Comparing time course profiles of immediate acetylene reduction by grasses and legumes

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.