We observed a close correlation between the inhibition of photosystem II and the oxidation of polyphenols during an acute oxidative stress in sunflower leaf discs. To assess the physiological significance of polyphenols as antioxidants in planta, we compared the kinetics of polyphenols oxidation with the inhibition of the photosynthetic apparatus in sunflower leaf discs exposed to an acute photooxidative stress. Illumination of leaf discs in the presence of methyl viologen induced a rapid and large non-photochemical quenching of chlorophyll-a fluorescence, which was reversed after 4 h of treatment as indicated by the ≈ 30% increases of the steady-state (Fs) and maximal (Fm′) levels of chlorophyll-a fluorescence relative to the first hour of treatment. This event coincided with the accelerated decreases of the maximum (Fv/Fm) and effective (∆F/Fm′) quantum yields of photosystem II, and also with the beginning of polyphenols oxidation, estimated by the UV absorbance of methanolic leaf extracts, and supported by the Folin–Ciocalteu method and cyclic voltammetry. The decreases of Fv/Fm and the concentrations of reducing polyphenols were highly correlated (R2 = 0.877) during the experiment. Coherent with the decrease of UV absorbance of methanolic extracts, polyphenol oxidation resulted in a marked decrease of UV absorbance of leaf epidermis. Also, polymerization of oxidized polyphenols caused the accumulation of brown pigments in the MV-treated leaf discs, decreasing leaf reflectance, especially at 550 and 740 nm. Fluorescence intensities were also decreased during the MV treatment. Interestingly, the emission fluorescence ratio F740/F684 (excitation at 550 nm) decreased similarly to Fv/Fm (R2 = 0.981) due to the brown pigments. Moreover, the excitation fluorescence ratio F484/F680 (emission at 740 nm) was linearly correlated (R2 = 0.957) to ∆F/Fm′, indicating a decrease of efficiency of energy transfer between the antenna pigments to the photosystem II reaction center during the oxidative stress. These results support the view that polyphenols can be effective antioxidants protecting the plants against reactive oxygen species.

中文翻译：

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急性光氧化应激下多酚的氧化和光系统Ⅱ的抑制

我们观察到在向日葵叶盘的急性氧化应激过程中，光系统 II 的抑制与多酚的氧化之间存在密切的相关性。为了评估多酚在植物中作为抗氧化剂的生理意义，我们比较了多酚氧化的动力学与暴露于急性光氧化应激的向日葵叶盘中光合装置的抑制作用。在甲基紫精存在下对叶盘进行照明诱导了叶绿素-a 荧光的快速且大的非光化学猝灭，在处理 4 小时后逆转，如稳态 (Fs) 增加约 30% 和相对于处理的第一小时，叶绿素-a 荧光的最大 (Fm') 水平。这一事件与光系统 II 的最大 (Fv/Fm) 和有效 (ΔF/Fm') 量子产率的加速下降以及多酚氧化的开始相吻合，通过甲醇叶提取物的紫外吸光度估计，以及由 Folin-Ciocalteu 方法和循环伏安法支持。在实验过程中，Fv/Fm 的降低与还原性多酚的浓度高度相关（R2 = 0.877）。与甲醇提取物的紫外吸光度降低相一致，多酚氧化导致叶表皮的紫外吸光度显着降低。此外，氧化多酚的聚合导致 MV 处理的叶盘中棕色色素的积累，降低了叶片反射率，尤其是在 550 和 740 nm 处。在MV处理期间荧光强度也降低了。有趣的是，由于棕色颜料，发射荧光比 F740/F684（在 550 nm 处激发）与 Fv/Fm（R2 = 0.981）相似地降低。此外，激发荧光比 F484/F680（在 740 nm 处发射）与 ΔF/Fm' 呈线性相关（R2 = 0.957），表明天线色素到光系统 II 反应中心的能量转移效率降低。氧化应激。这些结果支持了这样的观点，即多酚可以成为有效的抗氧化剂，保护植物免受活性氧的侵害。表明在氧化应激期间天线色素到光系统 II 反应中心的能量转移效率降低。这些结果支持了这样的观点，即多酚可以成为有效的抗氧化剂，保护植物免受活性氧的侵害。表明在氧化应激期间天线色素到光系统 II 反应中心的能量转移效率降低。这些结果支持了这样的观点，即多酚可以成为有效的抗氧化剂，保护植物免受活性氧的侵害。