7-fold in 18 months and showed significant potential in the aforementioned plant-growth-promoting mechanisms. Therefore, the ability of such highly effective endophytic diazotrophic bacteria to form beneficial ecological associations with pine and spruce trees could explain their sustained growth on extremely nutrient-limited soils of the West Chilcotin region. Furthermore, their effectiveness with a foreign host indicates the lack of plant x microbe specificity, raising the possibility of their utilization as potential biofertilizers to regenerate trees in disturbed and nutrient-poor ecosystems.

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