Cyclic electron flow around PSI monitored by afterglow luminescence in leaves of maize inbred lines ( Zea mays L.): correlation with chilling tolerance.

Maize ( Zea mays L.) inbred lines of contrasting chilling sensitivity (three tolerant, three sensitive lines) were acclimated to 280 μmol photons m⁻² s⁻¹ white light at a 17°C sub-optimal temperature. They showed no symptoms of photoinhibition, despite slight changes in photosystem II (PSII) fluorescence and thermoluminescence properties in two tolerant lines. A luminescence “afterglow” emission [Bertsch and Azzi (1965) Biochim Biophys Acta 94:15–26], inducible by a far-red (FR) illumination of unfrozen leaf discs, was detected either as a bounce in decay kinetics at constant temperatures or as a sharp thermoluminescence afterglow band at about 45°C, in dark-adapted leaves. This band reflects the induction by warming of an electron pathway from stromal reductants to plastoquinones and to the Q_B secondary acceptor of PSII, resulting in a luminescence-emitting charge recombination in the fraction of centres that were initially in the S_2/3Q_B non-luminescent state. A 5-h exposure of plants to growth chamber light shifted this luminescence emission towards shorter times and lower temperatures for several hours in the three chilling-tolerant lines. This downshift was not observed, or only transiently, in the three sensitive lines. In darkness, the downshifted afterglow band relaxed within hours to resume its dark-adapted location, similar for all maize lines. A faster dark re-reduction of P700⁺ oxidized by FR light (monitored by 820-nm absorbance) and an increase of photochemical energy storage under FR excitation (determined by photoacoustic spectroscopy) confirmed that a cyclic pathway induced by white actinic light remained activated for several hours in the tolerant maize lines. [ABSTRACT FROM AUTHOR]