Oxidation of polyphenols and inhibition of photosystem II under acute photooxidative stress

Abstract

Main conclusionWe 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 presence of methyl viologen induced a rapid and large non-photochemical quenching of chlorophyll-a fluorescence, which was reversed after four hours 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 (ΔF/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 (R 2 = 0.877) during the experiment. Coherent with the decrease of UV absorbance of methanolic extracts, polyphenols 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 MVtreated 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 F 740 /F 684 (excitation at 550 nm) decreased similarly to Fv/Fm (R 2 = 0.981) due to the brown pigments. Moreover, the excitation fluorescence ratio F 484 /F 680 (emission at 740 nm) was linearly correlated (R 2 = 0.957) to ΔF/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.

Genetic modification of the flavonoid pathway alters growth and reveals flexible responses to enhanced UVB – Role of foliar condensed tannins

“…For example, decreased photosystem II efficiency and net photosynthesis in the DFRi plants (Figure 3b,c) indirectly supports the proposal that condensed tannins participate in regulation of the oxidative state of photosynthetic cells (Gourlay & Constabel, 2019;Harding, 2019). This functionality would fit the suggested polymerization of condensed tannins in chloroplast-derived tannosomes (Brillouet et al, 2013) and the recently identified links between polyphenol oxidation and acute oxidative stress in chloroplasts of Helianthus annuus L. (Samson et al, 2020).…”

Uncoupling effect of lipid peroxidation in spinach thylakoids exposed to peroxyl radicals generated by 2,2′-azobis (2-amidinopropane) dihydrochloride

“…Although inhibition of ATP synthesis can also result from the inactivation of the F-ATPase and from the inhibition of the photosynthetic electron transport, only the uncoupling of thylakoids can cause an inhibition of non-photochemical quenching (qN) accompanied by an overreduction of the photosynthetic electron transport chain (i.e., a decrease of photochemical quenching qP). Such simultaneous decreases of both qN and qP have been reported in a variety of environmental stresses: low temperatures (Strand and Öquist 1988), ozone (Calatayud et al 2002), copper (Pádua et al 2010), acid rain (Qiu and Liu 2002), and methyl viologen (Samson et al 2020). To further support our conclusions, it would have been interesting to directly measure the inhibition of qN formation in thylakoids treated with low to moderate AAPH concentrations.…”

Photoprotective Role of Photosynthetic and Non-Photosynthetic Pigments in Phillyrea latifolia: Is Their “Antioxidant” Function Prominent in Leaves Exposed to Severe Summer Drought?

“…The high daily turnover of quercetin and luteolin derivatives observed here poses the question whether the flavonoids have been degraded because of the combined action of high light and elevated leaf T [ 91 ] or have instead been oxidized by the excess of ROS (or through electron donation to vacuolar peroxidase)and not fully recycled to their reduced forms [ 77 , 92 , 93 ]. Addressing this complex issue requires further investigations aimed at exploring changes in both the sub-cellular distribution of ascorbate (and glutathione as well) [ 94 , 95 ] and the activity of mono-dehydroascorbate [ 93 ] in leaves facing severe drought stress.…”