Abstract
Symbiotic efficiencies and compatibilities between native rhizobia and 17 soybean cultivars including Asian and US ones were investigated in a farmer field in a traditional soybean-growing area of northern Thailand where inoculation had never been practiced. The field is representative of the soybean cultivation area in the cool-dry season with irrigated lowland rice-based system in the upper northern region. Nodule dry weight of the tested soybean cultivars revealed good nodulation ability of the native rhizobia with both Asian and US soybean cultivars. This result indicates the presence of a wide genetic host range of the native rhizobial populations in this soybean-growing area. Relative ureide index, nitrogen-fixing efficiency and seed yield of the tested soybean cultivars indicated that on the average, “Asian-type” soybeans exhibited a higher symbiotic performance than the “US-type” soybeans with the native rhizobia. Among all the tested soybean cultivars, Dempo, a local Indonesian cultivar, produced maximum seed yield (1.8 t ha−1). Among the tested US culti vars, Bossier showed the highest symbiotic performance in terms of nodule dry weight, shoot dry weight and total nitrogen accumulation. However, the highest seed yield was found in another US soybean cultivar, Valder. This was probably due to the new development of high nodulation at later growth stages, i.e. the pod-filling stage. Two Thai breeding cultivars, CM60 and CMU001 derived from US cultivars, produced a higher seed yield (1.5 and 1.8 t ha−1, respectively) than their parent cultivars, Williams and Pakchong (0.9 and 1.1 t ha−1, respectively). Due to the high effectiveness of the native rhizobial populations, inoculation may not be necessary for many soybean cultivars grown in this area. However, it appeared that the symbiotic efficiency depended largely on the soybean cultivars. Therefore, when new breeding cultivars are released, the symbiotic performance of these new cultivars with the native rhizobia should be assessed.
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