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

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 I 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 (Phi(PSII)) was accompanied by an increase in the quantum yield for dissipation by down regulation in PSII (Phi(NPQ)). The concomitant decrease in the quantum yield of non-regulated energy loss in PSII (Phi(NO)) 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 (Q(A)) 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 Phi(PSII), the simultaneous decrease of Phi(NPQ), and the concomitant decrease of Phi(NO). Yet, ETR began to increase, as well as the fraction of open PSII reaction centers. (C) 2014 Elsevier Inc. All rights reserved.