Azolla is a genus of small aquatic ferns with a world-wide distribution in temperate and tropical regions. In Azolla's leaf cavities, a symbiotic bluegreen algae-Anabaena azollae fixes molecular nitrogen and supports host growth on nitrogen-free medium. Their nitrogen fixing rate is as high as 450 kg N. ha^<-1>. year^<-1> and the symbiotic nitrogen fixing activity of the algal cells is much higher than that of bacteroids in nodules. Optimum temperature of A. pinnata, a subtropical species, is about 25℃, so relatively low tolerance for high temperature restricts wide application of Azolla in the tropics. Phosphorus nutrition is the most critical to Azolla growth in paddy field. In the most suitable conditious, 1kg P_2O_5 of superphosphate can produce more than 2kg of nitrogen in Azolla body. In rice culture, Azolla is extensively used as an effective green manure in China and Vietnam. Currently, the interest in this plant has increased in other countries. Azolla is grown either before or after transplanting of rice. Agronomic and academic studies on Azolla must be strengthened in view of high potential for nitrogen fixation by the Azolla-Anabaena complex.

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