PHOTOSYNTHETIC OXYGEN EVOLUTION WITHIN SESBANIA-ROSTRATA STEM NODULES

The tropical wetland legume, Sesbania rostrata Brem. forms N-2-fixing nodules along its stem and on its roots after infection by Azorhizobiu m caulinodans. The N-2-fixing tissue is surrounded by a cortex of unin fected cells which, in the stem nodules (but not the root nodules), co ntain chloroplasts. The photosynthetic competence of these chloroplast s was assessed through a novel technique involving image analysis of c hlorophyll a fluorescence. Calculation of the quantum efficiency of ph otosystem II (PS II) photochemistry from these images indicated that m ost of the chloroplasts with potential for non-cyclic photosynthetic e lectron-transport were concentrated within the mid-and inner-cortex, c lose to the edge of the N-2-fixing tissue. PS II activity in the corti cal cells was confirmed in vivo using O-2-specific microelectrodes whi ch showed that the concentration of O-2 (PO2) in the outer cortex coul d rise from less than 1% up to 23.4% upon increased irradiance of the nodule, but that the pO(2) of the inner cortex and infected tissue rem ained less than 0.0025%. Nitrogenase activity of stem nodules, as meas ured using a flow-through acetylene reduction assay (no H-2 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 pO(2) of 40%.