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Proanthocyanidins (PAs) are the main products of the flavonoid biosynthetic pathway in many plants. However, their biological function during environmental stresses in plants is rarely reported. In the present study, the effects of pretreatment with PAs on the response of cucumber ( Cucumis sativus L.) seedlings to high irradiance (HI), polyethylene glycol (PEG), and cold stress were investigated. The PAs pretreament alleviated stress-induced oxidative damage in plant cells and increased the activity of alternative oxidase (AOX) and content of abscisic acid (ABA). Furthermore, PAs-pretreated seedlings suffered less damage by the stress conditions, maintained higher content of chlorophyll a+ b and AOX proteins in comparison with the control. Therefore, our findings suggest that PAs might contribute to plant tolerance to environmental stresses. [ABSTRACT FROM AUTHOR]

The effects of exogenous salicylic acid (SA) on photosystem 2 (PS 2) in barley (Hordeum vulgare L.) seedlings were investigated. SA pretreatment provided protection against subsequent osmotic stress. The highest protective effect of 0.25 mM SA was confirmed by determination of chlorophyll fluorescence, electrolyte leakage, malonyldialdehyde contents, PS 2 mRNAs and proteins. SA pretreatment increased reactive oxygen species (ROS), decreased net photosynthetic rate and stomatal conductance immediately, but prevented ROS accumulation during subsequent osmotic stress by activating antioxidant enzymes. Elimination of H2O2 during SA pretreatment inhibited almost all above mentioned SA effects. Therefore, SA pretreatment enhanced osmotic stress tolerance in barley seedlings mainly through ROS signals, rather than SA itself. The only SA-dependent and ROS-independent effect of exogenous SA on PS 2 was reduction of non-photochemical quenching. [ABSTRACT FROM AUTHOR]