DAYLENGTH RANGE FOR CIRCANNUAL RHYTHMICITY IN PTERYGOPHORA-CALIFORNICA (ALARIACEAE, PHAEOPHYTA) AND SYNCHRONIZATION OF SEASONAL GROWTH BY DAYLENGTH CYCLES IN SEVERAL OTHER BROWN-ALGAE

The growth rhythm of the kelp Pterygophora californica Ruprecht could be synchronized to periods of 3 months by daylength cycles of sinusoidal or rectangular shape. Arhythmic, continuous growth was found at 8 and 9 h light per day, while rhythmic and synchronized growth cycles with short resting phases occurred during periodic daylength shifts from 8 upward to 10, 12, 14 or 16 h light per day, or from 16 downward to 12 or 10 h light per day. Cultivation at constant 10, 12, 14 or 16 h light per day permitted free-running growth rhythms. These results suggest that a critical daylength at 8-9 h light per day sets a lower limit to circannual rhythmicity in P. californica and is at the same time used by the alp as a strong Zeitgeber signal for synchronizing the circannual growth rhythm to the natural year. The growth rhythms of Agarum cribrosum (Mertens) Bory, Pleurophycus gardneri Setchell et Saunders, Laminaria saccharina (Linnaeus) Lamouroux, L. bongardiana Postels et Ruprecht (= L. groenlandica Rosenvinge sensu Druehl 1968) and Desmarestia aculeata (Linnaeus) Lamouroux could be synchronized by artificial daylength cycles, but proof of free-running, circannual growth rhythms is still lacking for these species.