Composition and distribution of nitrogen and population of bacteroids in nodules of Chinese milk vetch, red clover, and soybean

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Abstract
Generally, in Chinese milk vetch (CV) and red clover (RC) the symbiotic N2 fixation activity is higher than in soybean (SB) per unit of nodule mass. To analyze the cause of this phenomenon, the composition and distribution of the nodule nitrogen and population as well as some biochemical properties of the nodule bacteroids of the above plants cultured in vermiculite media were investigated. Mature nodules contained 10.2 (CV), 8.6 (RC), and 3.5 (SB) mg of total cytosolic soluble proteins per g fresh weight, respectively. The content of leghemoglobin as a component of cytosolic soluble protein was 0.92 (CV), 0.62 (RC), and 0.74 (SB) mg per g fresh nodule, which corresponded to 9.0, 7.2, and 21.1% of the total cytosolic soluble proteins, respectively. The number of bacteroidal cells of the nodules was 38.2 × 109 (CV), 32.7 × 109 (RC), and 115.5 × 109 (SB) and the total content of bacteroidal soluble proteins of the nodules was 16.2, 11.6, and 10.4 mg per g fresh weight, respectively, indicating that the protein content per bacteroidal cell in the CV and RC nodules was 4–5 times higher than in SB. It was found that about 35 (CV), 28 (RC), and 24 (SB)% of the total nodule nitrogen was present in the form of cytosolic and bacteroidal soluble proteins. Within the plant species, the contents of amino acids in the nodules of RC and ureides in the nodules of SB were significantly high. Nodule bacteroids of CV and RC showed a lower specific gravity than that of SB. The activity of β-hydroxybutyrate dehydrogenase in relation to the bacteroidal soluble protein content was significantly higher in the nodules of CV than in those of RC and SB. A considerably higher activity of cytochrome c oxidase was detected in the bacteroids of CV than in those of RC and SB. The bacteroids of all the plant species displayed a catalase activity. Since the nitrogenase activity of SB per mg nodule leghemoglobin was much lower than that of CV and RC, in spite of the comparatively high concentration of leghemoglobin per g fresh nodule, it was likely that other factors than the nodule leghemoglobin concentration affected the N2 fixation activity of SB.