CRITICAL LEVELS OF LIGHT AND TEMPERATURE REGULATING THE GAMETOGENESIS OF THREELAMINARIASPECIES (PHAEOPHYCEAE)1

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Abstract
Gametophytes of threeLaminariaspecies occurring near Helgoland, North Sea, were cultivated 4 wk in a 12:12 LD regime at different temperatures in artificial light fields, and in the sea at different water depths. In the artificial light fields underwater spectral distribution was simulated according to Jerlov water Types 5, 7, 9. Blue light in the simulated light fields amounted to 17, 12 or 4% of total quanta. The rate of vegetative growth did not depend on spectral distribution, was light‐saturated at 4–6 W · m−2, and increased with temperature up to 15 C.L. saccharina(L.) Lamour. exhibited the highest tolerance towards temperature, light and UV. Gametophytes survived 1 wk at 21 C ± 0.1, but not 22 C ± 0.1. Gametophytes ofL. hyperborea(Gunn.) Fosl. andL. digitata(Huds.) Lamour. survived 1 wk at 20 C ± 0.1, but not at 21 C ± 0.1. In sunlight, and in the light field of a xenon lamp, 50% ofL. saccharinagametophytes were killed by a quantum dose of 50 μEin · cm−2, and 100% of the plants by 90 μEin · cm−2. Approximately half of these quantum doses killed the corresponding percent of the other species gametophytes. Appreciably higher quantum doses were survived in visible light, with red being the most detrimental. Fertility depended on a critical quantum dose of blue light which decreased almost exponentially with decreasing temperature. The quantum dose (400–512 nm) required for induction of fertilization of 50% of the female gametophytes (males react similarly) was 90 μEin · cm−2at 5 C, 110 μEin · cm−2at 10 C, 230 (560 inL. digitata)μEin · cm−2at 15 C, and 560 (L. hyperborea)or about 850 (other 2 species) μEin · cm−2at 18 C. In the sea the gametophytes survived the dark winter months in the unicellular stage, with almost no vegetative growth of the primary cell, due to lack of light. In early spring the female gametophytes matured in the unicellular, and the males in a few‐celled stage at the depth of 2 m, as did the laboratory cultures under conditions inducing maximal fertility.