Increased resistance of photosystem II and photosystem I photochemistry to high light stress correlates with altered redox properties of QB following cold acclimation in Arabidopsis thaliana

Exposure of control (non-hardened) Arabidopsis leaves to high light stress at 5° C resulted in a decrease of PSII photochemical efficiency to 55% compared to the control. This was accompanied by a parallel 65% inhibition of PSI photochemistry measured as D A820/A820, by an increase of the intersystem electron pool size as well as an acceleration of PSI-dependent cyclic electron transport. In contrast, cold acclimated leaves exhibited only 22% and 35% decrease of PSII and PSI photochemistry, respectively, under the same photoinhibitory conditions. Thermoluminescence measurements of PSII revealed that S2/S3QB- recombinations were shifted to lower temperatures in cold acclimated plants. This corresponded to lower activation energy of the S2QB- peak, thus narrowing the gap in the redox potentials between QA and QB electron acceptors. The greater resistance to high light stress will be discussed in terms of possible alternative nonradiative charge recombination pathways in cold acclimated Arabidopsis plants.