Photosynthesis Research 36: 119-139, 1993. © 1993 Kluwer Academic Publishers. Printed in the Netherlands. Regular paper Theoretical assessment of alternative mechanisms for non-photochemical quenching of PS II fluorescence in barley leaves Robin G. Waiters & Peter Horton Robert Hill Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, P.O. Box 594, Firth Court, Western Bank, Sheffield $10 2UH, UK Received 15 September 1992; accepted in revised form 18 February 1993 Key words: energy dissipation, photoinhibition, photosynthesis, Photosystem II, quantum yield, state transition Abstract The components of non-photochemical chlorophyll fluorescence quenching (qN) in barley leaves have been quantified by a combination of relaxation kinetics analysis and 77 K fluorescence measurements (Waiters RG and Horton P 1991). Analysis of the behaviour of chlorophyll fluorescence parameters and oxygen evolution at low light (when only state transitions - measured as qN t - are present) and at high light (when only photoinhibition- measured as qN i -is increasing) showed that the parameter qN t represents quenching processes located in the antenna and that qNi measures quenching processes located in the reaction centre but which operate significantly only when those centres are closed. The theoretical predictions of a variety of models describing possible mechanisms for high-energy-state quenching, measured as the residual quenching, qNe, were then tested against the experimental data for both fluorescence quenching and quantum yield of oxygen evolution. Only one model was found to agree with these data, one in which antennae exist in two states, efficient in either energy transfer or energy dissipation, and in which those photosynthetic units in a dissipative state are unable to exchange energy with non-dissipative units. Abbreviations: Fo, F m - room-temperature chlorophyll fluorescence yield with all centres open, closed: F v - variable fluorescence yield = Fr, - Fo; LHC II - light-harvesting chlorophyll-protein complex of PS II; PS I, PS I!- Photosystem I, II; P700, P680- primary donor in Photosystem I, II; QA- primary electron acceptor of PS II; Pmax- maximum quantum yield of oxygen evolution; qN- coefficient of non-photochemical quenching of variable fluorescence; qN e, qNt, qN i - coefficient of non-photochemi- cal quenching due to high-energy-state, state transition, photoinhibition; qO- coefficient of quenching of dark level fluorescence; qP-coefficient of photochemical quenching of variable fluorescence; • p - 'intrinsic' quantum yield of open PS II reaction centres = qbs/qP; ~PS 2 - - quantum yield of PS II = qP × Fv/Fm; dPs -quantum yield of oxygen evolution = rate of oxygen evolution/light intensity Introduction The fate of light energy absorbed by a photo- synthetic pigment bed depends on the availabili- ty of reaction centres which are in a photo- synthetically active state ('open'). At low light intensities, energy is efficiently used in photo- chemistry, since open reaction centres are freely available. In contrast, where photosynthetic capacity is exceeded, the excess energy must be safely dissipated; without an efficient sink for the excess excitation, a substantial population of