Constant short-day treatment of outdoor-cultivated Laminaria digitata prevents summer drop in growth rate

Previous laboratory studies in species of the Laminariales have revealed th at both the onset of growth in early winter and the summer drop in growth r ate are controlled by the annual course of daylength synchronizing endogeno us, circannual clocks within the thallus. Moreover, it is known for some la minarian species that cultivation in the laboratory in constant short days (SD) leads to arhythmic, continuous growth activity of the blade throughout the year. Such a prolonged SID treatment has now been performed for the fi rst time in outdoor-cultivated Laminaria digitata, Field-grown sporophytes were collected in May from the sea near Helgoland (North Sea) and cultivate d on the island of Sylt (North Sea) in temperature-controlled outdoor tanks (300 l) at 10 degreesC for 1.5 years either in a constant 8 h of light per day, controlled by an automatic blind on top of the tank. or in ambient da ylengths. In constant SD, the growth rate remained at a steady-state level of approximately 0.4 cm day(-1) from the end of June until mid-October, whe reas growth rate in ambient daylengths declined steadily after June to half the rate in SID in October. Growth became light-limited between October an d February in both treatments and, in the second year from July onwards, hi gher growth rates were again observed in constant SID than in ambient dayle ngths. Further work is required to find out whether SID treatment in summer would also prevent the summer drop in growth rate in other perennial seawe ed species, e.g. commercially valuable red algae. This would potentially in crease the chance of more constant biomass production throughout the year, and decrease the danger of the cultivated perennial algae being overgrown b y annual epiphytes in summer.