Summary: The possibility that glutamate might serve as a source of reducing power, supporting fixation of N to NH by bacteroids in soybean root nodules, was investigated. Suspensions of bacteroids were retained within a reaction chamber supplied with a flow of solution containing dissolved air, buffer, oxyleghaemoglobin and either disodium glutamate (1 or 10 mM) or no added substrate (2 or 20 mM-NaCl). The solution passing out of the chamber was analysed for dissolved NH and proportional oxygenation of leghaemoglobin was determined spectrophotometrically. Rates of production of NH and of consumption of O and the concentrations of free, dissolved O prevailing during steady states were calculated. At 1 mM, glutamate stimulated O demand of the bacteroids and enhanced NH production. However, increased production of NH was shown to arise entirely from deamination of [N]glutamate and not from N fixation. At 10 mM-glutamate, the proportion of NH arising from [N]glutamate increased with increased steady state concentrations of free, dissolved O in the chamber, but there was also significant and efficient stimulation of N fixation when the concentration of free, dissolved O was between 10 and 70 nM. The kinetics of O consumption by bacteroids using endogenous substrates or 10 mM-glutamate were similar.


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