Abstract: Continuous cropping,which is commonly practiced in the tobacco cultivation areas of china,brought about a number of problems on tobacco production,such as growth inhibition,yield decline and quality deterioration.Allelopathy of root exudates is regarded as a reason for these problems in some crops;however,the role of root exudates on tobacco production is still not very clear.We conducted a hydroponics experiment to study the effects of root exudates on the growth and uptake nutrients of tobacco seedling.Root exudates were obtained directly from the culture solution of adult tobacco seedling,and then fractionated into acid soluble,alkaline soluable and neutral groups.Therefore,these fractionated solutions with the un-fractionated root exudates solution were added into the culture solutions of young seedlings with four leaves,respectively.The control treatment was also set up without root exudates component added.The results showed that un-fractionated root exudates inhibited greatly the growth and root activities of the seedlings from day 20,and the effect of root exudates increased dramatically with increasing amount the root exudates added.The added fractionated root exudates,especially in neutral one,depressed root activities. All of these fractionated materials declined significantly NO3-,H2PO4-,K+ uptake.It was observed that the most depressed NO3- and K+ uptake was occurred with the addition of neutral acid fraction,respectively.As a consequence, It was inferred that several chemical inhibitors of growth and nutrient uptake of tobacco seedlings should be contained in root exudates.