Studies On The Autotoxic Substances Of Tobacco And The Effects Of Autotoxic Substances On The Rhizospheric Microbiology

To study the autotoxic substances of the tobacco rhizospheric soil, some experiments such as evaluation on the allelopathic effect of the rhizospheric soil in cropping sequence for different years, separation and identification of substances extracted from rhizospheric soil under flue-cure tobacco were conducted in this study. At the same time, another experiment ,in which tobacco soil substances (collected by Macroporous resin of D101) and hydroxybenzoic acid were added to the soil under tobacco planted in the pot, was conducted to study how the exogenous substances including tobacco soil substances and hydroxybenzoic acid affect the tobacco and the change in the microorganism by T-RFLP to further confirm the autotoxic effect of hydroxybenzoic acid on tobacco and procedure for the continuous cropping obstacle of tobacco caused by the autotochemicals and microorganism . The results were summarized as follows:The results of the evaluation on allelopathic effect of tobacco rhizosphere soil showed that there were some acidic polarity autotoxic compounds in the soil which could inhibit the growth of the tobacco and lettuce at high concentrations and promoted it at low concentrations. The analysis of GC-MS indicated that the tobacco rhizospheric soil contained lots of organic acids which were 50 times to the control and some compounds such as benzoic acid ,hydroxybenzoic acid ,vanillic acid and vanillin reported as allelochemicals were detected in the soil samples. The result of bioassay showed that benzoic acid, hydroxybenzoic acids had higher inhibitory effect on tobacco than vanillic acid and vanillin. The results of the exogenous-adding-experiment showed that both tobacco soil substances and hydroxybenzoic acid had influence on the growth of tobacco directly, including inhibiting the elongation of roots and decreasing the lateral roots, chlorophyll contents, photosynthesis intensity and potassium level as well as destroying the protective enzymes in tobacco. The correlative analysis of microorganism mediated by exogenous-adding-substances with autotoxic effect showed that there were 42 negatively and significantly related common T-RFs and 10 positively related common T-RFs. There were more positively correlative T-RFs in the soil mediated by exogenous-adding-substances and many pathogens such as Pirellula staleyi, Rickettsia prowazekii, anaerobes such as Clostridium bifermentans,Clostridium argentinense were found in them. The soil of CK had more negative T-RFs and many microorganism of soil nutrition cycle such as Porphyrobacter tepidarius related to nitrogen cycle, Desulfotomaculum geothermicum,Ectothiorhodospira marismortui related to sulfur cycle were found in the negative T-RFs, at the same time, other microorganism such as Shewanella related to improvement of soil texture, Bacillus pumilus,Bacillus thuringiensis related to restrict the pathogen were found also.Accordingly, author strongly suggested that the soil under tobacco contain autotoxic substances which could inhibit the growth of the tobacco and destroy the balance of soil micro-system, and then led to the continuous cropping obstacle of tobacco. The procedure of the continuous cropping obstacle of tobacco caused by autotoxic substances and microorganism resulted by autotoxic substances was confirmed by the exogenous-adding-experiment and the experiment also confirmed that phenolic acid played an important role in continuous cropping obstacle of tobacco.