Photosynthesis Research 54: 169–183, 1997. 169 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Regular paper The effects of elevated light on Photosystem II function: A thermoluminescence study Robin G. Walters 1 & Giles N. Johnson 2 1 Robert Hill Institute, Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court, Western Bank, Sheffield S10 2TN, UK; 2 SBE, Bˆ at. 532, CE-Saclay, 91191 Cedex, France (Present address: School of Biological Sciences, University of Manchester, 3.614 Stopford Building, Oxford Road, Manchester, M13 9PT, UK) Received 17 September 1996; accepted in revised form 14 April 1997 Key words: chlorophyll fluorescence quenching, photoinhibition, photoprotection, photosynthesis Abstract We have used the technique of thermoluminescence (TL) to investigate high-light-inducedchlorophyll fluorescence quenching phenomena in barley leaves, and have shown it to be a powerful tool in such investigations. TL measurements were taken from wild-type and chlorina f2 barley leaves which had been dark-adapted or exposed to 20 min illumination of varying irradiance or given varying periods of recovery following strong irradiance. We have found strong evidence that there is a sustained trans-thylakoid ∆pH in leaves following illumination, and that this ∆pH gives rise to quenching of chlorophyll fluorescence which has previously been identified as a slowly-relaxing component of antenna-related protective energy dissipation; we have identified a state of the PS II reaction centre resulting from high light treatments which is apparently able to perform normal charge separation and electron transport but which is ‘non-photochemically’ quenched, in that the application of a light pulse of high irradiance cannot cause the formation of a high fluorescent state; and we have provided evidence that a transient state of the PS II reaction centre is formed during recovery from such high light treatments, in which electron transport from Q A to Q B is apparently impaired. Abbreviations: Q A – primary quinone electron acceptor of PS II; Q B – secondary quinone electron acceptor of PS II; qE – ∆pH-dependent protective energy dissipation; qN – coefficient of non-photochemical quenching of chlorophyll a fluorescence; qN f , qN m , qN s – fast, medium and slow-relaxing components of qN; qO – coefficient of quenching of dark-level chlorophyll a fluorescence; PS II – Photosystem II; TL – thermoluminescence; T TLmax – temperature at which a given TL signal is a maximum Introduction The effects of extreme light on the photosynthetic apparatus of higher plants are well documented. Inci- dent light in excess of that which can be used in pho- tosynthesis leads to damage to Photosystem II (PS II), resulting in the subsequent degradation and resynthe- sis of the D1 polypeptide (for a review see Aro et al. 1993; Barber 1995). Plants have evolved mechanisms to prevent such damage, and several possible protec- tive processes have been identified as a result of analy- sis of chlorophyll a fluorescence quenching (Demmig- Adams and Adams 1992; Krieger and Weis 1992; Hor- ton et al. 1996). At room temperature, chlorophyll flu- orescence is emitted almost exclusively by PS II and so changes in the yield of that fluorescence provide infor- mation on the function of PS II. In vitro evidence indi- cates that several different biochemical mechanisms