Abstract: ObjectiveEffects of decomposed liquids of Chenopodium ambrosioides L.on photosynthesis characteris-tics and guard cells viabilities were studied to provide reference for revealing invasion mechanism of C. ambrosioides.MethodThe changes of plant height,fresh weight and dry weight of aboveground parts,leaf areas,actual photochemical efficiency(Φ PsⅡ)and maximal photochemical efficiency(Fv/Fm)of broad bean seedlings treated by C.ambrosioides de-composed liquids were analyzed by using the epidermal strips bioassay and pot culture experiment.Guard cells viabilities and abnormal rate of nucleus were also detected.ResultAfter treated by C.ambrosioides decomposed liquids,leaf areas of broad bean seedlings did not change greatly,but above-ground biomass and plant height significantly decreased(P<0.05,the same below),and was time-and concentration-dependent.Chlorophyll fluorescence parameters including Fv/Fmand ΦPs of broad bean leaves changed at various extents,but did not reach significant level(P>0.05)compared with control (CK). Under the treament of C. ambrosioides decomposed liquids,guard cell viabilities decreased,abnormal nucleus features including malposition,pyknotic,stretch and degradation occurred.The cell mortality rates and aberration rates significantly increased as the decomposed liquid concentration increased.ConclusionC. ambrosioides decomposed liquids have slight influence on photoreaction of broad bean seedlings. But strong cytotoxicity lead to aberration of leaf stomatal guard cells,decrease of cell viabilities,which inhibits stomatal movement of the seedlings,hinders CO2ab-sorption ability,decreases photosynthetic efficiency,and eventually influencing seeling growth.