By spraying different concentration of CaCl₂,gibberellin (GA₃),salicylic acid (SA) and paclobutrazol (PP₃₃₃),we studied and compared the effects of the pretreatments with four chemical regulators on chilling tolerance and photosynthetic characteristics in tobacco(Nicotiana tabacum L.) seedlings.The results showed that:(1)As compared with the control spraying distilled water,the pretreated seedlings with the four chemical regulators generally had higher dry weight,root vigor,proline content and net photosynthetic rate,and lower malondialdehyde (MDA) content,which totally enhanced the quality of tobacco seedlings.(2)When transferred to chilling stress at 10 ℃ for 20 d,as compared with the control,the pretreated seedlings with the four chemical regulators could maintain higher dry weight,relative growth rate,dry matter accumulation rate,root vigor,proline and chlorophyll content,net photosynthetic rate,intercellular CO₂ concentration,stomatal conductance and transpiration rate,reduced MDA content and lipid perioxidation,which finally enhanced chilling tolerance of the tobacco seedlings.(3)The pretreatments with the four chemical regulators all could increase chilling tolerance in tobacco seedlings to different contents,in general,SA pretreatment displayed the best effect.