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A field experiment was carried out to compare the responses to ozone (O3) in two common herbaceous plant species,Plantago major L.and Sonchus oleraceus L.,by building open-top growth chambers in situ to simulate O3 stress (+O3,85 ± 5 ppb,9 hr/day for 30 days) in a lowland habitat in Inner Mongolia,Northem China.Responses to O3 of gas exchange,chlorophyll a fluorescence,leaf pigment content,antioxidant capability,soluble protein content,membrane lipid peroxidation and dark respiration (Rd) were analyzed.Results showed that elevated O3 exposure significantly reduced the light-saturated net photosynthesis (PNsat),stomatal conductance (gs) and transpiration rate (E) in both species.Although non-significant interactive effect between species and O3 on PNsat was analyzed,the reduction in PNsat in S.oleraceus might be due primarily to the higher fraction of close PSII reaction centers and impaired activities of plant mesophyll cells as evidences by decreased maximum efficiency of PSII photochemistry after dark adapted state (Fv/Fm) and unchanged intercellular CO2 concentration (Ci).Besides,biochemical analysis showed that S.oleraceus had lower antioxidant ability compared to P.major.As a result,S.oleraceus was damaged to the larger extent in terms of lipid peroxidation and visible O3 injury,indicating that S.oleraceus was more sensitive to O3 than P.major.Our results indicated that wild herbaceous plant species growing in a lowland habitat in sandy grassland were sensitive to O3 stress and S.oleraceus can be considered as one of the bio-indicators for high O3concentration in semi-arid grassland of north China.