Leghemoglobins and nitrogenase activity during soybean root nodule development

We have examined the changes in soybean leghemoglobins in relation to plant age, nodule size, and nitrogenase (EC 1.7.99.2) activity. The ratio of nitrogenase activity to leghemoglobin content declines throughout plant development and senescence. This results from a slower rate of increase of leghemoglobin compared with that of nitrogenase activity during early nodule development and a more rapid decline in nitrogenase activity after flowering. Larger nodules were generally found to have higher leghemoglobin contents but lower nitrogenase activity per unit fresh weight than nodules of intermediate size. The concentration of leghemoglobin in the nodules increases until the plants have flowered, at which time it represented 40% of the total soluble protein of the nodules. The proportions of the two major leghemoglobin species separable by electrophoresis (LbF and LbS) were measured and it was found that although LbF was the major component of all samples examined, the ratio of LbF:LbS decreased with increasing nodule size and age.