Mesophyll versus epidermal anthocyanins as potential in vivo antioxidants: evidence linking the putative antioxidant role to the proximity of oxy-radical source

The hypothesis that anthocyanins in red leaves may be potential in vivo antioxidants whose efficiency is linked to their proximity with the oxy-radical source was tested. Advantage was taken of intra-individual and intra-species variations in the anthocyanic trait and green and red leaves on the same individuals or leaves of green and red phenotypes were compared for the extent of PSII damage by reactive oxygen species generated by methyl viologen treatment in the light. Two species possessing anthocyanins in the mesophyll (Cistus creticus and Photinia x fraseri) and two in the epidermis (Rosa sp. and Ricinus communis) were used, while red actinic light (which is not absorbed by anthocyanins) allowed discrimination between an indirect sunscreen and a direct antioxidant function. Red leaves whose anthocyanins were located in the mesophyll were more resistant to methyl viologen treatment than their green counterparts. In one of these species (Cistus creticus), where anthocyanins are induced in some individuals within the natural population after bright cool days in winter, both green and future-red morphs displayed the same sensitivity to methyl viologen before anthocyanin induction. Immediately after reddening, however, resistance to methyl viologen was considerably increased in the red morphs. By contrast, red leaves whose anthocyanins were restricted to epidermal cells were more sensitive to the herbicide. Total leaf phenolic levels in green/red pairs were similar. The results indicate that vacuolar anthocyanins may be an effective in vivo target for oxy-radicals, provided that the oxy-radical source and the anthocyanic detoxifying sink are in close vicinity.