NANOSECOND FLUORESCENCE OF CHLOROPLASTS AS A PROBE FOR ELECTRON-TRANSFER DISRUPTION IN PHOTOSYSTEM-II

The relative amplitude of fluorescence (lifetime, 1.2 ns) of wheat chloroplasts illuminated at low light intensity is increased 35-50 times on addition of inhibitors of electron transfer on the acceptor side of photosystem II (PS II) (diuron, dinoseb and 2-n-heptyl-4-hydroxyquinoline-N-oxide (HOQNO)), but is insensitive to cytochrome b6f complex inhibitors (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and 2-iodo-6-isopropyl-3-methyl-2,4,4'-trinitrodiphenyl ether (DNP-INT)) and to photophosphorylation uncouplers. At high light intensities, the relative amplitude of the 1.2 ns fluorescence component increases to the same extent as observed at low intensities in the presence of diuron. The nanosecond fluorescence of chloroplasts exposed to high intensity light is decreased 3-4-fold by carbonylcyanide m-chlorophenylhydrazone (CCCP). The data obtained show that the PS II fluorescence of chloroplasts in the nanosecond region is sensitive to a change in the electron transport rate at the level of the quinone acceptors of PS II. The nanosecond fluorescence of chloroplasts illuminated at high light intensity reflects the state of the electron transport system on the donor side of PS II.