Chlorophyll thermoluminescence of leaf discs: simple instruments and progress in signal interpretation open the way to new ecophysiological indicators.

Abstract

Luminescence from photosynthetic material observed in darkness following illumination is a delayed fluorescence produced by a recombination of charge pairs stored in photosystem II, i.e. the back-reaction of photosynthetic charge separation. Thermoluminescence (TL) is a technique consisting of a rapid cooling followed by the progressive warming of a preilluminated sample to reveal the different types of charge pairs as successive emission bands, which are resolved better than the corresponding decay phases recorded at constant temperatures. Progress in thermoelectric Peltier elements and in compact light detectors made the development of simple, affordable and transportable instruments possible. These instruments take advantage of multifurcated light guides for combined TL, fluorescence and absorbance/reflectance measurements. Meanwhile, experiments on unfrozen leaf discs, with excitation by single turn-over flashes or far red light and infiltration by specific inhibitors/uncouplers, have led to a better understanding of in vivo TL signals. Much like chlorophyll fluorescence and in a complementary way, TL in the 0-60 degrees C temperature range not only informs on the state of photosystem II in leaf tissues and its possible alterations, but also gives a broader insight into the energetic state inside the chloroplast by probing (1) the light-induced or dark-stable thylakoid proton gradient through the protonation of the Mn oxygen-evolving complex, (2) the induction of cyclic/chlororespiratory electron flow towards the plastoquinone pool, (3) the [NADPH+ATP] assimilatory potential. By a different mechanism, warming above 60 degrees C without preillumination reveals chemiluminescence high temperature thermoluminescence (HTL) bands due to the radiative thermolysis of peroxides, which are indicators of oxidative stress in leaves.