Studies On Mechanism Of Continuous Angelica Sinensis Cropping Obstacle And Its Preparatory Bioremediation

Angelica sisnensis is a perennial herb of Umbelliferae,and has been cultivated for more than 1700 years.Its root is used frequently as a crude drug in traditional Chinese medicine.However,the contradiction between limited suitable areas and market supply all year long resulted in the fact that continuous cropping obstacles become common and serious more and more,but its reason is not clear up to now.To explore the origin of its occurrence mechanisms,field and pot experiment were conducted in Min county of Gansu province during 2006-2008.Regarding allelopathic substances and microorganisms in rhizosphere soil as the ligament of many factors of continuous cropping obstacles,we first detected the variation of rhizosphere microbial population in response to continuous A.sinensis cropping,then collected and identified the autotoxic effect of plant and soil arounding A.sinensis.In addition,we also trialed the effect of allelochemicals in continuous cropping soil on seedlings growth,rhizosphere microbial and Ditylenchus destructor,and the interactions of allelopathy and microbial from continuous cropping soil on A.sinensis growth.At last we trialed the effect of intercropping on A.sinensis growth and the microflora in rhizosphere soil of A.sinensis meanwhile try to supply the scientific bases for the alleviation or elimination of A.sinensis continuous cropping obstacles.The main results are as follows:1.Through decreasing the activity of protective enzymes and their ability of cleaning free radicle, continuous cropping made lots of free radicle remain in plant so that induced membrane lipid peroxidization,electrolytic leakage became heavier,content of proline and soluble sugar increased.The external manifestation of this influences were that plant growth was inhibited,content of photosynthetic pigments decreased,so did the intensity of photosynthesis and respiration,content of dry mass.In continuous A.sinensis cropping system,SOD,POD and CAT activity decreased by 29.13%,10.87%and 43.83%respectively compared with those in non-replant A.sinensis;The content of proline,soluble sugar and MDA increased by 121.95%,29.20%and 16.00%;The content of chlorophyll a,chlorophyll b and photosynthetic rate decreased by 39.78%,46.90%and 34.23%respectively;The yield,essential oil content and extract content decreased by 29.52%,28.42%and 58.82%respectively,and the incidence of D.destructor increased by 2.52 fold.Both of the decrease or increase extent showed significance at P＜0.05.2.Plate count analysis revealed a reduction trend in the number of culturable microorganism in rhizosphere as A.sinensis cropping continued.Bacteria and actinomyces had an apparent decrease extent in CFU number,while fungi had an apparent increase extent.In the second cropping season,the number of culturable bacteria,actinomyces,and diversity index in rhizosphere soil were decreased by 37.22%, 43.68%and 32.09%respectively compared with those in non-replant A.sinensis soil,while the number of fungi was increased by 106.64%.An obvious decrease tread was also found in the number of aerobic cellulose-decomposing bacteria and inorganic phosphorus-solubilizing bacteria,while great increase in ammonifying bacteria.The variation of organic phosphorus-solubilizing bacteria and kalium-solubilizing bacteria changed with the change of growth stage.Of the essential soil enzymes,urease and neutral phosphatase activity significantly declined in the continuous cropping,but the catalase and polyphenol oxidase activity rise little.3.All of the aqueous extracts from A.sinensis tissues and rhizosphere soil had inhibitory effects on A.sinensis germination and seedling growth,and this inhibitory effect generally increased with increased extract concentration at a certain ranges of extract concentrations.Under lower concentration,the inhibition wasn't shown,indeed,the promotion was exhibited.With the concentration increasing,the inhibition was enhanced.Inhibitory effects of plant above ground extracts were more evident than that of root extracts.Inhibitory effects were greater for all extracts at the rootstock thickening than the seedling stage.It may be concluded that soil previously planted with A.sinensis as well as A.sinensis plants themselves contain allelopathic substances.They are toxic to the growth of A.sinensis and other plants. The chemical compounds may play some part in growth inhibition due to the continuous cropping of A. sinensis.4.Soil sterilization could improve growth of A.sinensis plants to greater extent and for example, plant height,yield,essential oil content and extract content of A.sinensis grown in sterilized soil were significantly higher than those in non-sterilized soil.SOD and POD activities in the leaves of A.sinensis plants grown in sterilized soil were higher than those in non-sterilized soil,while CAT activity and MDA content were significantly lower.The number of bacteria in sterilized soil was increased by 173.82% compared with that in non-sterilized soil,while the number of actinomyces and fungi were decreased by 63.23%and 37.87%respectively.The number,abundance,community diversity,evenness and dominance concentration of some functional groups were changed greatly in sterilized soil.The abundance of aerobic cellulose-decomposing bacteria,inorganic phosphorus-solubilizing bacteria and kalium-solubilizing bacteria in sterilized soil had changed to tends to those in rotation system.The diversity index in sterilized soil decreased by 31.04%at seedling stage,but increased by 65.51%and 27.51%at rootstock thickening and harvest stage compared with those in non-sterilized soil.The change range of diversity index and population of functional groups in rhizosphere soils at the rootstock thickening was higher than at seedling and harvest stages.The incidence of D.destructor of A.sinensis grown in sterilized soil decreased by 75.01%compared with that in non-sterilized soil.Although there are more improving in factors affected growth of A.sinensis,the A.sinensis continuous cropping obstacle had not been eliminated completely by soil sterilization.5.The occurrence and aggravation of A.sinensis continuous cropping obstacle was resulted from both allelopathy and soil biota factors.The growth,yield and quality of A.sinensis plants were affected by the allelopathy and the soil biota which in continuous cropping soil through the change of plant physiological and biochemical characteristics and soil microenvironment of plant growth.The dry weight of A.sinensis was significantly depressed in the treatments other than in the control,and the inhibitory effect of allelopathy on growth of A.sinensis was higher than that of deleterious microorganisms in early growth period,whereas the inhibitory effect showed a contrary tendency in late growth period.Compared to the control,for the inhibition percentage of biomass,the proportion of T₁ treatment and T₂ treatment was 61.90%and 38.10%for shoot dry weight,80.00%and 20.00%for root dry weight at seedling stage;The proportion of T₁ treatment and T₂ treatment was 58.97%and 41.03%for shoot dry weight,41.02%and 58.98%for root dry weight at rootstock thickening.The percentage was 48.29%and 51.71%for yield,21.74%and 78.26%for essential oil content,47.16%and 2.84%for extract content respectively.For the protective enzymatic activity,compared to the control,SOD,POD and CAT activity were increased by 12.50%,60.47%and 6.43%in the leaves of A.sinensis plants as the T₁ treatment,MDA content was increased by 14.67%.SOD and CAT activity were decreased by 20.59%and 28.22%,while POD activity and MDA content were increased by 38.22%and 32.00%as the T₂ treatment,and the differences were significant all.The number,abundance,community diversity,evenness and dominance concentration of some functional proups also were changed markedly in the effect of allelopathic compounds and deleterious microorganisms in replant soil.Compared with the control,allelopathic compounds and deleterious microorganisms in the replant soil seemed to stimulate the reproduction of rhizosphere soil fungi,and restrain the reproduction of rhizosphere soil actinomyces,ammonifying bacteria,aerobic cellulosedecomposing bacteria,organic phosphorus-solubilizing bacteria and kalium-solubilizing bacteria at rootstock thickening and harvest stage.T₁ and T₂ decreased the population of bacteria and increased that of inorganic phosphorus-solubilizing bacteria significantly at rootstock thickening,whereas they increased the population of bacteria and decreased that of inorganic phosphorus-solubilizing bacteria significantly at the harvest stage.The abundance of aerobic cellulose-decomposing bacteria and kalium-solubilizing bacteria in T₁ and T₂ A.sinensis rhizosphere soils changed at the rootstock thickening and harvest stage. The total population of functional groups,community diversity and evenness decreased and dominance concentration increased in T₁ and T₂ A.sinensis rhizosphere soils compared with the control.The change range of diversity index and population of functional groups in rhizosphere soils at the rootstock thickening was higher than at the harvest stage,compared to the control,The total population and community diversity of functional groups were decreased by 1.92%and 21.97%in T₁ and 25.13%, 43.88%in T₂ at the rootstock thickening.6.The microflora in rhizosphere soil of A.sinensis was significantly improved in intercropping system by restraining pathogen growth and enhancing community diversity and evenness of some functional proups.Which alleviated the A.sinensis continuous cropping problem by restraining diseases and regulating the protective enzymes system.A.sinensis plant growth status was improved in intercropping system.Compared with the monocropping system,plant height,shoot dry weight and root dry weight were increased by 12.34%, 36.92%and 9.15%respectively.SOD,POD and CAT activity were increased by 80.00%,243.19%and 37.13%respectively,while MDA content was decreased by 11.67%.Lower D.destructor incidence and higher yield and quality of A.sinensis were observed in intercropping system compared to those in monocropping system.The population of bacteria,aerobic cellulose-decomposing bacteria,organic phosphorus-solubilizing bacteria,inorganic phosphorus-solubilizing bacteria and kalium-solubilizing bacteria in rhizosphere soil could be promoted in intercropping in vigorous growth stage.The number of bacteria,aerobic cellulose-decomposing bacteria,organic phosphorus-solubilizing bacteria and inorganic phosphorussolubilizing bacteria in intercropping were enhanced by 53.52%,23.19%,103.44%and 49.46% respectively compared with the monocropping.In harvest stage,the number of bacteria,fungi and ammonifying bacteria significantly decreased by 31.71%,22.05%and 47.26%,while the population of inorganic phosphorus-solubilizing bacteria,kalium-solubilizing bacteria increased by 20.05%and 75.59%. The abundance of aerobic cellulose-decomposing bacteria,organic phosphorus-solubilizing bacteria and kalium-solubilizing bacteria in intercropping A.sinensis rhizosphere soils changed at the rootstock thickening.Although the total population of functional groups in intercropping A.sinensis rhizosphere soils was lower than in monocroping system,community diversity and evenness increased and dominance concentration decreased.The change range of diversity index and population of functional groups in intercropping A.sinensis rhizosphere soils at the rootstock thickening was higher than at the seedling and harvest stage.The diversity index was significantly increased by 11.25%,80.81%and 44.97% respectively at the seedling,rootstock thickening and harvest stage in intercropping system compared with the control.7.Based on the experimental results described above,we concluded the allelopathic chemicals and deleterious microorganisms affected A.sinensis growth and cause continuous A.sinensis cropping problem. Why was the A.sinensis growth inhibited severely when it was continuously cropped in the same field? It is supposed that after the whole growth stage in the first year,the rhizosphere ecology was changed so greatly that it was no longer suitable for A.sinensis growth.One result was that a lot of allelochemicals were produced directly or indirectly in four ways including leaching from the shoot,shoot and root residues,root exudates and metabolites of microorganisms in rhizosphere,which will take effect immediately after seedling was cultivated in the following year.The other result indicated that the quality and quantity of microorganisms in the rhizosphere were changed greatly,including the deleterious microorganism of A.sinensis.In the second year,after cultivating,the allelopathic substances would be absorbed by A.sinensis.Some of the deleterious microorganisms would also take action at this time. Therefore,in the early stage the A.sinensis development would be retarded,caused low protective enzyme activity and shoot biomass.The plant developed in the condition will have weaker resistance and is easier for pathogens to infect than normal A.sinensis in shis stage.At the following stage,more pathogens will also infect A.sinensis and develop.During the process,they will cause the decline of yield and quality through change the physiological character.In a word,all the factors collectively caused the continuous A.sinensis cropping problem.In intercropping system,continuous A.sinensis cropping obstacle was significantly alleviated by changing the microflora in rhizosphere soil and regulating the protective enzymes system of A.sinensis.