Regulation of Reproduction in Delayed Gametophyte of Saccharina japonica (Laminariales, Phaeophyceae): Effects of Light Intensity, Quality and Photoperiod

Saccharina japonica gametophytes can survive a long period under unfavorable environmental conditions, while they also delay in growth and/or reproduction. Although the reproduction in delayed gametophyte of S. japonica was known to be strongly influenced by light intensity, light quality, and photoperiod, no previous studies have evaluated their interactive effects on gametogenesis. To evaluate these effects, we used an orthogonal experiment to expose delayed gametophytes of S. japonica to different light intensities, light qualities, and photoperiods for 12 days. The results showed that changes in light intensity rather than light quality and photoperiod significantly affected the relative growth rates of the delayed gametophytes. Blue light had the greatest promotion on reproduction rate. The optimal light conditions in the early vegetative growth phase in gametogenesis induction for the delayed gametophytes were at 60–80 µmol photons m −2 s −1 with daylength of 12 or 16 hours under white or blue light. When the delayed gametophytes were maintained in a constant light condition from delayed state to gametogenesis, the beneficial photoperiods for vegetative growth and reproductive rate were both 16L (16 hours of light): 8D (8 hours of dark). However, when the delayed S. japonica gametophytes achieve the optimal growth state during the first 6 days and then they were cultured at different light conditions for the following 6 days, the reproduction rate increased as the daylength decreased and attained a peak value in group of 8L:16D photoperiod, indicating that photoperiod adjustment at the transition period is crucial in the gametogenesis induction process of delayed gametophyte of S. japonica .