In dark-adapted spinach leaves approximately one third of the Photosystem II (PS II) reaction centers are impaired in their ability to transfer electrons to Photosystem I. Although these 'inactive' PS II centers are capable of reducing the primary quinone acceptor, QA, oxidation of Q~, occurs approximately 1000 times more slowly than at 'active' centers. Previous studies based on dark-adapted leaves show that minimal energy transfer occurs from inactive centers to active centers, indicating that the quantum yield of photosynthesis could be significantly impaired by the presence of inactive centers. The objective of the work described here was to determine the performance of inactive PS II centers in light-adapted leaves. Measurements of PS II activity within leaves did not indicate any increase in the concentration of active PS II centers during light treatments between 10 s and 5 min, showing that inactive centers are not converted to active centers during light treatment. Light-induced modification of inactive PS II centers did occur, however, such that 75% of these centers were unable to sustain stable charge separation. In addition, the maximum yield of chlorophyll fluorescence associated with inactive PS II centers decreased substantially, despite the lack of any overall quenching of the maximum fluorescence yield. The effect of light treatment on inactive centers was reversed in the dark within 10-20 mins. These results indicate that illumination changes inactive PS II centers into a form that quenches fluorescence, but does not allow stable charge separation across the photosynthetic membrane. One possibility is that inactive centers are converted into centers that quench fluorescence by formation of a radical, such as reduced pheophytin or oxidized P680. Alternatively, it is possible that inactive PS II centers are modified such that absorbed excitation energy is dissipated thermally, through electron cycling at the reaction center.
Abbreviations: AA518-absorbance change at 518 nm, reflecting the formation of an electric field across the thylakoid membrane; AAFL1 -amplitude of the fast (< 100 ms) phase ofAA518 induced by the first of two saturating, single-turnover flashes spaced 30 ms apart; AAFL2 -amplitude of the fast (< 100 ms) phase of AA518 induced by the second of two saturating, single-turnover flashes spaced 50 ms apart; DCBQ-2,6-dichloro-p-benzoquinone; Fo-yield of chlorophyll fluorescence when QA is fully oxidized; Fm-yield of chlorophyll fluorescence when QA is fully reduced; Fx-yield of chlorophyll fluorescence when QA is fully reduced at inactive PS II centers, but fully oxidized at active PS II centers; Pheo-pheophytin; P680-the primary donor of Photosystem II; PPFDphotosynthetic photon flux density; QA-primary quinone acceptor of PS II; QB -secondary quinone acceptor of PS II