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

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 KNO₃. Foliar chlorosis decreased with increasing $NO_3^ - $ application whereas nodule mass per plant was generally increased by $NO_3^ - $application. Total amounts of nodular RT remained constant or increased with $NO_3^ - $ 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.