The Relationship Between Induction Of Photoprotective Mechanisms And Antioxidant Nutrient Quality Of Leafy Lettuce(Lactaca Saiva L. Var. Capirate DC.) Under Different Light Intensity

In the absence of other environmental stress factors, leaves acclimate to increasing photosynthetic photon flux density (PPFD) by increasing their maximal photon synthetic capacites. However, no leaf can utilize all of the light aborbed during exposure to high light for synthesis. The absorption of excess light can be deleterious since it can potentially result in the production of singlet oxygen and reactive oxygen species such as superoxide and H₂O₂. Xanthophyll cycle-dependent energy dissipation in the light-harvesting antennae and photoreduction of oxygen species by the chloroplast's antioxidant systems are thoght to play a photoprotective role in leaves by mitigating oxidative damage cellular macromolecules. Quantifying the fate of absorbed light energy of leaves has become an important aspect of plant photosynthetic research. The contribution of the various processes, e.g., photosynthesis, photorespiration and the water-water and xanthophyll cycles to the photoprotection of the photosynthetic apparatus can be categorised as either photochemical or thermal dissipation processes. The acclamation of photochemistry, xanthphyll cycle-dependent energy dissipation, and antioxidants was characterized in leaves of leafy lettuce that were grown under photosynthetic photon flux densities(PPFDs) by shading to artificial light supplement in the greenhouse. Accordingly, the effect of photosynthetic photon flux densities(PPFDs) on photosynthesis , chlorophyll fluorescence and antioxidant capacities were investigated. The results are as follows:1) The effects of different irradiance on gas exchange, biomass and nutritive quality of leave lettuce were studied. Increasing light irradiance from natural light level all significantly decreased the photosynthetic rate (Pn), stomatal conductance and intracellular CO₂ concentration. In conparison, the decrease in Pn under low light irradiance was followed by increased Ci. The fresh weight ang dry weight of leaf lettuce incresed with the increase in light intensity, and same was for the changes in contents of soluble protein and carbohydrate in leaves..2) The effect of different irradiance on chlorophyll fluorescence, light allocation, xanthophyll cycle pool size and the content of chlorophyll and carotenoid were investigated in leaf lettuce plants grown in a greenhouse. High light significantly reduced the efficiency of energy capture by open PSII reaction centres (Fv/Fm), indicating that photoinhibition occurred during plant growth. High light treatment had lower quantum yield of PSII electron transport (OPSII) than the controls, which was mainly due to both decreasess in the photochemical quanching (qP) and the efficiency of excitation energy capture by open PSII reaction centre (Fv'/Fm'). OPSII, qP and Fv'/Fm' increased with the decrease in light intensity. Under high light, the allocation of light absorbed by PSII antennae to the photochemical reaction (JPSII) decreased, but the allocation of absorbed light to thermal dissipation (JNPQ) and excessive energy (Jf, D) increased. High light also increased the ratio of (A+Z)/(V+A+Z), suggesting that xanthophyll cycle plays an important role in photo-protection during high-light stess As the light intensity decreased, the chlorophyll contents(Chl. ) decreased, and the chlorophyll a /b ratio (Chila /b) aslo declined. Our results support the idea that light requirements for the regeneration of leaf lettuce under different light intensity are related to their characteristics of light energy utilization and dissipation.3) The effect of light irradiance on electron transport, producing of reactive oxide species and contents of antioxidants and activity of antioxidant enzyms in leaf lettuce grown under different photo intensity were studied. High light significantly increased the total electron transport through PSII (Je (PSII)), followed by increased electron flux for the photosynthetic carbon reduction (Je(PCO)) and electron flux for the photorespiratory carbon oxidation (Je(PCR)), and increased alternative electron flux(Ja). Shading decreased the total electron transport through PSII, followed by decreased electron flux for the photosynthetic carbon reduction, electron flux for the photorespiratory carbon oxidation and alternative electron flux. Low light intensity decreased the O2" producing rate and H2O2 content in leaf lettuce leaves and reduced the lipid peroxidation. The MDA content, as an index of lipid peroxidation, dramatically increased with increasing light intensity. As a result, the contents ofGSHk a—tocopherol increased with increasing of growing light intensity. High light led to an elevation in activities of SOD, which play an important role in removing O2 . CAT were sensitive to excess light energy, and their activity significantly increased under high light energy, and activities of GPOD, APX, DHAR and GR all increased under high light, which indicated their important roles in scavenging reactive oxygen species in leaf lettuce.HbCh accumulation was closely related to the capacity of the ascorbate/glutathione cycle, which contributes to determining variation in sensitivity to chilling. Compared to controls, high light significantly increased ascorbate and glutathione content, which indicated that the ascorbate-glutathione cycle played key role in removing reactive oxygen species in leaf lettuce under high light. Low light intensity decreased the activity of antioxidant enzymes, which adapted to the low O2 producing rate and H2O2 content. All these results indicated that growing plant under high light stressful conditions resulted in enhenced role of photoprotection but was associated with increased level of nutritional factors.