Photosynthetic oxygen evolution within Sesbania rostrata stem nodules

The tropical wetland legume, Sesbania rostrata Brem. forms N₂-fixing nodules along its stem and on its roots after infection by Azorhizobium caulinodans . The N₂-fixing tissue is surrounded by a cortex of uninfected cells which, in the stem nodules (but not the root nodules), contain chloroplasts. The photosynthetic competence of these chloroplasts was assessed through a novel technique involving image analysis of chlorophyll a fluorescence. Calculation of the quantum efficiency of photosystem II (PS II) photochemistry from these images indicated that most of the chloroplasts with potential for non-cyclic photosynthetic electron transport were concentrated within the mid- and inner-cortex, close to the edge of the N₂-fixing tissue. PS II activity in the cortical cells was confirmed in vivo using O₂-specific microelectrodes which showed that the concentration of O₂ (pO₂) in the outer cortex could rise from less than 1% up to 23.4% upon increased irradiance of the nodule, but that the pO₂ of the inner cortex and infected tissue remained less than 0.0025%. Nitrogenase activity of stem nodules, as measured using a flow-through acetylene reduction assay (no H₂ evolution was evident), showed a reversible increase of 28% upon exposure of the nodules to supplemental light. This increase resembled that obtained with stem nodules upon their exposure to an external pO₂ of 40%.