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Abstract
Symbiotic efficiency of 50 native bradyrhizobium isolates from northern Thailand with 12 soybean cultivars of Asian, Nigerian, or US origin was evaluated under laboratory conditions. The isolates were obtained from fields in upland rainfed areas of northern Thailand where soybeans had been traditionally cultivated. Twelve soybean cultivars of Asian or US origin were grown in a nitrogen-free nutrient solution with inoculation of each of the isolates. The symbiotic efficiency of the isolates was evaluated based on the total amount of nitrogen accumulation by host plants in comparison with uninoculated control plants or plants inoculated with Bradyrhizobium japonicum USDA110. Almost all the isolates were able to nodulate all the soybean cultivars. Compared with control soybean cultivars grown in a nitrogen-free nutrient solution (- N - R), forty-four isolates induced a high nitrogenfixing ability (E category) in association with at least one soybean cultivar, Coc Chumhat. More than half of the tested isolates induced effective nitrogen fixation in this category in association with almost all the tested soybean cultivars (except for Peking (46%) and Bossier (20%)). Unique character of the tested isolates was that most of them were effective in association with a relatively wide range of soybean cultivars, i.e. “promiscuous.” This character was not observed in the native isolates from irrigated areas in our previous study. Three soybean cultivars, Coc Chumhat, CMU0001, and IITA medium, showed a high symbiotic performance (E + e category of the isolates) with most of the tested isolates (90, 84, and 78%, respectively), suggesting their “promiscuous” character. From the results of our three experiments (one field and two laboratory experiments), it was concluded that: 1) the native rhizobial populations in the traditional soybean-growing area of northern Thailand showed a high level of effectiveness and effectiveness in association with a wide range of soybean genotypes, 2) when a proper soybean cultivar is used, adequate amount of nitrogen fixation for soybean would be expected from these native rhizobial populations, and 3) various symbiotic characteristics of the native rhizobial populations obtained from field and laboratory experiments provided useful background information for the justification of soybean cultivation and the need for inoculation in northern Thailand.