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

summary 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 N2 fixation than A. filiculoides with homologous Anabaena. A microphylla with Anabaena from A. filiculoides grew more slowly and fixed less N2 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.