Studies of the Azolla-Anabaena Symbiosis Using Azolla mexicana, I. Growth in Nature and Laboratory

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Abstract
Azolla mexicana Presl. was evaluated as a model for future studies in N-fixation. In field studies, floating sporophytes resulted from temperature-dependent (15.degree. C) germination of zygotes in the bottom mud. High light intensity resulted in senescence. Sexual reproduction in June appeared photoperiodic. In the laboratory, optimal conditions were 14 h light/10 h dark; 12 klx cool white fluorescent light; 23.degree. C day/ 18.degree. C night; pH 4.2-8.0 with no definite optimal pH. The plants were quite tolerant of NaCl concentration and grew, though decreasingly, down to -5 bars stress with polyethylene glycol. Light intensity of 20 klx induced senescence in the laboratory, and there was no growth at 0.75 klx. A. mexicana exhibits a doubling time of 68-75 h in a new selective medium: about 48 h less than that reported for other species from North America and equal to a reported tropical species.

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