Responses to high temperature of the Azolla-Anabaena association, determined in both the fern and in the cyanobacterium.

The responses to high temperature of the cyanobacterial symbiont and the host fern in tile Azolla-Anabaena symbiosis were studied by using Azolla strains with heterologous Anabaena azollae Strasburger of without Anabaena. At 26/18°C, symbiotic Azolla grew better than the corresponding Anabaena-free Azolla. A high temperature-sensitive species Azolla filiculoides Lamarck, produced smaller biomass under conditions of 37 °C (day)/29 °C (night) than the corresponding Anabaena-free Azolla grown in N-containing medium. A. filiculoides with Anabaena from temperature tolerant Azolla microphylla Kaulfuss was more effective at N fixation than A. filiculoides with homologous Anabaena. A microphylla with Anabaena from A. filiculoides grew more slowly and fixed less N than the strains with homologous Anabaena. In the presence of combined N at 37°/29 °C, A. microphylla grew better than A. filiculoides with Anabaena from A. microphylla. Tolerance of the Azolla-Anabaena association to high temperature is, therefore, determined both in the host fern and symbiont Anabaena.