PHOTOINHIBITION AND RECOVERY AFTER HIGH LIGHT STRESS IN DIFFERENT DEVELOPMENTAL AND LIFE-HISTORY STAGES OF LAMINARIA SACCHARINA (PHAEOPHYTA)1,2

The capacity to cope with high light stress was investigated in different life-history and developmental stages of Laminaria saccharina Lamour. sporophytes and gametophytes. Changes in photosynthetic efficiency and in the level of photoinhibition were measured by in vivo fluorescence changes of photosystem II. Pigment content was studied using high performance liquid chromatography. Additionally, the morphology of the various developmental stages during the life cycle was studied by light microscopy in relation to the photosynthetic parameters. High light stress (2 h, 500 μmol.m-².s^?1) induced photoinhibition of photosynthesis with fast kinetics in older sporophytes and gametophytes. In contrast, the absolute degree of photoinhibition after light stress was higher in younger than in older sporophytes. Photosynthesis recovered faster in older sporophytes and gametophytes compared to young sporophytes. In very young sporophytes, photosynthesis did not recover fully even after 12 h exposure to low light, indicating severe photodamage. Kinetics of recovery in old sporophytes and in gametophytes showed a fast and a slow phase, whereas younger sporophytes recovered only with a slow phase, The fast phase is indicative of a decline of the photoprotective process, whereas the slow phase indicates a recovery from photodamage. The capacity to cope with high light stress in Laminaria sporophytes increased with increasing age of the thalli. The gametophytes are less sensitive to high light stress and may be selected to endure unfavorable white light conditions. Investigation of the xanthophylls showed that the higher resistance to high light is not caused solely by a higher content of xanthophyll cycle pigments. Additionally, changes in the thallus structure during the development of the sporophytes seemed to cause a higher resistance to high light. The observed changes in the ability to cope with high light in the different life-history and developmental stages of Laminaria saccharina may influence the distribution of the species on the shore.