The Research On Main Continuous Cropping Obstacles In Nicotiana Tobacco L.

Serious continuous-cropping obstacle in tobacco crop, which resulted in reducing tobacco leaves yield and lowering the leaves quality, has observed for years in Hezhou, Guangxi is serious. Overcome of the obstacle is beneficial to the sustainable development of tobacco agriculture. Understanding the causes for tobacco continuous cropping obstacle is the precondition to solve continuous cropping obstacle problem. Therefore, in this research, exudates from tobacco roots were separated into different components and the bio-toxicity of the components was compared. The phenolic acids secreted from tobacco roots and the relationship between phenolic acids and continuous cropping obstacle were further studied.The main results are listed as follows:(1) Tobacco continuous cropping obstacle generally existed in all of the tobacco planting areas in Hezhou. Under the continuous cropping conditions, the tobacco was short and the old period came in advance.(2) The extract of decomposed roots collected in the mature stage was applied to tobacco crop. As a result of the application, the growth of tobacco was promoted. The value of plant height, leaf number, maximum leaf area, aboveground fresh weight and leaf weight were1.71,1.25,1.55,1.94and1.76times as CK respectively. These results indicate tobacco root extract is not the main continuous cropping obstacle of tobacco.(3) The concentrate solution of tobacco root exudates was added into the nutrient solution. After fourteen days the tobacco growth condition became poor. The value of the plant height, maximum leaf area, leaf number, were35.29%、29.90%、71.43%as CK respectively. The seed germination and radicle length were inhibited by root exudates too. These results indicate tobacco root exudates have toxic effect on the later cropping tobacco.(4) The root exudates were separated into different components by C18column, silica gel column, anion resin and cationic resin. The components which were adsorbated by C18column, not adsorbated by silica gel column and adsorbated by the anion resin had significant inhibition on germination rates, radicle length and the growth of leaves. And the inhibition caused by the component adsorbated by C18column was most obvious. Meanwhile, the biotoxicity of root exudates was disappear completely when the component adsorbated by C18column was removed. The radicle length of the seeds soaked in the root exudates without the component adsorbated by C18column is close to CK. It is thus clear the component adsorbated by C18column is the main toxicant in root exudates.(5) When ferulic acid aqueous solution flowed into the C18column,82.44±0.56%of it was adsorbated by C18column. There were some phenolic acids in tobacco root exudates, such as p-hydroxybenzoic acid, vanillic acid, ferulic acid, benzoic acid, cinnamic acid. And when the seeds soaked in the five kinds of phenolic acids (100μg/mL), root growth was inhibited in different extents. However, the main phenolic acid sereted from root exudates was ferulic acid. Its content in the root exudates was6.8or365.0times as other phenolic acids. Meanwhile, a large number of the ferulic acid existed in the tobacco planting soil, the content was4.45times as rice planting soil. Ferulic acid has obvious biological toxicity. When seeds soaked with ferulic acid, the maximum and minimum value of radicle length was84.5%and16.5%as CK respectively. These results indicate ferulic acid may be the main tobacco continuous cropping obstacles in tobacco root exudates.(6) The biological toxicity of root exudates supplied with extract solution of microbial fertilizer for37days was reduced significantly.The radicle length ranged from2.61±0.32mm to6.63±0.48mm. The BIO microbial fertilizer water solution was added into the phenolic acids solution, and the mixed solution was cultured. After37days, p-hydroxybenzoic acid, vanillic acid, benzoic acid and cinnamic acid were decomposed completely and the degradation rate of ferulic acid was96.6%. These results implied applying microbial fertilizer can reduce the toxicity of root exudates and decompose the phenolic acids. So, applying microbial fertilizer may be the effective measurfe to reduce tobacco continuous cropping obstacle.(7) The biological toxicity strength in sweet potato root exudates was near to that in tobacco root exudates. The radicle length RI value of the seeds soaked in sweet potato root exudates (the carbon content was20μmol/mL) was-60.0%, which was close to the RI value(-53.8%) of that soaked in tobacco root exudates. So, under the tobacco-potato-tobacco cropping patterns, the sweet potato root exudates may inhibit the growth of tobacco in the later crop.In conclusion, the ferulic acid may be the main continuous cropping obstacles in tobacco root exudates, and the microbial fertilizer is beneficial for disintegrating the toxic substance in root exudates.