Effects of carbohydrate on the internal oxygen concentration, oxygen uptake, and nitrogenase activity in detached pea nodules

The interaction between carbon substrates and O₂ and their effects on nitrogenase activity (C₂H₂) were examined in detached nodules of pea (Pisum sativum L. cv “Sparkle”). The internal O₂ concentration was estimated from the fractional oxygenation of leghemoglobin measured by reflectance spectroscopy. Lowering the endogenous carbohydrate content of nodules by excising the shoots 16 hours before nodule harvest or by incubating detached nodules at 100 kPa O₂ for 2 hours resulted in a 2- to 10-fold increase in internal O₂, and a decline in nitrogenase activity. Conversely, when detached nodules were supplied with 100 millimolar succinate, the internal O₂ was lowered. Nitrogenase activity was stimulated by succinate but only at high external O₂. Oxygen uptake increased linearly with external O₂ but was affected only slightly by the carbon treatments. The apparent diffusion resistance in the nodule cortex was similar in all of the treatments. Carbon substrates can thus affect nitrogenase activity indirectly by affecting the O₂ concentration within detached nodules.