Oxidation of polyphenols and inhibition of photosystem II under acute photooxidative stress
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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.
KeywordsAntioxidants Brown pigments Chlorophyll fluorescence Excitation spectra Reflectance spectra Helianthus annuus Photosynthesis Polyphenols
We are grateful to Lionel Saunois and Amandine Dubois from the greenhouse facility for taking care of the sunflower plants, and to Gwendal Latouche for his technical assistance. GS acknowledges his sabbatical leave from the Université du Québec à Trois-Rivières.
Author contribution statement
GS and ZGC conceived research, conducted most experiments and analyzed the data. WMAER and PM conducted experiments with voltammetry and analyzed the data. GS prepared the figures and wrote most of the manuscript. All authors reviewed and approved the manuscript.
Compliance with ethical standards
Conflict of interest
ZGC declares a link to the company FORCE-A as one of the co-authors of the Multiplex patent that the company exploits. Other authors have no competing interests.
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