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

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Abstract
The time course profiles of C 2 H 2 reduction by intact Scirpus olneyi (bulrush), Oryza sativa (rice) and Spartina alterniflora (cordgrass) with roots in atmospheres of N 2 and 30-day-old Glycine max (soybean) in air were all immediately linear. This is the first report of immediately linear rates of C 2 H 2 reduction by grass roots removed from soil. The immediately linear profile of C 2 H 2 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 O 2 above normal environmental levels for 15 min before assay exhibited a short delay in C 2 H 2 reduction. These initially nonlinear rates of C 2 H 2 reduction are attributable to transient O 2 inhibition of nitrogenase. Initial nonlinear rates of C 2 H 2 reduction were also observed with immature soybean plants and with intact plant assays of O. sativa and S. olneyi in which C 2 H 2 was injected into cylinders surrounding the plant tops. These results indicate that, apart from O 2 inhibition of nitrogenase, the diffusion of C 2 H 2 and C 2 H 4 between the nitrogen-fixing sites and the sampling ports may cause initial nonlinear rates of C 2 H 2 reduction. We conclude that in situ plant-associated nitrogenase activity should result in immediate reduction of C 2 H 2 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.