Changes in the redox potential of primary and secondary electron-accepting quinones in photosystem II confer increased resistance to photoinhibition in low-temperature-acclimated arabidopsis

• Exposure of control (non-hardened) Arabidopsis leaves for 2 h at high irradiance at 5 degreesC resulted in a 55% decrease in photosystem II (PSII) photochemical efficiency as indicated by FcFm. In contrast, cold-acclimated leaves exposed to the same conditions showed only a 22degreesC decrease in FupsilonFm. Thermoluminescence was used to assess the possible role(s) of PSII recombination events in this differential resistance to photoinhibition. Thermoluminescence measurements of PSH revealed that S(2)QA(-) recombination was shifted to higher temperatures, whereas the characteristic temperature of the S(2)Q(B)(-) recombination was shifted to lower temperatures in cold-acclimated plants. These shifts in recombination temperatures indicate higher activation energy for the S(2)Q(A)(-) redox, pair and lower activation energy for the S(2)Q(B) redoxpair. This results in an increase in the free-energy gap between P680(+)Q(A)(-) and P680(+)Pheo(-) and a narrowing of the free energy gap between primary and secondary electron-accepting quinones in PSH electron acceptors. We propose that these effects result in an increased population of reduced primary electron-accepting quinone in PSII, facilitating non-radiative P680(+)QA(-) radical pair recombination. Enhanced reaction center quenching was confirmed using in vivo chlorophyll fluorescence-quenching analysis. The enhanced dissipation of excess light energy within the reaction center of PSII, in part, accounts for the observed increase in resistance to high-light stress in cold-acclimated Arabidopsis plants.

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