The azolla-anabaena symbiosis: Morphology, physiology and use^1,2

Heterosporous ferns of the genus Azolla contain an N₂ -fixing cyanobacterium, designated Anabaena azollae, in specialized leaf chambers. Our studies of the symbiosis per se have been primarily restricted to A. caroliniana. However, comparative studies have encompassed four of the six known Azolla species: A. filiculoides, A. mexicana and A. pinnate, in addition to A. caroliniana. The physiological, biochemical, morphological and ultrastructural studies which support a current hypothesis of host-symbiont interactions and structure-function relationships are described. The latter is largely based on recent studies of the ontogenetic sequence of leaf development and associated events in the symbiotic cyanobacterium. Under optimal growth conditions the four Azolla species double their biomass within two days and maintain an N content of 5-6% of the dry weight with N₂ as the only N source. Results of subsequent comparative studies of photosynthesis, respiration, nitrogen fixation and associated processes in A. caroliniana as a function of 12 h, 16 h and continuous light regimes are summarized. The daily N input from light and dark fixation is estimated for the four Azolla species. The effects of several combined N sources on growth and physiological processes under a 16 h-8 h light-dark regime and estimates of the N inputs from combined N sources and N₂ fixation, as determined using isotope dilution techniques, are presented. The historical and current use 'of this association', as an alternative N source in rice culture is briefly described. Some of the more recent field studies by other workers are reviewed and the agronomic potential and limitations of Azolla as an alternative N source are discussed.