CHLOROPHYLL ALPHA FLUORESCENCE AND PHOTOCHEMICAL ACTIVITIES OF CHLOROPLAST FRAGMENTS

Comparative studies were made on the fluorescence characteristics of chlorophyll a at 20[degree] and -193[degree], and quantum efficiencies for P 700 oxidation and NADP+ reduction were measured in chloroplasts and chloroplast fragments obtained after incubation with 0.5% digitonin. Differences in the fluorescence yield of chlorophyll a in flowing stationary suspensions of untreated chloroplasts and of the large fragments are indicative of light-induced photoreduction of the quencher Q of chlorophyll a, associated with pigment System 2 (chlorophyll a2). The relatively low constant fluorescence yield of chlorophyll a in the small fragments indicates the absence of fluorescent chlorophyll a from these fragments and suggests that the low fluorescence is due to chlorophyll a, associated with pigment System 1 (chlorophyll a1). The ratio of the fluorescence yields of chlorophyll a1 and chlorophyll a2 is 0.45:1. In the large particles the concentration ratio of pigment System 1 and System 2 is 1:3. The efficiencies of quanta absorbed at 673, 683, and 705 nm for NADP+ reduction and P 700 oxidation in untreated chloroplasts and chloroplast fragments indicate that digitonin treatment results in a separation of System 2 from System 1 in the small fragments. Sonication does not cause such a separation. Under the conditions used P 700 oxidation and NADP+ reduction in the small fragments separated after digitonin treatment, occurred with maximal efficiency of 0.7 to 1.0 and 0.7, respectively. The constancy of the fluorescence yield of chlorophyll ai in the small fragments, under conditions at which P 700 is oxidized and NADP+ is reduced, is interpreted as evidence either for the hypothesis that the fluorescence of chlorophyll ai is controlled by the redox state of the primary photo-reductant XH, or alternatively for the hypothesis that energy transfer from fluorescent chlorophyll a1 to P 700 goes via an intrinsically weak fluorescent, still unknown, chlorophyll-like pigment. The low-temperature emission band around 730 nm is argued not to be due to excitation by System 1 only; the relatively large half width of the band, as compared to the emission bands at 683 and 696 nm, suggests that it is possibly due to overlapping emission bands of different pigments.