The effects of elevated light on Photosystem II function: A thermoluminescence study

We have used the technique of thermoluminescence (TL) to investigate high-light-induced chlorophyll 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_Ato Q_Bis apparently impaired.