Picosecond time-resolved fluorescence study of chlorophyll organisation and excitation energy distribution in chloroplasts from wild-type barley and a mutant lacking chlorophyll b

Abstract

Picosecond time-resolved fluorescence spectroscopy has been used to investigate the fluorescence emission from wild-type barley chloroplasts and from chloroplasts of the barley mutant, chlorina f-2, which lacks the light-harvesting chlorophyll a/b-protein complex. Cation-controlled regulation of the distribution of excitation energy was studied in isolated chloroplasts at the F 0 and F m levels. It was found that: 1. (a) The fluorescence decay curves were distinctly non-exponential, even at low excitation intensities (<2 × 10 14 photons · cm −2). 2. (b) The fluorescence decay curves could, however, be described by a dual exponential decay law. The wild-type barley chloroplasts gave a short-lived fluorescence component of approximately 140 ps and a long-lived component of 600 ps ( F 0) or 1300 ps ( F m) in the presence of Mg 2+; in comparison, the mutant barley yielded a short-lived fluorescence component of approx. 50 ps and a long-lived component of 194 ps ( F 0) and 424 ps ( F m). 3. (c) The absence of the light-harvesting chlorophyll a/b-protein complex in the mutant results in a low fluorescence quantum yield which is unaffected by the cation composition of the medium. 4. (d) The fluorescence yield changes seen in steady-state experiments on closing Photosystem II reaction centres ( F m/ F 0) or on the addition of MgCl 2 (+Mg 2+/−Mg 2+) were in overall agreement with those calculated from the time-resolved fluorescence measurements. The results suggest that the short-lived fluorescence component is partly attributable to the chlorophyll a antenna of Photosystem I, and, in part, to those light-harvesting-Photosystem II pigment combinations which are strongly coupled to the Photosystem I antenna chlorophyll. The long-lived fluorescence component can be ascribed to the light-harvesting-Photosystem II pigment combinations not coupled with the antenna of Photosystem I. In the case of the mutant, the two components appear to be the separate emissions from the Photosystem I and Photosystem II antenna chlorophylls.