Abstract
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%.
Published Version
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