Oxygen concentration within the leaf cavities from the apex to the base of Azolla filiculoides Lam. was measured by an oxygen microprobe. The oxygen values in the leaf cavities were lower than in the aerated water and were within the range 0.157–0.183 atm. At 140 µE m−2 s−1 irradiance, oxygen decreased in the leaf cavities where acetylene reduction activity and heterocyst frequency were highest. Oxygen evolution and oxygen consumption inhibition by KCN strongly suggest that oxygen concentration changes, in the leaf cavities, are due to both photosynthesis of Azolla and Anabaena and to respiration of the whole system.
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Caiola et al. (1989) studied this question.
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