| Photosynthesis is important for plant growth and survival. However, when the lightenergy absorbed by photosynthetic apparatus exceed that actually used for photosynthesis,the excessive light energy reduce the photosynthetic efficiency and result in photoinhibition,even photooxidative damage to the photosynthetic reaction center. In the nature condition,photoinhibition in plants is a common phenomenon. The environmental stresses such as lowand high temperatures, drought and salinity enhance photoinhibition. There are severalphotoprotection mechanisms in higher plants, of those the xanthophyll cycle has beenconceived to play a key role in photoprotection. The xanthophyll cycle comprisesintercoversions of three carotenoid pigments, violaxanthin, antheraxanthin and zeaxanthin，which catalysed by two enzyrne, violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase(ZE). Under the high light, violaxanthin is catalysed to zeaxanthin via antheraxanthincatalysed by violaxanthin de-epoxidase (VDE). However, when light stress disappeared,epoxidation of zeaxanthin to antheraxanthin and violaxanthin is catalysed by anotherenzyme，zeaxanthin epoxidase (ZE). Energy dissipation depended on xanthophyll cycle couldprotect photosynthetic apparatus against the harmful effects of excessive light. In fact, theseregulatory mechanisms are tightly integrated with photosynthesis itself, and there is emergingevidence that when these processes are altered, the ability of plants to assimilate carbon overlong time periods and to produce biomass may be affected.In this study, wild type (WT) tomato, sense transgenic lines [T2-7, T2-10and T2-11]were used to determine the contents of xanthophyll cycle pigments in thylakoid membrane,photosynthetic rate and chlorophyll a fluorescence during the photoinduction and salt stress.The results were showed as follows:(1) The results of xanthophyll cycle pigment analysis showed: overexpression ofviolaxanthin de-epoxidase gene (LeVDE) in tomato increased antheraxanthin (A) andzeaxanthin (Z) content and decreased violaxanthin (V) content compared with wild type (WT)plants during the light induction period and salt stress. (2) When the plants, grown under low light (600–800μmol m-2s-1PFD), were exposuredto1000μmol m-2s-1PFD, compared to wild type (WT), NPQ, ΦPSII and Pn in transgenicplants (WT) plants were faster and higher and Fv/Fm decreased more slowly duringphotoinduction.(3) When the plants, grown under nature conditions, were exposured to1000μmol m-2s-1PFD, compared to wild type, ΦPSII and Pn in transgenic plants plants were slower and lower,and Fv/Fm also decreased more slowly during photoinduction; While initial rate in NPQformation was no difference between WT and transgenic plants. After2.5mM DTT treatment,the highest and the final NPQ of WT and T2-11decreased by64.1%,45.2%,50%and40.9%,respectively. Compared with NPQ in T2-11, the initial rate of ΦPSII and the final ΦPSIIincreased after DTT treatment. This result indicated that VDE over-expressing enhancedthermal dissipation in the PSII antennae, so less excited energy entered into electron transport.Conversely, although the final NPQ level decreased to50%of that in the WT plants withouttreatment, ΦPSII didn’t increase in WT plants even decreased in a certain extent. This is dueto because that during the process of light induction of photosynthesis in WT plants withDTT treatment, the final NPQ level decreased oberviously and the increase in qI resulted inthe decrease in ΦPSII during the process of light induction of photosynthesis in WT plantswith DTT treatment.(4) Under long-term salt stress, the growth of WT and transgenic lines were all decreased.Under salt stress, Fv/Fm and Pnall decreased obviously, while NPQ all increased in WT andtransgenic lines, but qE increased more much and qI increased less in transgenic linescompared to WT. Meanwhile, WT plants accumulated much more O2and H2O2, and had alower Fv/Fm compared with transgenic lines.To conclude, the present results indicated that, during the light/dark transition, VDEover-expressing enhanced thermal dissipation in the PSII antennae, so less excited energyentered into electron transport and photoinduction extent was lower than that in WT. Inaddition, VDE over-expressing seemed to regulate the opening of stomata partly, but itrequires further study. Under salt stress, VDE over-expressing alleviated PSII photoibhibitionand enhanced salt tolerance. |
