Leghemoglobins and nitrogenase activity during soybean root nodule development

Abstract

Changes in soybean legHb in relation to plant age, nodule size and nitrogenase (EC 1.7.99.2) activity were examined. The ratio of nitrogenase activity to legHb content declined throughout plant development and senescence. This resulted from a slower rate of increase of legHb content compared to nitrogenase activity during early nodule development and a more rapid decline in nitrogenase activity after flowering. Larger nodules were generally found to have higher legHb content but lower nitrogenase activity per unit fresh weight than nodules of intermediate size. The concentration of legHb in the nodules increased until the plants flowered, at which time it represented 40% of the total soluble protein of the nodules. The proportions of the 2 major legHb species separable by electrophoresis (LbF and LbS, migrating rapidly and slowly, respectively) were measured. LbF was the major component of all samples examined. The ratio of LbF:LbS decreased with increasing nodule size and age.