Index >>Nitrogen Fixation >>Repression by Fixed Nitrogen

Repression by Fixed Nitrogen -
Repression of nitrogen fixation is brought about by the presence of available fixed nitrogen in the soil. Excess of NH₄⁺ or NO₃^- also prevents nodulation. Regulation of nitrogen fixation by fixed nitrogen appears to be at the level of nodulation and not at the level of nitrogenase synthesis.

Lectin binding is necessary for infection by Rhizobium. Fixed nitrogen appears to prevent the lectin from being available to the bacteria in R.trifoli and clover. It thus prevents infection of the plant roots, and consequently nodulation.

R. japonicum mutants lacking nitrate reductase (which breaks down nitrate) are unable to nodulate soybeans in the presence of high levels of nitrate.

This indicates that nitrates, or metabolic products of nitrate metabolism, are involved in the inhibition of nodulation.