| Cucumber(Cucunis sativus L.)Fusarium wilt,commonly known as the dead seedling disease,stem rot disease,wilting disease,is one of the major diseases in cucumber production,significantly limiting the yield and quality of cucumber.Cucumber Fusarium wilt is a kind of serious fungal soil borne disease,it was caused by the infection of Deuteromycotina Fusarium oxysporum which was a type of cucumber specialization(FOC,Fusarium oxysporum f.sp.Cucumerinum).At present,the research process for cucumber wilt disease pathogenesis is very slow.There are two views about the pathogenic mechanism at home and abroad.One is the plugging of xylem theory,which suggests that the vascular bundle system of pathogen infected plants are plugged by the growth of hypha sclerotia and the secretion of callose,gum,etc..These substances can impair nutrients and water transport,which results in chlorosis caused by nutrient deficiency and aboveground wilting caused by water deficiency,eventually withering death.Another is systemic toxicity theory,which argues that pathogenic fungus after engraftment in plants,can produce toxin(fusaric acid).The toxins can damage the plant cell membrane system,thus affecting the normal physiological metabolism,losing defense function,decreasing root activity,and inactivating the resistance related enzyme,eventually led to the death of plants.In this theis,hydroponic experiments supplied with different nitrogen forms were conducted in a greenhouse.The effects of FOC pathogen infection and low concentration fusaric acid toxin treatment on basic physiological and biochemical indexes,such as the cucumber plant water uptake,temperature characteristics and toxin action,were investigated.On the basis of previous research results and the two kinds of theory of pathogenic mechanism,we continued to study that how two different N forms,ammonium nitrogen and nitrate nitrogen,regulated the cucumber fusarium wilt.The water absorption relations,ammonium and nitrate nutrients metabolism,carbon and nitrogen metabolism,resistant enzyme activity,amino acid and organic acid metabolism,and toxin response ability to fusaric acid were analyzed when the cucumber plants were infected by FOC pathogen and treated by fusaric acid.We further discussed the mechanism that the plants supplied with ammonium were susceptible to disease and the plants supplied with nitrate were resistant to disease,and in order to provide certain theory foundation for prevention and control of cucumber wilting disease.The major results are list as follows:1.FOC pathogen and fusaric acid significantly decreased the biomass and water content of each part of the cucumber plants under ammonium nitrogen supplied condition,but had little influence on the nitrate nitrogen supplied plants.The water absorption of ammonium supplied plant was reduced significantly after the FOC pathogen infection,the reduction extent was increased with the increasing days of infection,while,there was little effect on the nitrate nitrogen supplied plants.The fusaric acid had similar effect on the water absorption as the pathogen infection,but had much more pronounced.The leaf temperature of ammonium supplied plants was increased after pathogen infection or fusaric acid treatment,and the response to fusaric acid was more obvious.Neither pathogen infection nor fusaric acid treatment had effect on leaf temperature of nitrate supplied plants.Thease results suggested that nitrate supplied plants had better resistance to pathogen or fusaric acid than ammonium supplied plants,and the fusaric acid had the stronger pathogenicity.2.The ammonium nitrogen uptake ability of ammonium supplied plants was significantly deceased after the FOC pathogen infection and fusaric acid treatment,the reduction extent was increased with the increasing days of infection,and the fusaric acid had much more pronounced than the pathogen infection.While,the pathogen infection had little effect on the nitrate uptake of the nitrate supplied plants,and the fusaric acid reduced the nitrate uptake.Compared with the ammonium supplied plants,the nutrient uptake of nitrate supplied plants was much higher under fusaric acid treatment.Moreover,FOC pathogen infection and fusaric acid significantly increased the ammonium transport rate and the root ammonium content of ammonium supplied plants,but had no effect on the nitrate supplied plants.The activity of CAT,POD,PPO and SOD in root of ammonium supplied plants was significantly increased,while there was no change in nitrate supplied plants.3.Under the non-inoculated condition,the soluble sugar content and soluble protein content in root of ammonium supplied plants were much higher than that in nitrate supplied plants,but there was no differene in soluble starch content between the two forms nitrogen supplied plants.The FOC pathogen infection and fusaric acid reduced the soluble sugar and soluble starch content,and increased the soluble protein content and the sugar content in phloem sap of ammonium supplied plants,which had no effect on the nitrate supplied plants.Glutamine synthetase(GS)is the key enzyme in the process of N assimilation,Through the determination of the enzyme activity,we expected to reveal the resistant mechanism to pathogen on enzyme levels.The results showed that the GS activity was significantly decreased in the root of ammonium supplied plants after the FOC pathogen infection and fusaric acid treatment,which was not changed in nitrate supplied plants.The carbon nitrogen ratio in leaf of ammonium supplied plants were much higher than that of nitrate supplied plants.4.Using high performance liquid chromatograph(HPLC)to identify the organic acid content of each part of the cucumber plants,we found that under the non-inoculated condition the contents of oxalic acid,malic acid,citric acid,succinic acid and fumaric acid in leaf and root of ammonium supplied plants were significantly lower than that of nitrate supplied plants.The FOC pathogen infection increased all kinds of organic acid content in leaves of ammonium supplied plants,except for the oxalic acid,and increased the succinic acid content and decreased the citric acid content in roots.The fusaric acid had the similar effects on the organic acid content in leaf and root of ammonium supplied plants.Either FOC pathogen infection or fusaric acid treatment had no influence on the organic acid content in leaf and root of nitrate supplied plants.5.The content of 17 amino acids in each part of cucumber plants was identified with automatic amino acid analyzer Biochrom30.The results showed that total free amino acid content in root of ammonium supplied plants was significantly higher than that of nitrate supplied plants.The FOC pathogen infection and fusaric acid significantly increased the total free amino acid content in root of both forms nitrogen supplied plants.Further detailed analysis found that under the non-inoculated condition,all kinds of amino acid contents in root of ammonium supplied plants were much higher than that of nitrate supplied plants,except for the cysteine and tyrosine.Compared with non-inoculated plants,after the pathogen infection,the ammonium supplied plants had an increase in content of all kinds of amino acid in roots,some kinds of amino acid contents were increased in the nitrate supplied plants root,and others remained unchanged.After the fusaric acid treatment,the variation trends of amino acid content of both forms nitrogen supplied plants were similar with the pathogen infection plants. |
PDF Full Text Request