Electron spin resonance spectroscopy was used to study the mechanism of the singlet oxygen(1O2*) production and scavenging in cytochrome b6f complex(Cyt b6f) of spinach.The results showed that under white-light illumination and aerobic conditions,1O2* was detected in isolated Rieske-depleted Cyt b6f and isolated Rieske Fe-S protein,indicating that both Chl a cofactor and Rieske Fe-S protein were responsible for 1O2* formation,with Chl a being the major source.This conclusion was further corroborated by 1O2* formation under selective excitation of Chl a by using monocolor red light at 675 nm.Furthermore,1O2* was scavenged by adding antioxidative substance such as L-histidine,ascorbate,β-carotene or glutathione.This offered protection against singlet oxygen oxidation of Cyt b6f complex.