NONPHOTOCHEMICAL QUENCHING OF EXCITATION-ENERGY IN PHOTOSYSTEM-II - APICOSECOND TIME-RESOLVED STUDY OF THE LOW-YIELD OF CHLOROPHYLL-A FLUORESCENCE INDUCED BY SINGLE-TURNOVER FLASH IN ISOLATED SPINACH THYLAKOIDS

Chlorophyll a fluorescence emission is widely used as a noninvasive me asure of a number of parameters related to photosynthetic efficiency i n oxygenic photosynthetic organisms. The most important component for the estimation of photochemistry is the relative increase in fluoresce nce yield between dark-adapted samples which have a maximal capacity f or photochemistry and a minimal fluorescence yield (F-o) and light-sat urated samples where photochemistry is saturated and fluorescence yiel d is maximal (F-m). However, when photosynthesis is saturated with a s hort (less than 50 mu s) flash of light, which induces only one photoc hemical turnover of photosystem II, the maximal fluorescence yield is significantly lower (F-sat) than when saturation is achieved with a mi llisecond duration multiturnover flash (F-m). To investigate the origi ns of the difference in fluorescence yield between these two condition s, our time resolved fluorescence apparatus was modified to allow coll ection of picosecond time-resolved decay kinetics over a short time wi ndow immediately following a saturating single-turnover flash (F-sat) as well as after a multiturnover saturating pulse (F-m), Our data were analyzed with a global kinetic model based on an exciton radical pair equilibrium model for photosystem II, The difference between F-m and F-sat was modeled well by changing only the rate constant for quenchin g of excitation energy in the antenna of photosystem II. An antenna-ba sed origin for the quenching was verified experimentally by the observ ation that addition of the antenna quencher 5-hydroxy-1,4-naphthoquino ne to thylakoids under F-m conditions resulted in decay kinetics and m odeled kinetic parameters very similar to those observed under F-sat c onditions in the absence of added quinone. Our data strongly support t he origin of low fluorescence yield at F-sat to be an antenna-based no nphotochemical quenching of excitation energy in photosystem II which has not usually been considered explicitly in calculations of photoche mical and nonphotochemical quenching parameters. The implications of o ur data with respect to kinetic models for the excited-state dynamics of photosystem II and the practical applications of the fluorescence y ield parameters F-m and F-sat to calculations of photochemical yield a re discussed.