Soybean response to nodulation by rhizobitoxine-producing bradyrhizobia as influenced by nitrate application

Abstract

Foliar chlorosis of soybean (Glycine max [L.] Merr.) resulting from nodulation by rhizobitoxineproducing (RT +) strains of Bradyrhizobium japonicum is commonly less severe in the field than under greenhouse conditions. Differences in nutritional conditions between the field and greenhouse may contribute to this phenomenon. In particular, field-grown plants obtain some N from soil sources, whereas in the greenhouse soybean is often grown in low-N rooting media to emphasize symbiotic responses. Therefore, we examined the effect of NO 3 on the expression of RT-induced symptoms. Soybean plants inoculated with RT + bradyrhizobia were grown for 42 days in horticultural vermiculite receiving nutrient solution amended with 0.0, 2.5, or 7.5 mM KNO 3. Foliar chlorosis decreased with increasing NO~ application whereas nodule mass per plant was generally increased by NO~ application. Total amounts of nodular RT remained constant or increased with NO~ application, but nodular concentrations of RT decreased. Chlorosis severity was negatively correlated with shoot dry weight, chlorophyll concentration, and total shoot N content. It was concluded that application of NO 3 can reduce the negative effects of RT production on the host plant. This suggests that any NO 3 present in field soils may serve to limit chlorosis development in soybeans.Abbreviations: R T -rhizobitoxine, RT + -rhizobitoxine-producing, Lb -leghemoglobin

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

“…Certain soybean genotypes possess the Rj4 gene, which has been shown to interdict nodulation by B. elkanii (Devine et al, 1990), but no evidence of this interdiction was observed for the cultivars included in this study. Reasons for the lack of correlation between the greenhouse and field studies for some cultivars are not known, but may include differences in plant growth stage at sampling, contrasting nutrient regimes between the greenhouse and field (Johnson et al, 1958;Teaney and Fuhrmann, 1993), and possible modifying effects of altered nodule functioning over time for the various cultivars examined (Teaney and Fuhrmann, 1992). Nevertheless, the high apparent sensitivity of many soybean cultivars to RT+ strains, combined with the high numbers of RT+ soybean bradyrhizobia present in many soils of the mid-Atlantic and southeastern USA (Caldwell and Hartwig, 1970;Fuhrmann, 1990;Keyser et al, 1984;Mpepereki and Wollum, 1991;Weber et al, 1989), justifies additional studies of the role of rhizobitoxine in the soybean-bradyrhizobia symbiosis.…”