Abstract
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°C), irradiance (33.2-116 μE•m-2•s-1) and photoperiods (16:8, 12:12 and 8:16h LD) in nutrientrich medium. Sorus formation occurred earlier at 15°C than at 5 and 10°C and were largest in the area at 15°C. Sori grew larger under high irradiance (116 μE•m-2•s-1) than under low irradiances (66.4 and 33.2 μE•m-2•s-1), but the initiation of sorus formation was not influenced by irradiance. Sorus formation was earlier in 8:16h LD than in 12:12 and 16:8h 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.
