Azolla: Botany, Physiology, and Use as a Green Manure 1 THOMAS A. LUMPKIN AND DONALD L. PLUCKNETT2 This is a comprehensive review of literature pertaining to the aquatic fern Azolla and its nitrogen-fixing algal symbiont, Anabaena azollae. The preceding decade has witnessed an explosive growth in research on Azolla, and hopefully this paper will facilitate those efforts. The paper is broken into three major categories: botany, physiology and bio- chemistry, and agriculture. The botany section includes a world distribution map and reference tables for the 6 Azolla species, and includes the first review of literature on Anabaena azollae. The physiology and biochemistry section covers the range of topics from en- vironmental factors to life processes and nitrogen fixation. Tables on the effect of growth regulators and on the rate of nitrogen fixation measured by acetylene reduction are presented. The agriculture section draws extensively from literature published in the Peo- ple's Republic of China and in the Democratic Republic of Vietnam. The major focus of this section is on the history and management practices for Azolla cultivation as a green manure for rice. The effect of weed suppression, use as a fish food and animal fodder, and the insects and diseases of Azolla are also discussed. Azolla has been of traditional interest to botanists and Asian agriculturists because of its symbiotic association with a nitrogen-fixing blue-green alga. Stim- ulated by the recent energy crisis, the interests of these two groups have merged, resulting in the publication of numerous articles in popular magazines and exten- sion bulletins. These articles have focused on the green manure, nitrogen fixation, and hydrogen production qualities of Azolla (Galston, 1975; Newton, 1976; Brill, 1977; Singh, 1977b). The intent of this paper is to provide a current and comprehensive survey of all available literature on the Azolla-Anabaena symbiosis, essentially up-dating and expanding the excellent review by Moore (1969). Some of the included ref- erences were published prior to 1969 but were hitherto unavailable (e.g., those from Vietnam and China). The most remarkable characteristic of Azolla is its symbiotic relationship with the nitrogen-fixing blue-green alga (cyanobacterium), Anabaena azollae. The del- icate Azolla (Fig. 1) provides nutrients and a protective cavity in each leaf (Fig. 2) to Anabaena colonies in exchange for fixed atmospheric nitrogen and possibly other growth-promoting substances (Schaede, 1947; Ashton and Walmsley, 1976). The rate of nitrogen fixation in the Azolla-Anabaena symbiosis rivals that of the Rhizobium-legume symbiosis. Talley et al. (1977) reported a daily fixation rate ' Submitted for publication August I, 1978; accepted August 7, 1978. Journal Series No. 2294 of the Hawaii Agricultural Experiment Station. This work was in part supported by AID grant/csd 2833. z Graduate Student (East West Center grantee) and Professor of Agronomy, respectively, Depart- ment of Agronomy and Soil Science, College of Tropical Agriculture, University of Hawaii, Honolulu, Hawaii. Economic Botany, 34(2), 1980, pp. 111-153 9 1980, by the New York Botanical Garden, Bronx, NY 10458