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• CGA reduces effects of methyl viologen (MV) stress in apple leaves. • CGA enhances the antioxidant capacity in apple leaves. • It is the first to confirm the in vivo antioxidant fuctions of CGA in plants. Considered to be an antioxidant, chlorogenic acid is involved in resistance to insects and pathogens, and beneficial effects on human health. However, little is known about the antioxidant functions of chlorogenic acid against oxidative stress in plants. In this study, we studied the potential role of exogenous chlorogenic acid in alleviating methyl viologen-mediated oxidative stress in detached leaves of apple (Malus domestica Borkh). The results showed that exogenous chlorogenic acid availably relieved the decreases in chlorophyll concentrations and maximum potential Photosystem II efficiency. Exogenous chlorogenic acid also effectively reduced membrane damage and lipid oxidation and stimulated activity of antioxidant enzymes, including peroxidase, catalase, and polyphenol oxidase. It is noteworthy that exogenous chlorogenic acid improves antioxidant activity of apple leaf under oxidative stress using DPPH, ABTS, and FRAP assays. In addition, the concentration of phenolics increased remarkably in response to chlorogenic acid application after methyl viologen exposure. Furthermore, exogenous chlorogenic acid also changed the transcription levels of related genes for antioxidant enzymes and phenolic metabolism. These results suggest that chlorogenic acid can be effective antioxidants protecting the plants against oxidative stress by enhancing antioxidant capacity. [ABSTRACT FROM AUTHOR]

• 5-aminolevulinic acid (ALA) alleviates the effect of methyl viologen-induced photooxidative stress on pear leaves. • ALA trigger endogenous H 2 O 2 accumulation. • NADPH oxidase-mediated H 2 O 2 signaling , which results in activated antioxidant enzyme activities, may plays an important role in mediating ALA-induced tolerance to photooxidative stress. The beneficial effects of 5-aminolevulinic acid (ALA) on abiotic stress have been demonstrated in several plant species. However, little is known about the signal transduction pathways triggered by ALA in the photooxidative stress response. In this study, we investigated the effects of ALA on methyl viologen (MV)-induced photooxidative stress tolerance in detached pear leaves (Pyrus ussuriensis). Furthermore, we also examined the involvement of NADPH oxidase (RBOH)-mediated H 2 O 2 as a signaling molecule in ALA-induced photooxidative stress tolerance. Our results showed that exogenous ALA alleviated the observed decreases in chlorophyll content and maximum potential photosystem II efficiency under MV-induced stress. Exogenous ALA also effectively reduced MV-induced oxidative damage and enhanced the activities of antioxidative enzymes (SOD and CAT). In situ determination of reactive oxygen species revealed that ALA induced H 2 O 2 production in the pear leaves. Importantly, ALA-induced tolerance was abolished by an RBOH inhibitor. Further investigation revealed that the transient overexpression of RBOHF in tobacco leaves alleviated MV-induced damage by enhancing the activities of SOD and CAT. This study provides new insights into the mechanisms by which ALA improves photooxidative tolerance in pear leaves. [ABSTRACT FROM AUTHOR]