The influence of summer/winter conditions of temperature and irradiance on diel oscillations in photosynthesis by Fragilaria crotonensis

SUMMARY. 1. The common pennate diatom (Bacillaryiophyta) Fragilaria crotonensis Kitton showed strong periodicity in its photosynthesis -irradiance (P—I) relationship under summer conditions of high irradiance and temperature which damped progressively throughout the growth phase. Under winter conditions of low irradiance and temperature, weak periodicity was observed.
2. The amplitude of diel oscillations in photosynthesis as a function of irradiance under winter conditions showed only slight variation throughout the growth phase. Under summer conditions, however, amplitudes for both α and P_max were highest in the early exponential phase, declining progressively during growth.
3. These results indicate that the occurrence of periodicity is dependent, in part, on the growth phase of the cells and, in part, on environmental parameters of irradiance and temperature variations under simulated summer and winter conditions.