Violaxanthin Cycle Of Sargassum Thunbergii In Intertidal Zone

Sargassum thunbergii(Sargassaceae, Phaeophyta), a common brown macroalgae, is widely distributed in the intertidal zone. To clarify the importance of violaxanthin cycle(V-cycle) for brown algae in resisting environmental stress, chlorophyll fluorescence and the de-epoxidation kinetics of V-cycle were examined in response to dehydration and light stresses. The main results showed as following:1. Different responses of the xanthophyll cycle to dehydration and light intensity in Sargassum thunbergiiPhotosynthetic parameters, pigment compositions and inhibitor effects of Sargassum thunbergii germlings were examined in response to dehydration and light intensity.Results showed that de-epoxidation state(DPS) in response to dehydration stress was only about one-tenth of those for medium and high light intensities, though the DPS increased with both stress intensity. Furthermore, dithiothreitol(DTT) inhibited 55% NPQ of dehydrated germlings indicating the limited proportion of V-cycle dependent quenching in NPQ. Thus, there was low NPQ in dehydrated germlings which was only about one-sixth of that induced by medium and high light intensities. The NPQ may be insufficient to protect the photosynthetic apparatus to maintain a high level of photosynthesis as evidenced by the significant suppressions of effective quantum yield of photosystem II(?PSII). During rehydration, we suggested the stable level of DPS was due to the retention of V-cycle pigments induced by severe dehydration since there was no significant increase of DPS and it was insensitive to DTT. However, the high NPQ dependent on V-cycle provided efficient protection of photosynthesis under light stress and returned to the unstressed level during recovery period. The present results suggested that the high capacity of photosynthetic apparatus in maintaining its function under light stress was based on light-sensitive V-cycle, which may be related to the high resistance to light-induced damage. However, dehydration-insensitive V-cycle was insufficient to protect the photosynthesis at a high and constant level under severe dehydration stress,which may be related to the non-tolerance to dehydration of S. thunbergii.2. Interactive effects of light and dehydration on xanthophyll cycle in Sargassum thunbergii(Sargassaceae, Phaeophyta)In this study, a three-way factorial experimental design was used to investigate the dynamic changes of chlorophyll fluorescence and the V-cycle de-epoxidation of the intertidal macroalga Sargasum thunbergii in response to light, dehydration anddehydration opportunity. Based on the magnitude of effect, dehydration was the predominant negative factor affecting the ?PSII. The NPQ and V-cycle were more susceptible to light. Preillumination enhanced significantly the levels of NPQ and V-cycle during dehydration, indicating preillumination could make the S. thunbergii more tolerant to dehydration. Moderate dehydration enhanced significantly the levels of NPQ and V-cycle during the subsequent light period, indicating moderate dehydration could make the S. thunbergii more tolerant to light. When light was combined with dehydration, S.thunbergii exhibited significantly low ?PSII, NPQ and DPS. Based on these results, it was hypothesized that retention of xanthophyll pigments could improved the tolerance of S.thunbergii to the following stress. To test this hypothesis, the combined high light and high degree of dehydration treatment was executed in S. thunbergii after preillumination.Preillumination resulted in the retention of xanthophyll pigments as evidenced by the high DPS. The following exposure to high light and high degree of dehydration induced significantly high NPQ and DPS. Thus, retention of xanthophyll pigments may allow for a faster response to environmental stress by accelerating V-cycle and NPQ induction. This character may represent an adaptive strategy of brown algae to cope with the rapidly changing stress conditions.3. Coordination between xanthophyll cycle and antioxidant system in Sargassum thunbergii(Sargassaceae, Phaeophyta) in response to high light and dehydration stressesThe differential contribution of the V-cycle and the antioxidant system to photoprotect the photosynthetic apparatus of Sargassum thunbergii to high light(HL),dehydration(DY), HL prior to DY(HL-DY) and HL in combination with DY stresses were investigated in this study. Both DPS and total antioxidant capacity(T-AOC) were up-regulated, response to the four kinds of stresses. The de-epoxidation of V-cycle was engaged earlier in response to stress exposure while significant change in antioxidant activities took a relatively long time. Under HL stress, the PSII photoinactivation was significant as suggested by low maximum PSII efficiency together with effective quantum yield and high excitation pressure. Simultaneously, V-cycle dependent thermal dissipation increased progressively, reaching a high value. The low oxidative stress indicated by low levels of superoxide radical and hydrogen peroxide was accompanied with a low T-AOC.However, DY treated samples exhibited severe oxidative stress and high capacity to detoxify reactive oxygen species, as evidenced by the high T-AOC, oxidized glutathione,and dehydroascorbate. The low activity of V-cycle was reflected by low DPS. Theseresults indicated that V-cycle was more sensitive to HL, while antioxidant system was more sensitive to DY. When blades were transferred to DY from HL, the enhancement of antioxidant system activity was greater than that of V-cycle de-epoxidation. There exists coordination between V-cycle and antioxidant system in response to varying environmental factors. In addition, as demonstrated in HL-DY stress, preillumination decreased the degree of photoinhibition and oxidative stress during dehydration, indicating that high light exposure could make the blades more tolerant to dehydration. This observation can be the bases to idinentify the species that have developed with higher competitive abilities in adapting to the intertidal area.