The relationship between superoxide and singlet oxygen in PSⅡ particle of spinach under strong illumination was studied with spin-trap ESR technique. The generation of superoxide was increased under D 2O environment which prolonged the half life of singlet oxygen. The generation of superoxide was decreased when histidine existed as a scavenger of singlet oxygen. It is highly possible that the superoxide generated in PSⅡ particle originates from singlet oxygen.
