The afterglow thermoluminescence band as an indicator of changes in the photorespiratory metabolism of the model legume Lotus japonicus

The afterglow (AG) luminescence is a delayed chlorophyll fluorescence emitted by the photosystem II that seems to reflect the level of assimilatory potential (NADPH+ATP) in chloroplast. In this work, the thermoluminescence (TL) emissions corresponding to the AG band were investigated in plants of the WT and the Ljgln2-2 photorespiratory mutant from Lotus japonicus grown under either photorespiratory (air) or non-photorespiratory (high concentration of CO2) conditions. TL glow curves obtained after two flashes induced the strongest overall TL emissions, which could be decomposed in two components: B band (t(max)=27-29 degrees C) and AG band (t(max)=44-45 degrees C). Under photorespiratory conditions, WT plants showed a ratio of 1.17 between the intensity of the AG and B bands (I-AG/I-B). This ratio increased considerably under non-photorespiratory conditions (2.12). In contrast, mutant Ljgln2-2 plants grown under both conditions showed a high I-AG/I-B ratio, similar to that of WT plants grown under non-photorespiratory conditions. In addition, high temperature thermoluminescence (HTL) emissions associated to lipid peroxidation were also recorded. WT and Ljgln2-2 mutant plants grown under photorespiratory conditions showed both a significant HTL band, which increased significantly under non-photorespiratory conditions. The results of this work indicate that changes in the amplitude of I-AG/I-B ratio could be used as an in vivo indicator of alteration in the level of photorespiratory metabolism in L. japonicus chloroplasts. Moreover, the HTL results suggest that photorespiration plays some role in the protection of the chloroplast against lipid peroxidation.