Formation of nitrosylleghemoglobin in nodules of nitrate-treated cowpea and pea plants

The formation of nitrosylleghemoglobin (LbNO) was examined in cowpea and pea nodules in relation to the inhibition of nitrogen fixation by nitrate. Leghemoglobin was of the ferrous type and was mainly converted to LbNO in cowpea nodules when the acetylene-reducing activity decreased to 45% of control values as a result of the supply of nitrate. In nodules of nitrate-treated pea plants, leghemoglobin was also of the ferrous type and LbNO was a minor component of leghemoglobin. The levels of LbNO isolated from nodules corresponded to the levels of LbNO calculated from equilibrium constants for LbNO and the concentration of nitrite in nodules. The dissociation rate constants for LbNO from both cowpea and pea were much smaller than those for LbO₂ or LbCO, as is also the case in soybean.These results indicate that the inhibition of the functions of leghemoglobin, due to the accumulation of LbNO, induces a decrease in nitrogen fixation in cowpea nodules, and that the inhibition of nitrogen fixation in pea nodules is not related to the formation of LbNO.