Physiological and biochemical aspects of nitrogen fixation by bacteroids in soybean nodule cells

Data are presented for relationships between concentrations of free, dissolved O2 ([O2]) and respiration and between respiration and nitrogen fixation by soybean bacteroids during microaerobic steady states in flow chamber reactions. In these, O2 was supplied in an air-saturated solution containing oxyleghaemoglobin and respiratory substrates. With all substrates tested at low but non-limiting concentrations and with endogenous respiration, rates of bacteroid respiration indicated an apparent Km of 26 nm O2 and Vmax of 21 nmol O2 min−1 (mg dry wt)−1. Provided that [O2] was <60 nm, rates of N2 fixation were positive linear functions of bacteroid respiration but the slopes of the regression lines indicated that the various substrates supported N2 fixation with different efficiencies. Further, with [O2] >60 nm, N2 fixation was negatively correlated with bacteroid respiration. The characteristics of these relationships were applied in a previously-published simulation of infected nodule cells to suggest probable effects of various levels of O2-supply on profiles of [O2] within and the consequent effects on N2-fixation. It was concluded that the infected cell would be able to sustain N2-fixation over a 100-fold range of [O2] in the cytoplasm adjacent to intercellular spaces but highest rates would occur when [O2] in this location was about 1 μm. The causes of these properties of the simulated cell are discussed and the results are compared with maximum rates of N2 fixation attained in field crops of soybeans. Although the latter are somewhat higher than simulated values, it is considered that the simulation gives realistic results for conditions likely to be encountered in vivo.