Electron spin resonance in zero magnetic field of triplet states of chloroplasts and subchloroplast particles.

Recently, the new technique of electron spin resonance in zero magnetic field, as observed by Microwave Induced changes in Fluorescence (MIF) has been applied to the study of the light-induced triplet state in chloroplasts under reducing conditions [ 1 ] . Two resonances were detected at 723 and 952 MHz. The resonances were observed also in samples in which the acceptor of photosystem 2 was reduced with light in the presence of 3-(3,Cdichlorophenyl)-l,ldimethylurea (DCMU) and hydroxylamine (NH,OH). This led to the conclusion that the observed triplet was linked to photosystem 2 (PS2). The remarkable narrowness of the lines suggested that it was located on pigment molecules in identical surroundings, i.e., on the reaction center chlorophyll. In order to verify these conclusions, we have carried out MIF experiments on subchloroplast particles enriched in either photosystem 1 (PSl particles) or photosystem (PS2 particles) by digitonin treatment. In contrast to the work on whole chloroplasts, the resonances described above could not be detected in the subchloroplast particles, even when the microwave power incident on the sample was greatly increased. Apparently, rupture of the photosynthetic complex of chloroplasts prevents the formation of this triplet. This means that in intact chloroplasts, it is formed by an excitation energy transfer from the PS2 units with closed traps to PSI units, and is trapped on a fraction of antenna chlorophyll that after digitonin treatment