• Carotenoids;
  • chlorophyll fluorescence;
  • singlet-oxygen;
  • xanthophyll cycle;
  • zeaxanthin

Carotenoids have two important roles in photosynthetic organisms. First, they act as accessory light-harvesting pigments, effectively extending the range of light absorbed by the photosynthetic apparatus. Secondly, they perform an essential photoprotective role by quenching triplet state chlorophyll molecules and scavenging singlet oxygen and other toxic oxygen species formed within the chloroplast. Only recently an additional, novel, protective role has been proposed for the carotenoid zeaxanthin, involving the dissipation of harmful excess excitation energy under stress conditions. Zeaxanthin may be formed through de novo synthesis in response to long-term environmental stress, and through the rapid enzymic de-epoxidation of the carotenoid violaxanthin (the xanthophyll cycle) in response to short-term alterations in the plant's light environment. Interspecific differences occur in the ability of plants and algae to produce zeaxanthin under stress conditions, and hence the ability to photoprotect the photosynthetic apparatus through this means varies from species to species. The ability of a plant to respond to light-mediated environmental stress by producing zeaxanthin may therefore affect, at least in part, the ability of that plant to inhabit or colonise certain habitats (e.g. sun or shade conditions).