Coordination between xanthophyll cycle and antioxidant system in Sargassum thunbergii (Sargassaceae, Phaeophyta) in response to high light and dehydration stresses

摘要

The differential contribution of the xanthophyll cycle (XC) 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. Both de-epoxidation state (DPS) and total antioxidant capacity (T-AOC) were upregulated in response to the four kinds of stresses. The de-epoxidation of XC 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, XC-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 TAOC. 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 XC was reflected by low DPS. These results indicated that XC was more sensitive to HL, while the antioxidant system was more sensitive to DY. When S. thunbergii blades were transferred to DY from HL, the enhancement of antioxidant system activity was greater than that of XC de-epoxidation. There exists coordination between XC 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 identify species that have developed with higher competitive abilities in adapting to the intertidal environment.