J. Photochem. Photobiol. B: Biol., 15 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK (1992) 63-74 63 Chlorophyll fluorescence analyses of photosystem II reaction center heterogeneity Kerry K. Karukstis Department of Chemistry, Harvey Mudd College, Claremont, CA 91711 (USA) (Received September 6, 1991; accepted March 20, 1992) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Abstract This paper summarizes recent fluorescence investigations which probe the heterogeneity displayed by photosystem II (PS II) reaction centers. Those studies which reveal PS II functional and structural heterogeneity using chlorophyll fluorescence transients and steady state intensities are highlighted. Chlorophyll fluorescence measurements are especially well suited for revealing the existence of heterogeneity within PS II reaction centers in terms of both their antenna composition and their efficiency of electron flow to the plastoquinone pool. The spectroscopic characteristics of chlorophyll fluorescence - in particular, the intensity and induction kinetics of the steady state room-temperature emission - reflect the fundamental properties of the chlorophyll molecules and their environment. Although numerous other effective tools for the elucidation of PS II heterogeneity are available, this discussion is limited to considerations of the de-excitation of electronically excited chlorophyll molecules via fluorescence emission as an indicator of PS II organization and function. Keywords: Chlorophyll a fluorescence, photosystem II heterogeneity, active and inactive photosystem II reaction centers, PS IIa and PS 116, Qa-reducing and non-reducing centers; quinones (spinach thylakoid). 1. The origin of in viva chlorophyll fluorescence at room temperature The fluorescence emission of chlorophyll molecules emanates from an electronic transition from the lowest excited singlet state of chlorophyll to the ground state. The first excited singlet state is obtained directly by absorption into the chlorophyll absorption band in the red spectral region and indirectly on rapid decay via heat dissipation arising from blue-light absorption in the Soret region. Although the light-harvesting pigment-protein complexes of higher plants are composed of both chlorophyll a (Chl a) and chlorophyll b (Chl b), the in viva chlorophyll fluorescence emission at room temperature arises only from Chl a. Chl b serves as an accessory pigment which transfers absorbed energy without loss to Chl u via exciton migration. Although the antenna complexes of reaction centers from both photosystems I and II (PS I and PS II) contain Chl a, the room-temperature chlorophyll fluorescence in chloroplasts is almost exclusively associated with PS II [l]. In functioning chloroplasts, deactivation of the electronically excited chlorophyll molecules occurs primarily via energy transfer to the reaction center. However, the loll-1344/92/$5.00 0 1992 - Elsevier Sequoia. All rights reserved