Experiment report of discovery and use of Fangstan county wild azolla.

In 1981, a wild species of azolla was discovered in the streams and rice fields at Dong-gan-chi village in Fangstan County while engaged in extending the cultivation of azolla in rice fields. It was learned from local farmers that the wild azolla strain had existed in the area for many years. The water temperature in springs and streams is a constant 13-14 deg C. The wild strain was identified as an ecological type of Azolla imbricata and designated it Azolla imbricata Fangstan. A. imbricata is the northernmost distribution boundary of Chinese wild azolla found to date, and its ecological adaptation to northern China is strong. It belongs to thermophilic Azolla sp. and its beginning growth temperature is about 10 deg C., higher than that of A. filiculoides. Its optimum growth temperature is 25-28 deg C. and its high temperature tolerance may exceed 40 deg C. The light requirement of A. imbricata is not very strict, but the most favorable light intensity ranged from 40 to 60 Klx. It grows better than A. filiculoides under both strong and weak illumination. A. imbricata is responsive to fertilization, growing faster and having higher nitrogenase activity in earthworm manure nutrient solution. Its N2-fixing capacity increased with the addition of P, K, and microelements Fe, Zn, Cu, and B. A. imbricata propagates primarily by vegetative reproduction. It can overwinter in streams and rice fields, but produces fewer sporocarps in the spring. A. imbricata recovers earlier and grows more vigorously after naturally overwintering than other azolla strains, although it does not perform as well as A. filiculoides in early spring and late autumn. A. imbricata reached its peak growth rate in the field between 25 and 28 May. Under optimum conditions its propagative index exceeded 0.3, higher than that of A. filiculoides by 3-14%, and its doubling time was 2 d. The oversummering ability of A. imbricata is strong, too. By cultivating A. imbricata year round, yields of 300-375 + azolla fresh wt/ha are possible, and in association with rice, yields of 75-150 t/ha are realized.