Inhibition of nitrogen fixation in soybean plants supplied with nitrate II. Accumulation and properties of nitrosylleghemoglobin in nodules

The accumulation of nitrosylleghemoglobin (LbNO) in nodules and the properties of LbNO in vitro were investigated in connection with the inhibition of nitrogen fixation in soybean nodules by nitrate.The leghemoglobin extracted under argon gas from nodules of plants supplied with nitrate consisted mainly of LbNO, as judged from the spectrum which corresponded to that of LbNO formed in vitro by the reaction of leghemoglobin with nitrite in the presence of dithionite or by the combination of ferrous leghemoglobin (Lb²⁺) with nitric oxide. Further, LbNO formed in vivo was easily dissociated by visible light, as was LbNO formed in vitro. Thus, authentic LbNO does actually accumulate in nodules.Most of the leghemoglobin was of the ferrous type in nodules of plants supplied with nitrate. Some LbNO appeared to be derived from LbO₂ which was deoxygenated by nitrite. The increase in levels of LbNO in nodules paralleled the decrease in acetylene reducing activity.These results indicate that the decrease in nitrogenase activity in nodules of soybean plants supplemented with nitrate is caused by the decrease in levels of LbO₂ that carries oxygen into bacteroids, which results from the formation of LbNO