Anthocyanin accumulation in poinsettia leaves and its functional role in photo-oxidative stress

The role of anthocyanin accumulation in poinsettia leaves exposed to photo-oxidative stress was evaluated by comparing green (anthocyanin less) and reddish (anthocyanin well equipped) leaves, co-occurring in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch). For the assessment we compared the photoprotective and antioxidant mechanisms in the two leaf types in response to photo-oxidative stress, generated in chloroplasts by the application of methyl viologen (MV). MV accepts electrons from photosystem I (PSI) with subsequent transfer to molecular oxygen to produce superoxide anions (O-2(center dot)-) that are converted by the chloroplast superoxide dismutase (SOD) to hydrogen peroxide (H2O2) that is reduced by ascorbate peroxidase (APX) to water and oxygen. At 90omin after MV application, the decrease in the activity of the H2O2-scavenging enzyme APX resulted in increased H2O2 levels in both leaf types, but to a significantly lower level in the reddish leaves, possibly due to the significantly higher anthocyanin and phenolic content that are considered as H2O2-scavengers. Reddish poinsettia leaves having higher antioxidant activity possessed more effective photoprotective and reactive oxygen species (ROS)-scavenging mechanisms, including higher enzymatic (SOD and APX), and non-enzymatic (anthocyanin and phenolic) content over the green ones. Anthocyanin well equipped poinsettia leaves were in advantage in response to photo-oxidative stress over the anthocyanin less ones that appeared not to be equally protected. This superiority in the avoidance of the photo-oxidative stress by the reddish leaves was also associated to a higher effective quantum yield of PSII photochemistry (Phi(PSII)), a lower excitation pressure (1- q(p)) and a lower H2O2 generation, compared to their green counterparts.