Rhizobitoxine-producing (RT +) strains of Bradyrhizobium japonicum, differing in their abilities to induce foliar chlorosis with 'Forrest' soybean (Glycine max [L.] Merr.), were evaluated for effects on short term shoot productivity, nodulation, N 2 fixation, and nodule protein production under greenhouse conditions. Soybeans were singly inoculated with washed suspensions of (Group II) USDA strains 31, 46, 76, 94, 110, 123 or 130. Strains USDA 110 and USDA 123 (Group I/Ia) were included as RT-controls. The plants were cultured in the absence of combined N in horticultural-grade vermiculite for 49 days. Beginning 21 days after planting, plants were evaluated weekly for chlorophyll, leaf protein and biomass accumulation, nodular contents of leghemoglobin, soluble protein and RT, and total shoot N content. Rhizobitoxine was detected in nodules of all RT + strains with the exception of USDA 31. However, only USDA 76 and USDA 94 produced both quantifiable concentrations of RT and symptoms of RT-induced chlorosis. Coincident with moderate to severe chlorosis were reductions in chlorophyll concentrations, shoot and nodule dry weight, leaf protein and total N 2 fixation. During extended periods of severe chlorosis, reductions in Lb and soluble nodular protein were observed. Based on carbon accumulation, all non-chlorotic treatments were statistically more productive than the chlorotic treatments. Similarly, non-chlorotic Group II treatments tended to fix less carbon relative to the RT-Group I/Ia controls, although these differences were not statistically significant. The results of this study suggest that, in the absence of discernable foliar chlorosis, the effect of RT + (Group II) nodulation on short term soybean productivity is minimal.

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