SINGLET MOLECULAR-OXYGEN AND PRIMARY MECHANISMS OF PHOTOOXIDATIVE DAMAGE OF CHLOROPLASTS - STUDIES BASED ON DETECTION OF OXYGEN AND PIGMENTPHOSPHORESCENCE

Photogeneration of singlet oxygen molecules (O-1(2)), their vibrationa lly excited state (O-1(2)) and dimols (O-1(2))(2) has been shown by m easuring photosensitised delayed luminescence in pigment-containing me dia. All singlet oxygen species are formed as a result of energy trans fer to O-2 from triplet pigment molecules. Monomeric pigment molecules are the most efficient singlet oxygen generators. The O-1(2) quantum yields are 40-80% in aerobic solutions of monomeric chlorophylls and p heophytins. Pigment aggregation causes a strong decrease in singlet ox ygen production. The O-1(2) quantum yield in chloroplasts has been est imated using literature and experimental data on formation of the chlo rophyll triplet states in the photosynthetic apparatus. The most proba ble value is 0.1%. One of the major sources of singlet oxygen is likel y to be the triplet states of newly formed pigment molecules which are not quenched by carotenoids and can be detected by measuring low-temp erature pigment phosphorescence. Quenching of singlet oxygen by the th ylakoid components has been analysed and the O-1(2) lifetime estimated . The data suggest that carotenoids and chlorophylls are the most effi cient physical O-1(2) quenchers and the O-1(2) lifetime is about 70 ns in thylakoids. The quantum yield of O-1(2)-induced pigment photodestr uction was estimated to be about 10(-6)-10(-5). This value is close to the quantum yield of chlorophyll photobleaching experimentally observ ed in aerobic suspensions of isolated chloroplasts. The intensity of p igment phosphorescence at 77 K correlates with the rate of chlorophyll photobleaching in plant materials. The data suggest that O-1(2) gener ation by the pigment triplet states is the most likely reason for chlo roplast photodamage. The intensity of pigment phosphorescence can be u sed as an index of the degree of plant photo-oxidative stress.