Analysis of Acetylene Reduction Rates of Soybean Nodules at Low Acetylene Concentrations

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Abstract
Acetylene reduction [assay for N2 fixation] data at subsaturating acetylene concentrations could be interpreted by use of the Michaelis-Menten equation, based on the acetylene concentration external to the nodules. One difficulty of this view is the assumption that the system is not diffusion limited is violated when studying intact nodules. The presence of a gas diffusion barrier in the nodule cortex leads to an alternate expression for the gas exchange rates at subsaturating gas concentrations. A theoretical comparison of the apparent Michaelis-Menten model and diffusion model illustrated the difficulties observed in the former model of overestimating the Michaelis-Menten coefficient and yielding a correlation between the Michaelis-Menten coefficient and the maximum rate. Use of a diffusion model resulted in estimates of the Michaelis-Menten coefficient consistent with enzyme studies, stability of the estimates of the Michaelis-Menten coefficient independent of treatment and a sensitivity of the diffusion barrier conductance to plant drought stress. All studies of nodule gas exchange need to consider possible effects caused by the presence of a diffusion barrier.