Flue-cured tobacco (Nicotiana tobacco) is an important economic crop in the southwest of China. Due to limited cultivated lands, continuous tobacco cropping is quite common in tobacco cultivation practices, which resulted in yield decline and quality deterioration. Scientists believe the accumulation of root exudates could be one of important reasons for obstructions ad problems in continuous cropping. But the components and function of exudates from flue-curved tobacco root are not clear and necessary to clarify. Furthermore, bio-measurements could be one of effective selections for overcoming the problems produced by continuous cropping. AM fungi could promote nutrition absorption by some crops and plant resistances to stress environments. It was found that synbioints could be formed between flue-curved tobacco and AM fungi in natural conditions but there are not reports of tobacco seedlings grown in trays floating in nutrient solutions. Therefore, we firstly extracted and fractionated exudates of flue-curved tobacco roots, then identified by high pressure liquid chromatograph (HPLC), and finally investigate their inhibition on seed germination in first experiment. Secondly, we studied the influences of AM inoculation on the growth and nutrition of flue-curved tobacco seedlings in order to select suitable fungi strains and make up nutrient solution suitable for culturing mycorrhizal tobacco seedlings as well as to supply as a method to solve problems produced by continuous cropping. Following results are obtained.In this test, flue-cured tobacco root exudates were absorbed and departed with 732-type cation exchange resin and 717-type anion exchange resin. The inhibition of root exudates to seed germination changes in the sequence: anion resin absorbed > unabsorbed by both anion and cation resin > cation resin absorbedRetention time of root exudates solution following passing through anion exchange resin was 2.03 min in chromatograph, quite closing to that of methyl benzol methanoic acid (2.05 min). It is reasonable to infer that there were methyl benzol methanoic acid or similar compounds in tobacco root exudates according to the principle of same compound with same retention time in chromatograph.The infection rates showed BEG-141>BEG-193>BEG-168. Nutrient supplies influenced the infection of AM fungi to tobacco roots of seedlings cultured in trays floating in nutrient solutions and the rate of infection was increased under low nutrient conditions. The results suggest that mycorrhizal infections could be improved by nutrient supplies including the time and the quantity and so forth.The biomass of mycorrhizal tobacco seedlings, much higher than non-mycorrhizal ones, changed in the sequence: BEG-141>BEG-168>BEG-193. The amount and pattern of nutrient supplies influenced significantly the growth of mycorrhizal seedlings. The biomass of mycorrhizal seedlings increased as the amount and times of fertilization increased. On the other hand, inoculation of suitable AM fungi decreased the amount of fertilizer used but increased fertilizer efficiency and promoted root growth including length and mass. BEG-141 showed better ability to promote root growth than BEG-168, which was similar to the role of mycorrhizal fungi in total biomass stimulation.Mycoorhzial fungi promoted significantly the absorption of nitrogen, phosphorus and potassium by tobacco seedlings in low nutrient conditions, particularly in low phosphorus, and the amount of nutrient absorption showed BEG-141>BEG-168>BEG-193>control (non-mycorrhizal inoculation). However, the nutrient absorption showed no differences among fungi species or strains and between mycorrhzail and non-mycorrhzial seedlings. Mycorrhzial inoculation also increased the neutral phosphatase activities in seedling culture mediums and significant differences were observed among fungus species, which showed BEG-168>BEG-193>control (without mycorrhizal fungi inoculated).Rational nutrient prescription is the precondition of successfully cultivating mycorrhizal seedlings. In general, suitable nitrogen and potassium and relative low phosphorus could be beneficial to mycorrhzial infection and seedling health. Simultaneously, light base fertilizer, enough dresser, rational amount and time of fertilizer supplies should be adopted in culture mycorrhizal seedlings. Application of nutrient prescription (N_1P_2K_2) obtained in present experiment could decrease 30% of nitrogen, 65% of phosphorus and 15% of potassium on the base of original standard fertilization and produce healthy seedlings with strong root systems and large biomass.In general, AM fungus inoculation combined with suitable nutrient supplies could promote the growth of tobacco seedlings, increase nutrient absorption and fertilizer efficiencies seedling in trays floating in nutrient solution. It is theoretically and practically meaningful to grow mycorrhizal tobacco seedlings in field as one of measurements to overcome the problems produced by continuous tobacco cropping

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