Inducible conditions for sorus formation of the sporophyte discs of Laminaria japonica Areschoug (Phaeophyceae)

Optimal conditions for sorus formation of Laminaria japonica were investigated by culturing discs of the sporophyte in bottles. All discs formed sori in the seven combinational conditions of temperature (5-15 degrees C), irradiance (33.2-11.6 mu E.m(-2).s(-1)) and photoperiods (16:8, 12:12 and 8:16 h LD) in nutrient-rich medium. Sorus formation occurred earlier at 15 degrees C than at 5 and 10 degrees C and were largest in the area at 15 degrees C. Sori grew larger under high irradiance (116 mu E.m(-2).s(-1)) than under low irradiances (66.4 and 33.2 mu E.m(-2).s(-1)), but the initiation of sorus formation was not influenced by irradiance. Sorus formation was earlier in 8:16 h LD than in 12:12 and 16:8 h LD. Rapid formation of large sori occurred at high temperatures even in a long photoperiod. Sorus formation in discs in nutrient-poor medium was slower than that in discs in nutrient-rich medium, in which all discs formed large sori. Most discs in nutrient-poor medium formed very small sori. These results indicate that water temperature and nutrient conditions strongly affect both to the induction and expansion of sorus formation.