Limitations of Symbiotic and Associative Nitrogen Fixation by Developmental Stages in the Systems Rhizobium japonicum with Glycine max and Azospirillum brasilense with grasses, e.g., Triticum aestivum

Abstract

By the oxygen enhancement effect of nitrogenase activity four stages (I–IV) of nodule development in Glycine max are separated. This effect correlates with an increase of leghemoglobin content above 7×10-2 μmol per g nodule fresh weight. Nodules fall into distinct size classes. Four maxima can be separated for the nodule population in stages II to IV, indicating that age and size are rather different parameters to characterize nodule development. A drop in nitrogenase activity in stage II is correlated with the exhaustion of starch granules in the amyloplast-mitochondrion associations and with space-filling multiplication of the bacteroids in the infected cells. During stage III with a constant nitrogenase activity at a low level, the bacteroids continue to accumulate poly-3-hydroxybutanoic acid (PHBA). In pure culture of Rhizobium japonicum maximum nitrogenase activity of 100– 150 nmol C2H4·mg protein-1·h-1 appears suddenly at a certain culture age as well as depending on the oxygen concentration. Some activities of other enzymes of N-metabolism are compared for bacteroids from effective and ineffective nodules with nitrogenase repressed and derepressed free-living cells. This indicates that the Rhizobium cells in pure culture can behave similarly to the bacteroids in the nodule cells. However, we have almost no information about the actual composition of the infection vacuoles surrounding the bacteroids in the nodule cells.