A photosynthetic organism is subjected to photo-oxidative stress when more light energy is absorbed than is used in photosynthesis. In the light, highly reactive singlet oxygen can be produced via triplet chlorophyll formation in the reaction centre of photosystem II and in the antenna system. In the antenna, triplet chlorophyll is produced directly by excited singlet chlorophyll, while in the reaction centre it is formed via charge recombination of the light-induced charge pair. Changes of the mid-point potential of the primary quinone acceptor in photosystem II modulate the pathway of charge recombination in photosystem II and influence the yield of singlet oxygen production. Singlet oxygen can be quenched by beta-carotene, alpha-tocopherol or can react with the D1 protein of photosystem II as target. If not completely quenched, it can specifically trigger the up-regulation of the expression of genes which are involved in the molecular defence response of plants against photo-oxidative stress.