Phenolic Compound Diversity Explored in the Context of Photo-Oxidative Stress Protection.

INTRODUCTION: Phenolic compounds are a chemically diverse group of plant secondary metabolites with important roles both in plant stress defence and human nutrition.

OBJECTIVE: To explore structure-function relations potentiating phenolic compounds to promote leaf acclimation to light stress by excess photosynthetically active radiation (photoinhibition) and by solar ultraviolet (UV) radiation.

METHODOLOGY: We report singlet oxygen and hydrogen peroxide antioxidant capacities and UV-absorbing properties of 27 flavonoids and 11 phenolic acids. Correlations of these characteristics in the whole data set and related activity-structure relationships in flavonoid data were investigated using simple statistical methods.

RESULTS: In comparison to flavonoids, phenolic acids are relatively ineffective reactive oxygen neutralising antioxidants; and - with the exception of gallic acid - have poor reactivity to hydrogen peroxide. Singlet oxygen and hydrogen peroxide detoxifying capacities of flavonoids are positively correlated, largely due to the strong positive effect of the hydroxylation of the C-ring in position-3. 3-O-Glycosylation halves reactive oxygen species (ROS) reactivities of quercetin and myricetin but eradicates the hydrogen peroxide reactivity of kaemferol. B-ring polyhydroxylation (cathecol structure) increases the hydrogen peroxide antioxidant function but decreases UV-B (280-315 nm) absorption. UV-A (315-400 nm) absorption is increased by the B-ring C2-C3 double bond either in itself or in combination with the C4 oxo-group.

CONCLUSION: Among the studied compounds, anthocyanins and flavonols were the strongest singlet oxygen and hydrogen peroxide scavengers, and are thus capable of supporting defence against both photoinhibition by visible light and UV stress in leaves, while flavanols may only be effective against the latter. Copyright © 2017 John Wiley & Sons, Ltd.