Photosynthetic responses to sudden exposure to high light stress (600 pinol photons m(-2) s(-1)) and the potential for subsequent recovery were assessed in Sargassum thunbergii germlings grown under three different light intensities of 10 mu mol photons m(-2) s(-1) (low light, IL), 60 pmol photons m(-2) s(-1) (moderate light, ML) and 300 mu mol photons m(-2) s(-1) (high light, HL). The photosynthetic activity (maximum photochemical efficiency, F-v/F-m; rapid light curves, RLCs; non-photochemical quenching, NPQ) was estimated by chlorophyll fluorescence using a pulse amplitude modulated fluorometer. All treatments exhibited high capacity for dynamic photoinhibition, with the fas.t reaction kinetics of F-v/F-m during both inhibition and recovery period, and with the rapid induction of maximum NPQ (within minutes). HL-germlings characteristically demonstrated a high NPQ value of approx. 5.5, allowing a flexible and reversible response to stress. Besides the significant contribution of NPQto photoprotection, photosynthetic capacity (ETRmax) in LL-germlings was as great as that in HL-germling,s, suggesting that energy dissipation through photochemical electron transport system could also reduce probability of photodamage. NPQ in S. thunbergii germlings appeared to be not directly controlled by a transthylakoid proton gradient (Delta pH) due to the lack of "light activated state". Furthermore, inhibition of xanthophyll cycle with DTT considerably blocked NPQ(pre) induction of preillumated germlings, and a slow NPQ relaxation occurred upon disruption of Delta pH by NH4Cl, collectively indicating the importance of xanthophyll cycle to NPQ. These results suggested that S. thunbergii germlings could tolerate sudden high light by down-regulation of photosynthetic capacity, based on highly efficient photoprotective responses, including energy dissipation through xanthophyll cycle and photosynthetic electron transport. The photoprotection was efficiently independent on the light history of germlings. The high photosynthetic plasticity with immediate response to rapidly changing light may be a central feature explaining the survival of germHings in highly variable light environments of intertidal habitat. (C) 2013 Elsevier B.V. All rights reserved.
