ABSTRACT
The availability of usable forms of nitrogen is limiting to cereal-grain crop growth in field situations. The contribution of biological nitrogen fixation is, however, limited by a number of factors that are not clearly understood. Root nodules form on soybeans as a result of invasion by the soil bacterium Rhizobium japonicum. The rhizobia, now called bacteroids, are observed as membrane-enclosed structures usually with several bacteroids per structure. This membrane is referred to as the peribacteroid membrane. The role of leghemoglobin is well documented. Studies on nitrogenase activity with isolated soybean bacteroids have generated results that could help explain some of the phenomena observed with intact soybeans. The development of the acetylene reduction technique as an assay for nitrogenase activity greatly accelerated the bacteroid studies. The rapid recovery observed with bacteroids suggests again that protein synthesis is not responsible.
