PHYSIOLOGICAL AND BIOCHEMICAL ASPECTS OF NITROGEN-FIXATION BY BACTEROIDS IN SOYBEAN NODULE CELLS

Data are presented for relationships between concentrations of free, d issolved O-2 ([O-2]) and respiration and between respiration and nitro gen fixation by soybean bacteroids during microaerobic steady states i n flow chamber reactions. In these, O-2 was supplied in an air-saturat ed solution containing oxyleghaemoglobin and respiratory substrates. W ith all substrates tested at low but non-limiting concentrations and w ith endogenous respiration, rates of bacteroid respiration indicated a n apparent K-m of 26 nM O-2 and V-max of 21 nmol O-2 min(-1) (mg dry w t)(-1). Provided that [O-2] was <60 nM, rates of NZ fixation were posi tive linear functions of bacteroid respiration but the slopes of the r egression lines indicated that the various substrates supported Nz fix ation with different efficiencies. Further, with [O-2] >60 nM, N-2 fix ation was negatively correlated with bacteroid respiration. The charac teristics of these relationships were applied in a previously-publishe d simulation of infected nodule cells to suggest probable effects of v arious levels of O-2-supply on profiles of [O-2] within and the conseq uent effects on N-2-fixation. It was concluded that the infected cell would be able to sustain N-2-fixation over a 100-fold range of [O-2] i n the cytoplasm adjacent to intercellular spaces but highest rates wou ld occur when [O-2] in this location was about 1 mu M. The causes of t hese properties of the simulated cell are discussed and the results ar e compared with maximum rates of N-2 fixation attained in held crops o f soybeans. Although the latter are somewhat higher than simulated val ues, it is considered that the simulation gives realistic results for conditions likely to be encountered in vivo. (C) 1997 Elsevier Science Ltd.