Variability among Soybean Genotypes in Response to Nodulation by a Rhizobitoxine‐Producing Strain of Bradyrhizobia

Abstract

Nodulation by rhizobitoxine‐producing (RT+) strains of Bradyrhizobium elkanii (syn. Bradyrhizobium japonicum; DNA homology group II) has been shown to reduce shoot growth, N2 fixation, and seed yield of ‘Forrest’ soybean (Glycine max [L.] Merr.). We conducted greenhouse and field experiments to assess the frequency of these negative responses among selected soybean cultivars. In the greenhouse, 38 cultivars representing a range of yield potentials, maturity groups, and growth habits were inoculated with B. elkanii strain USDA 94 (RT+) or B. japonicum strain USDA 110 (RT−). Large differences in tolerance to nodulation by RT+ bradyrhizobia were noted, as estimated visually and by comparing shoot weights (40 d after planting) of plants inoculated with the two strains. For the field study, two tolerant (Stafford and SS FFR 565), two moderately sensitive (Pharaoh and Williams 82), and two highly sensitive cultivars (Forrest and Hutcheson) were chosen. Seeds sown to vermiculite under greenhouse conditions were inoculated with either USDA 94 or a suspension of soil taken from the study site containing the indigenous bradyrhizobia. After 23 d, the nodulated seedlings were transplanted to 1.8‐m2 field microplots. Shoots were harvested at the R7 growth stage and analyzed for various response parameters. Only Forrest, Hutcheson, and Pharaoh were significantly affected by inoculation with USDA 94, which decreased seed yield, pod number, vegetative growth, and the amount of N2 fixed. These results support the hypothesis that significant genotypic variability exists in the response of soybeans to nodulation by RT+ bradyrhizobia. The greenhouse screening procedure can provide useful predictions of the field response of tolerant and highly sensitive cultivars, but may not be accurate for moderately sensitive cultivars.

Differential Response of Cowpea (<i>Vigna unguiculata </i>L.<i>)</i> Genotypes to Native Rhizobia in Tarai Region of Uttarakhand, India

“…The observed significantly better performance of PGCP-4 and PGCP-6, show variable differences in the physiological structures, which made these cultivars readily able to absorb nutrients and water from the soil, and to effectively carry out the photosynthetic process and ultimately, to accumulate the higher amount of photosynthates. This observation was in congruence with the earlier reports of Fuhrmann and Vasilas (1994), which concluded that the cultivars respond differently to various environmental factors, and this differential response is primarily driven by their genetic makeup and their adaptive capabilities. The dehydrogenase activity is linked with the microbial activity, related with the physiological efficiency of microorganisms associated with plants, which is mainly host-dependent as they releases nutrients for microbes in the form of root exudates.…”

Inheritance of rhizobitoxine‐induced chlorosis in soybean

“…The degree of susceptibility in their study ranged from highly resistant to highly susceptible. Fuhrmann and Vasilas (1994) also showed variability among soybean genotypes in response to RT. They evaluated 38 soybean cultivars in the greenhouse for their response to RT and reported that cultivars ranged from tolerant to very sensitive.…”