In order to assess its response to the herbicide, sethoxydim (SEY), seedlings of two foxtail millet (Setaria italica) hybrids were exposed to 0.75, 1.5, 3, and 6 L(SEY active ingredient, ai) ha -1 for 7 and 15 d. Our results showed that SEY reduced photosynthesis and oxidative stress in the hybrid millet (Zhangza) at the dosage below 1.5 L(ai) ha -1 (i.e., recommended dosage), whereas it caused death of Jingu 21 at all treatment dosages. In addition, we further explored the effect of SEY on PSI

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... and PSII; the hybrid millet showed a greater tolerance to SEY and also the ability to recover. In conclusion, the hybrid millet seems to possess certain photosynthetic protection mechanisms which could reduce or eliminate the herbicide stress by increasing nonphotochemical quenching for dissipating excessive light energy under SEY-induced oxidative stress. Additional key words: chlorophyll fluorescence; P700 parameters; reactive oxygen species; spring hybrid millet. Abbreviations: ai -active ingredient; Chl -chlorophyll; ETRI -PSI electron transport rate; ETRII -PSII electron transport rate; Fv/Fm -maximum photochemical efficiency of PSII; Fv/F0 -potential activity of PSII; FM -fresh mass; MDA -malondialdehyde; Pm -maximal P700 change; qP -photochemical quenching coefficient; ROS -reactive oxygen species; SEY -sethoxydim; YIphotochemical quantum yield of PSI; YII -PSII effective quantum yield; YNA -quantum yield of nonphotochemical energy dissipation due to accepter side limitation in PSI; YND -quantum yield of nonphotochemical energy dissipation due to donor side limitation in PSI; YNO -quantum yield of nonregulated energy dissipation in PSII; YNPQ -quantum yield of regulated energy dissipation in PSII. Acknowledgments: Conceived and designed the experiments by Y.G. WANG and P.Y. GUO; data analysis: X.E. SONG and Y.Y. WEN; contributed reagents/material/analysis tools: S.Q. DONG; performed experiments and writing the paper by M.J. GUO.