The plastoquinone (PQ) pool mediates electron flow and regulates photoacclimation in plants. Here we report the action spectrum of the redox state of the PQ pool in Arabidopsis thaliana, showing that 470-500, 560 or 650-660 nm light favors Photosystem II (PSII) and reduces the PQ pool, whereas 420-440, 520 or 690 nm light favors Photosystem I (PSI) and oxidizes PQ. These data were used to construct a model predicting the redox state of PQ from the spectrum of any polychromatic light source. Moderate reduction of the PQ pool induced transition to light state 2, whereas state 1 required highly oxidized PQ. In low-intensity PSI light, PQ was more oxidized than in darkness and became gradually reduced with light intensity, while weak PSII light strongly reduced PQ. Natural sunlight was found to favor PSI, which enables plants to use the redox state of the PQ pool as a measure of light intensity.

Main conclusions Low temperature decreases PSII damage in vivo, confirming earlier in vitro results. Susceptibility to photoinhibition differs among Arabidopsis accessions and moderately decreases after 2-week cold-treatment. Flavonols may alleviate photoinhibition. Abstract The rate of light-induced inactivation of photosystem II (PSII) at 22 and 4 °C was measured from natural accessions of Arabidopsis thaliana (Rschew, Tenela, Columbia-0, Coimbra) grown under optimal conditions (21 °C), and at 4 °C from plants shifted to 4 °C for 2 weeks. Measurements were done in the absence and presence of lincomycin (to block repair). PSII activity was assayed with the chlorophyll a fluorescence parameter F v / F m and with light-saturated rate of oxygen evolution using a quinone acceptor. When grown at 21 °C, Rschew was the most tolerant to photoinhibition and Coimbra the least. Damage to PSII, judged from fitting the decrease in oxygen evolution or F v / F m to a first-order equation, proceeded more slowly or equally at 4 than at 22 °C. The 2-week cold-treatment decreased photoinhibition at 4 °C consistently in Columbia-0 and Coimbra, whereas in Rschew and Tenela the results depended on the method used to assay photoinhibition. The rate of singlet oxygen production by isolated thylakoid membranes, measured with histidine, stayed the same or slightly decreased with decreasing temperature. On the other hand, measurements of singlet oxygen from leaves with Singlet Oxygen Sensor Green suggest that in vivo more singlet oxygen is produced at 4 °C. Under high light, the PSII electron acceptor Q A was more reduced at 4 than at 22 °C. Singlet oxygen production, in vitro or in vivo, did not decrease due to the cold-treatment. Epidermal flavonols increased during the cold-treatment and, in Columbia-0 and Coimbra, the amount correlated with photoinhibition tolerance. Electronic supplementary material The online version of this article (10.1007/s00425-020-03423-0) contains supplementary material, which is available to authorized users.

 Low temperature decreases PSII damage in vivo, in agreement with earlier in vitro results. Susceptibility to photoinhibition differs among Arabidopsis accessions and moderately decreases after cold-acclimation. Flavonols may alleviate photoinhibition. AbstractTo understand the effects of low temperature and cold-acclimation on reactive oxygen species and photoinhibition of photosystem II (PSII), light-induced inactivation of PSII was measured at 22 and 4 °C from four Arabidopsis thaliana accessions (Rschew, Tenela, Columbia-0 and Coimbra) grown under optimal conditions. Photoinhibition was also measured at 4 °C from plants cold-acclimated at 4 °C for two weeks. Measurements were done in the absence and presence of lincomycin that blocks PSII repair, and PSII activity was assayed with the ratio of variable to maximum chlorophyll a fluorescence (FV/FM) and with light-saturated rate of oxygen evolution using a quinone acceptor. Of the nonacclimated accessions, Rschew was the most tolerant to photoinhibition and Coimbra the least; the rate constants of photoinhibition of the most sensitive accession were 1.3-1.9 times as high as those of the tolerant ones. The damaging reaction of photoinhibition in non-acclimated plants was slower or equal at 4 °C than at 22 °C. The rate constants of photoinhibition of cold-acclimated plants, at 4 °C, were 0.55 to 1.25 times as high as those of non-acclimated plants; the protective effect of coldacclimation on photoinhibition was consistent in Columbia-0 and Coimbra whereas Rschew and Tenela were either slightly more tolerant or susceptible, depending on the method used to assay photoinhibition. Production of singlet oxygen, measured from thylakoid membranes isolated from non-acclimated and cold-acclimated plants, did not decrease due to cold-acclimation, nor did singlet oxygen production correlate with the rate of photoinhibition or with flavonol contents of the leaves.

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