Maackia amurensis Rupr. & Maxim. is a leguminous tree species possessing meritorious ornamental characteristics and is confirmed to associate with rhizobia that fix nitrogen, but few attempts to isolate symbiotically superior rhizobia have been made. Our goals were to isolate rhizobia from the root zones of indigenous trees of M. amurensis in two ecologically distinct forests in the Heilongjiang Province of China, characterize the rhizobia, and compare their effectiveness at causing nodulation of this host plant. Rhizobia were isolated and cultured from nodules that formed on seedlings grown in soils collected in May 1998, from the Maoershan (45°N, 127°E) and Liangshui (47°N, 128°E) Research Forests. Inoculants from each of the 160 isolates were applied to seedlings. A subset of 48 isolates that evoked the most nodules was partitioned by cluster analysis into 12 similarity groups based on measures of number of nodules (17.9 ± 6.5), the ratio of growth rate on two distinct media (2.26 ± 1.8), pH reaction as measured by absorption at 614 nm of bromthymol blue (0.98 ± 0.36), and tolerance to sodium chloride at 15 g/L (23 out of 48). By using single-isolate cultures of similar cellular concentration as inoculants, one isolate from each group and USDA 4349, an isolate obtained during previous research, are being compared for their capacity to infect and nodulate seedlings.
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