Morpho-functional patterns of photosynthesis in the south pacific kelp Lessonia nigrescens:: Effects of UV radiation on 14C fixation and primary photochemical reactions

The morpho-functional patterns of photosynthesis, measured as C-14-fixation and chl fluorescence of PSII, also as affected by different doses of UV radiation in the laboratory were examined in the South Pacific kelp Lessonia nigrescens Bory of the coast of Valdivia, Chile (40 degrees S). The results indicated the existence of longitudinal thallus profiles in physiological performance. In general, blades exhibited higher rates of carbon fixation and pigmentation as compared with stipes and holdfasts. Light-independent C-14 fixation (LICF) was high in meristematic zones of the blades (3.5 mu mol C-14.g(-1) fresh weight [FW].h(-1)), representing 2%-16% (percentage ratio) of the photosynthetic C-14 fixation (20 mu mol C-14.g(-1) FW.h(-1)). Exposures to UV radiation indicated that biologically effective UV-B doses (BEDphotoinhibition300) of 200-400 kJ.m(-2) (corresponding to current daily doses measured in Valdivia on cloudless summer days) inhibit photosynthetic C-14 fixation of blades by 90%, while LICF was reduced by 70%. The percentage ratio of LICF to photosynthetic C-14 fixation increased under UV exposure to 45%. Primary light reactions measured as maximum quantum yield (F-v/F-m) and electron transport rate (ETR) indicated a higher UV susceptibility of blades as compared with stipes and holdfasts: after a 48 h exposure to UV-B, the decrease in the blades was close to 30%, while in the stipes and holdfasts it was < 20%. The existence of translocation of labeled carbon along the blades suggests that growth at the meristem may be powered by nonphotosynthetic processes. A possible functional role of LIFC, such as during reduction of photosynthetic carbon fixation due to enhanced UV radiation, is discussed. These results in general support the idea that the UV-related responses in Lessonia are integrated in the suite of morpho-functional adaptations of the alga.