Comparative effects of four herbicides on non-photochemical fluorescence quenching in Lemna minor

Aquatic ecosystems are exposed to an increasing contamination of pesticides such as herbicides through water runoff. The pulse-amplitude-modulation (PAM) fluorometric method, as a sensitive and rapid method, was used to evaluate toxic effect of these pollutants in Lemna minor. Four herbicides (paraquat, norflurazon, flazasulfuron and atrazine) often found in outdoor water samples and inducing specific changes in the yield of the in vivo chlorophyll a fluorescence of PSII were selected. These herbicides affected photosynthesis via different ways by: accepting electron from PSI, inhibiting carotenoids and protein biosynthesis or blocking PSII electron transport. Data revealed that photosynthetic parameters based on fluorescence emission were modified with the increase of herbicide concentration. The toxicity of these compounds was as follows (from greatest to least): paraquat>norflurazon>atrazine>flazasulfuron. Growth rate and photosynthetic pigments analysis confirmed the results obtained with PAM fluorometry. We found that among the fluorescence parameters the non-photochemical quenching was the most appropriate indicator for the effects of herbicides. The components of non-photochemical quenching were then resolved by examination of relaxation kinetics of quenching upon DCMU addition and light saturation pulse in the entire plant. Three kinetically distinct phases were observed which have previously been identified in thylakoids (Horton and Hague, 1988) as being due to energy-state quenching (qE), state transition (qT) and photoinhibition (qI). These examined NPQ components showed different levels of sensitivity to the effect of herbicide. It was found that: (i) qE was the major NPQ component; (ii) qE was affected by all the selected herbicides; (iii) qT was significantly modified by paraquat and atrazine; (iv) qI was affected by norflurazon and flazasulfuron. We interpreted these results by the pesticide mode-of-action. This study shows that the use of NPQ as a biomarker may be appropriate in laboratory and field herbicide bioassays. Moreover, application of non-photochemical quenching analysis may allow a better understanding of the mechanism of herbicide action.