Relationship between nitrogen content and sorus formation in the brown alga Laminaria japonica cultivated in southern Hokkaido, Japan

Seasonality of reproduction in sporophytes of Laminaria japonica was investigated on the basis of the nitrogen content of thalli cultivated in the field in southern Hokkaido, Japan. Fertile plants occurred throughout most of the year. Sorus formation was greatest in the basal parts of thalli in fall (September). The secondary peak of sorus formation occurred in the apical parts of thalli in winter (February and March). The seasonality of sorus formation was linked closely to the nitrogen content in the thalli. The nitrogen content per volume of thalli declined from January to April and then increased to a maximum in September. In January and February, the nitrogen content was higher in the apical part (ca. 2 mg.cm(-3)) than in the basal part of thalli. After June, however, it was higher in the basal part. Sorus area increased in association with the nitrogen content per volume of thalli regardless of the thallus part or season. In addition, although sorus formation took more than 100 days after April, it needed only about 50 days in February. These results suggest that the low nitrogen content in summer delayed sorus formation and decreased the area of the sori. The onset and position of sorus formation depended on the growth and the nitrogen content per volume of each thallus.