Studies of the Azolla Pinnata‐anabaena Azollae Symbiosis: Growth and Nitrogen Fixation

SUMMARY Azolla pinnata grown in a standard medium and on floodwater under the high light (30 to 120 klx) and high temperature (26 to 35°C) conditions of Malaysia doubled its fresh wt in about 3 and 5 days respectively. The main factor limiting growth and acetylene reduction activity (ARA) of the A. pinnata‐Anabaena assollae symbiotic association on floodwater was phosphorus. Addition of phosphorus (P) to P‐deficient Azolla in floodwater resulted in an immediate increase in ARA followed by an increase in the growth rate. Minus P treatment caused a 50% reduction of ARA but had little effect on the growth rate of Azolla in the first week. Continued culture of Azolla in the minus P medium caused a significant reduction in growth rate, ARA, and the formation of anthocyanin pigment in the fern. Subsequent transfer into a standard medium did not restore its ARA. Full sunlight also reduced the growth and ARA of Azolla, and P modified the response of Azolla to light.