The effects of inhibition of mitochondrial alternative oxidase (AOX) respiratory pathway on photosynthetic apparatus in Rumex K-1 leaves were studied. Under high irradiance, the inhibition of AOX pathway caused over-reduction of photosystem (PS) 2 acceptor side, a decrease in the energy transfer in the PS 2 units, damage of donor side of PS 2 and decrease in pool size of electron acceptors. The inhibition of AOX pathway also decreased photosynthetic performance index (PI ABS ), actual photochemical efficiency (Φ PS2 ), photochemical quenching (qP) and photosynthetic O 2 evolution rate. The results demonstrate that mitochondrial AOX pathway plays a vital role in photoprotection of photosynthetic apparatus.Additional key words: chlorophyll a fluorescence, high irradiance, JIP-test, photosystem 2.

Oxybenzone (OBZ), a widely used sunscreen additive, has damaging effects on corals and plants. This study demonstrated that OBZ significantly inhibited photosynthesis and growth of cucumber (Cucumis sativus L.) seedlings. Combined analyses of chlorophyll fluorescence and 820-nm reflection suggested that OBZ severely inhibited photosynthetic electron transport (PET) from QA to cytochrome b6f (Cyt b6f) complex, restricting the reduction of the reaction center in PSI. Separate measurements of PSI and PSII PET in chloroplasts indicated that OBZ inhibited the PET of neither PSII nor PSI, but it severely inhibited the whole linear PET. We thus confirmed that the inhibition of PET by OBZ was due to the inhibition of plastoquinones (PQs) and/or the Cyt b6f complex between PSI and PSII. PET is essential for producing ATP and NADPH used in CO2 assimilation. Therefore, limiting the use of sunscreens containing OBZ is significant in protection of aquatic and terrestrial ecosystems and ensuring the safety of agricultural production.