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A comparison has been made between the changes in absorption spectra and chlorophyll fluorescence emission occurring upon the induction of non-photochemical dissipation of excitation energy (qE) in isolated thylakoids and those accompanying the aggregation of detergent-solubilised spinach light-harvesting complex (LHCII). In support of a recent hypothesis for the mechanism of qE (Horton et al. (1991) FEBS Lett. 292, 1–4), it was found that absorbence changes at 530 nm were associated with qE and LHCII aggregation. Antimycin A inhibited these changes and prevented LHCII aggregation, as indicated by the electrophoretic mobility of the complex and its low-temperature fluorescence spectrum. An antimycin-insensitive partial aggregation of LHCII was associated with an absorbance change at 505 nm. Low concentration of detergent caused disaggregation of LHCII and the reversal of qE. These data are discussed in terms of the relationship between structural change in LHCII and the mechanism of non-photochemical quenching of chlorophyll fluorescence in thylakoids.

Thylakoids isolated from light-treated spinach leaves (‘light’ thylakoids) which contain significant zeaxanthin levels show amplification of high-energy-state chlorophyll fluorescence quenching (qE) with respect to the transthylakoid pH gradient, when compared to thylakoids from dark-adapted leaves (‘dark’ thylakoids) which contain no zeaxanthin. In 77 K fluorescence emission spectra q E in ‘dark’ thylakoids shows maximal quenching with respect to fluorescence yield near 680 nm, whereas in ‘light’ thylakoids additional quenching occurs near 700 nm, which is associated with the appearance of a chlorophyll species absorbing at long wavelengths. It is suggested that this long-wavelength species is associated with an aggregated state of LHC II.

The parameter фp (= rate of photosynthetic O2evolution(incident light × qQ) reflects the efficiency of O2 evolution by PS II reaction centres with respect to incident light, independent of the redox state of the primary electron acceptor QA (Weis, E. and Berry, J.A. (1987) Biochim. Biophys. Acta 894, 198–208). During whole chain electron transfer to methyl viologen in spinach thylakoids decreases in фp of more than 50% were observed in the presence of a transthylakoid pH gradient. Evidence was obtained that there are two components of фp decrease; one is associated with non-photochemical dissipation of chlorophyll excitation, observed as nonphotochemical fluorescence quenching (qNP), while a second component is not associated with fluorescence changes. The evidence for the latter was obtained using thylakoids that were suspended in low magnesium, or in the presence of antimycin A, so that they would develop only low levels of qNP. It is proposed that this component is due to electron cycling around the PS II reaction centre, and that the cycle is closely related to the transthylakoid pH gradient. The cycle can be observed during electron flow through PS II only, to o-phenylene diamine, and therefore does not appear to be controlled by the redox state of components beyond PS II in the electron transfer chain. Reversible фp decreases observed in uncoupled thylakoids at low pH suggest that the cycle may be controlled by the pH of the intrathylakoid space, and not the pH gradient per se. Measurements made during the approach to steady state, in which the development qNP is slower that the formation of the pH gradient, are consistent with the suggestion that the contribution to фp changes by qNP can be determined by the ratio of variable to maximum fluorescence yield (Genty, B., Briantais, J.-M. and Baker, N.R. (1989) Biochim. Biophys. Acta 990, 87–92).