Photoprotective mechanism of the non-target organism Arabidopsis thaliana to paraquat exposure

• We propose the photoprotective mechanism of Arabidopsis thaliana to paraquat.
• Arabidopsis is protected from paraquat by increasing the non-photochemical quenching.
• The protection mechanism decreased the electron transport rate.
• This photoprotective mechanism was efficient since it kept reaction centers open.
• Chlorophyll fluorescence imaging revealed spatiotemporal heterogeneity of Pq effects.

The response of photosystem II (PSII), of the non-target organism Arabidopsis thaliana, to paraquat (Pq) exposure was studied by chlorophyll fluorescence imaging. Effects of 1 mM Pq application by spray on A. thaliana leaves were monitored as soon as 20 min after application at the deposit areas of the droplets. A decline in the effective quantum yield of photochemical energy conversion in PSII (ΦPSII) was accompanied by an increase in the quantum yield for dissipation by down regulation in PSII (ΦNPQ). The concomitant decrease in the quantum yield of non-regulated energy loss in PSII (ΦNO) pointed out a quick effective photoprotection mechanism to Pq exposure. Even 1 h after Pq spray, when the maximum Pq effect was observed, the decrease of electron transport rate (ETR) and the increase in non-photochemical quenching (NPQ) resulted to maintain almost the same redox state of quinone A (QA) as control plants. Thus, maximal photoprotection was achieved since NPQ was regulated in such a way that PSII reaction centers remained open. Arabidopsis plants were protected from Pq exposure, by increasing NPQ that dissipates light energy and decreases the efficiency of photochemical reactions of photosynthesis (down regulation of PSII) via the “water–water cycle”. PSII photochemistry began to recover 4 h after Pq exposure, and this was evident from the increase of ΦPSII, the simultaneous decrease of ΦNPQ, and the concomitant decrease of ΦNO. Yet, ETR began to increase, as well as the fraction of open PSII reaction centers.

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