TAXONOMY, PHYSIOLOGY, AND AGRONOMIC POTENTIAL OF AZOLLA SPP. (CHINA, VIETNAM)

The freefloating aquatic fern Azolla pinnata has been grown for centuries in northern Vietnam and isolated areas of southeastern China as a nitrogen-fixing green manure for rice.;Adaptation of the known species of Azolla to a wide range of climatic conditions was evaluated by growing cultures on nitrogen-free nutrient solution at a site where air temperatures ranged from 0(DEGREES)C in the winter to 40(DEGREES)C in the summer. Relative growth rates (RGR) were calculated from increases in dry weight over growth cycles of one week to one month. The RGRs of individual species were most closely related to minimum water temperature during spring and fall (R('2) = 0.67 to 0.99); minimum water temperature and daylength for spring plus fall (R('2) = 0.73 to 0.92); and minimum water temperature, solar radiation, and relative humidity during summer (R('2) = 0.74 to 0.91).;The agronomic potential of azolla accessions and azolla management treatments were evaluated in a field experiment with spring rice conducted at Hangzhou, China. A. caroliniana, two varieties of A. filiculoides, A. pinnata var. imbricata, and A. rubra were grown as a monocrop before rice, as an intercrop with rice, and as a monocrop and intercrop. Grain yields averaged across species increased in the order: combination > monocrop > intercrop. Grain yields were correlated with accumulated N within azolla accessions across management treatments (r = 0.94 to 0.99). However, yields were poorly or negatively correlated across accessions within a management treatment (monocrop, r = -0.27; intercrop, r = 0.20; combination, 4 = -0.61).;The known species of Azolla were collected from their native habitats. Morphology of Azolla caroliniana, A. filiculoides, A. mexicana, A. microphylla, A. nilotica, A. pinnata var. pinnata, A. pinnata var. imbricata, and A. rubra was studied using light and scanning-electron microscopy. An improved identification key was developed based on reproductive and vegetative tissues. Floats of the megaspores can be used for identification to the section of the genus, and the sporoderm of the megaspores and the glochidia of the massulae can be used for identification of the species. Trichomes on the leaf lobes and rhizomes, and growth patterns of crowded plants are important vegetative features for identification.