Objective  Auxin IAA-induced sporulation of Azolla was investigated.

  Method  Under natural conditions in a netted chamber, auxin IAA solutions in concentration of 0 (control), 1, 5, 10, 15, or 20 μg·mL⁻¹ was sprayed once-a-week on Azolla filiculoides Lamarck 1001, A. caroliniana wild 3001, and A. microphylla Kaulfus 4018 during vegetative growth period for the duration until sporocarps appeared .

  Result  After five months of spraying with 1, 5, or 10 μg·mL⁻¹ IAA solution, sporocarps began to form only on Azolla 1001. The sporulation rate, spore count, and female/male ratio were 15.0%, 6.31, and 0.62 ∶ 1, respectively, under the 5 μg·mL⁻¹ application, which significantly differed from 6.3%, 4.12, and 0.53 ∶ 1, respectively, under the 10 μg·mL⁻¹ application, 4.7%, 3.90, and 0.51 ∶ 1, respectively, under the 1 μg·mL⁻¹ application, and control. In addition, the size and C/N ratio of Azolla 1001 significantly increased by the spraying, as compared to control, whereas none observed on the other two ferns.

  Conclusion  Auxin IAA could induce the sporulation of A. filiculoides Lamarck 1001. Spraying IAA at the concentration of 5 μg·mL⁻¹ on the ferns appeared feasible for field application.