Comparing Time Course Profiles of Immediate Acetylene Reduction by Grasses and Legumes †

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The time course profiles of C2H2 reduction by intact Scirpus olneyi (bulrush), Oryza sativa (rice) and Spartina alterniflora (cordgrass) with roots in atmospheres of N2 and 30-day-old Glycine max (soybean) in air were all immediately linear. This is the first report of immediately linear rates of C2H2 reduction by grass roots removed from soil. The immediately linear profile of C2H2 reduction by soil-free grass roots was achieved by preventing contact between the roots and air. Roots of soybeans and S. olneyi receiving pretreatments of O2 above normal environmental levels for 15 min before assay exhibited a short delay in C2H2 reduction. These initially nonlinear rates of C2H2 reduction are attributable to transient O2 inhibition of nitrogenase. Initial nonlinear rates of C2H2 reduction were also observed with immature soybean plants and with intact plant assays of O. sativa and S. olneyi in which C2H2 was injected into cylinders surrounding the plant tops. These results indicate that, apart from O2 inhibition of nitrogenase, the diffusion of C2H2 and C2H4 between the nitrogen-fixing sites and the sampling ports may cause initial nonlinear rates of C2H2 reduction. We conclude that in situ plant-associated nitrogenase activity should result in immediate reduction of C2H2 and that linear rates are observed when the proper assay conditions are used. Our data suggest that nitrogen fixation is closely associated with the roots of S. olneyi, O. sativa, and S. alterniflora growing in salt marsh sediment.

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