Effect of photon irradiance and photoperiod on young sporophytes of four species of the Laminariales

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Abstract
Four kelp species (Alaria esculenta, Laminaria digitata, L. hyperborea and L. saccharina) were cultured in the laboratory and pretreated so that gametophytes became mature and sporophytes were produced. Young sporophytes less than 3 weeks old were exposed to various light conditions and their survival or relative growth rate in length (R L), and width (R W) in one experiment, determined. Sporophytes of L. digitata survived poorly in the dark compared with L. hyperborea. The minimum photon irradiance allowing growth in L. hyperborea was about 1 μmol m-2 s-1; 1–2 μmol m-2 s-2 sustained growth in all the species. The growth of all four species was saturated at 20–30 μmol m-2 s-1 of continuous photon irradiance. In shorter daylengths the growth rate was lower both below and above light saturation. Above saturation the growth was considerably faster than would be expected if no photosynthetic products were carried over from the light to the dark period. There was evidence that 2–3 times as much light was necessary to saturate photosynthesis as to saturate growth. Although there were significant differences between the species in R L during the first 10 days, differences in R W and in sporophyte age made it impossible to distinguish fast- or slow-growing species during the first 3 weeks. Single-dimension growth measurements at this developmental stage can thus be misleading. Young sporophytes of L. digitata were more tolerant of strong sunlight than those of L. hyperborea, but in both the survival was higher following acclimation to higher irradiance.