Photosynthetic Oxygen Evolution within Sesbania rostrata Stem Nodules

The tropical wetland legume, Sesbania rostrata Brem. forms nitrogen-fixing nodules along its stem and on its roots after infection by Azorhizobium caulinodans. The central 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 pO2 in the outer cortex could rise from less than 1% to over 8% upon increased irradiation of the nodule, and that this rise in pO2 was immediately reversible when the extra light was removed; increased irradiance resulted in hyperoxic pO2s of up to 23.4%. The inner cortex and infected tissue in the stem nodules had a pO2 of less than 0.0025%, but the pO2 in these regions were unaffected by increased irradiance. Possible mechanisms preventing O2 influx into the infected tissue, including a variable O2 diffusion barrier in the cortex, are discussed. Nitrogenase activity of stem nodules, as measured using a flow-through acetylene reduction assay (no H2 evolution was evident), showed an increase of 28% within 30 minutes of exposure of the nodules to supplemental light, corresponding with a rapid decline (37%) in CO2 output from the nodulated stem.