During the 6- to 7-week active life of soybean root nodules produced by a single strain of , the respiratory activity of the bacteria isolated from the nodules changed according to a definite pattern: a depression occurred 2 weeks after nodules first appeared, followed by a rise, and with a further sharp rise 1 week before nodule breakdown. The initial depression coincided with the onset of the bacteroid condition and the beginning of nitrogen fixation. The limits within which respiration varied depended on host-plant growth. The nitrogen-fixation rate remained constant from 2-3 weeks after nodule appearance until a few days before nodule decay began. The bacteroids isolated from the nodule did not oxidize substrates of the tricarboxylic acid cycle to completion, but the number of mole O consumed/mole substrate did not change during nodule life, and were similar to values obtained for cultures of the free living bacteria. Changes in bacteroid dry weight and nucleic acid content were observed; these were within the limits of dry weight and nucleic acid content of the same bacterial strain grown in liquid culture. The ratio acid soluble nucleotides: nucleic acid for bacteroids was between 0·12 and 0·13 during nitrogen fixation; these values are characteristic of bacteria of the same strain from the mid-logarithmic phase of growth in culture in spite of the fact that bacteroids are non-proliferating cells.


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