Abstract: Nostoc flagelliforme is a terrestrial nitrogen-fixing cyanobacteria, which has strong xeric ecological adaptability. The ultrastructure and physiological resistance of N.flagelliforme under drought stress were studied. The results showed that with increasing drought stress, the cell size and wall thickness were not significantly changed, the sheath tended to be shrunk, the arrangement of thylakoids tended to be disordered, and the polyhedron tended to be blurred and even disappeared, the amount of glycogen granules were decreased, but the amount of structural granules did not change significantly. SOD and CAT activities were firstly increased and then decreased, SOD activity reached the peak when the content of water reached 120%, but CAT activity reached the peak when the content of water reached 445%. With the increasing of drought stress, MDA content and oxygen free radical level were increased, H₂O₂ content firstly increased and then declined, it reached the peak when water content reach 120%. There were obvious differences between N.flagelliforme which dried storage for 1 year and resuscitative N.flagelliforme, the SOD and CAT activities, MDA and H₂O₂ contents were all lower than those of resuscitative N.flagelliforme, but oxygen free radical level reached the highest value. The results laid the foundation for the further studies on tolerance mechanism in N.flagelliforme.

Key words: Nostoc flagelliforme, drought stress, ultrastructure, stress physiology