Anthocyanin biosynthesis increases under environmental and biotic stresses in leaves. These same factors stimulate the production of reactive oxygen species (ROS). We have tested the theory that anthocyanins protect leaves from ROS, and that red leaves are better suited to scavenge ROS than are green leaves. Total antioxidant activities of leaf extracts from red and green morphs of Quintinia serrata (Escalloniaceae) and Elatostema rugosum (Urticaceae) were quantified using (i) their abilities to scavenge the stable DPPH radical, and (ii) cyclic voltammetry. They were also analysed for their compositions of flavonoids, hydroxycinnamates and antioxidant enzymes (superoxide dismutase, catalase, and ascorbate peroxidase). Red morphs of the shade plant, E. rugosum were more effective at scavenging ROS than the green morphs; with anthocyanins being the most active phenolic antioxidant tested. Anthocyanins appear to be synthesised in juvenile leaves to provide protection until a sufficient pool of enzymic antioxidants can be created. In contrast, leaves from red and green morphs of the sun plant Q. serrata had comparable antioxidant activities. Q. serrata appears to employ other phenolic compounds in addition to anthocyanin to provide adequate low molecular weight antioxidant (LMWA) protection; the hydroxycinnamates were found to be the most active components.

Key words: anthocyanins, Elatostema rugosum (Urticaceae), low molecular weight antioxidants (LMWA), phenolic compounds, Quintinia serrata (Escalloniaceae), reactive oxygen species (ROS)