Azolla Lam. is an aquatic fern taxon, which grows in symbiotic association with Anabaena azolla Strasburger (Nostocales: Nostocaceae) within the dorsal leaf lobe cavities (Ashton and Walmsley, 1976, 1984). Anabaena azolla can fix atmospheric nitrogen and is able to fulfill nitrogen requirements of Azolla, making it able to thrive in nitrogen-deficient waters (Ashton, 1974, 1978). Azolla is economically important and has been used in Southeast Asia as a green manure associated with wetland rice cultivation for the last 200 years (Lumpkin and Plucknett, 1982). However, the wider utilization of Azolla for agricultural purposes has been constrained by various biological factors including low tolerance to high temperatures and insect damage (Van Cat et al., 1989) and application of ammonia-based fertilizers (Lumpkin and Plucknett, 1982).
Early classifications of Azolla were based mainly on vegetative characteristics, in particular, using the form and size of leaves (Svenson, 1944). This, however, has led to considerable confusion, since the phenotypes of Azolla are plastic, varying under environmental influences (Ashton, 1978; Wantanabe and Berja, 1983; Moretti and Gigliano, 1988). Zimmermann et al. (1989) reclassified Azolla using electrophoretic techniques, whereas Nayak and Singh (1989) used cytological techniques. Traditionally 25 fossil and seven extant species of Azolla are recognized (Hills and Gopal, 1967; Lumpkin and Plucknett 1980; Ashton and Walmsley, 1984), which are divided into two sections:
As to the status of A. pinnata, some confusion prevails. Initially it was regarded as a complex of A. pinnata, A. africana Desv. and A. imbricata Roxb. ex Griff., but later reduced to one species with two varieties, A. pinnata var. imbricata and A. pinnata var. africana (also called var. pinnata) (Sweet and Hills, 1971; Stergianaou and Fowler, 1990).