| Banana wilt disease was caused by the Fusarium oxysporum f.sp. cubense (FOC), which was a causal agent of vascular wilt and leaf chlorosis of banana plants. This soil-borne disease spreads quickly, and no efficient methods have yet been developed for its thorough control.Currently, the pathogenesis mechanism of Fusarium wilt was attributed to plugging or systemic toxicity, and the argumentation why Foxysporum-caused Fusarium wilt occurs still remains open. The plugging theory suggested that the vessels of infected plants were plugged by fungal hyphae, callose, tylose and gel. These plugging substances could impair water transport, and finally resulted in the water deficiency of infected plants. However, the toxin theory suggested that the phytotoxin produced by pathogens could disturb the water metabolism of the infected plants, alter the membrane permeability and disturb the water balance by uncontrolled water loss from injured cells. With the study depth of wilt disease, the effect of toxin on disease development became a major topic. However, the domestic study about the toxin role was relatively less. The present study was to study the physiological and biochemical processes relative to Fusarium wilt, and finally to provide the theoretical basis for the prevention and control the banana wilt disease.In this thesis, hydroponic experiments were conduced to study the effect of F.oxysporum on water relationships, photosynthetic characters, toxin production and the relative mechanisms. In addition, different nitrogen forms were applied to compare their effect on banana wilt disease. Furthermore, different concentration combinations of iron and boron were also studied to clarify the resistance mechanism. The major results were listed as follows.1. The leaf average temperature (Avg), stomatal conductance (gs), leaf water potential and leaf water content were significantly decreased after FOC infection. However, E/gs ratio of infected plant was higher than that of healthy plants, indicating non-stomatal water loss. In red ink absorpthion experiments, leaves of infected banana plants accumulated a higher amount of ink. Furthermore, the leaf relative conductivity, the temperature frequency distribution and maximum temperature difference were increased afthe pathogen infection. The results showed that leaf water supply was not restricted in infected plant. Leaf membrane was damaged seriously by FOC infection, which could induce uncontrolled water loss from damaged. cells and destroyed the water balance in banana leaf, ultimately resulting in plant wilt.2. The pathogen was not detected in infected leaf. However, the fusaric acid was detected in both pathogen culture medium and disease banana plants. Also, with the increase of disease severity, the fusaric acid content was increased in infected plant. Fusaric acid treatment could induce the leaf stomatal close and temperature increment. Furthermore, compared with control plants, water loss of FA-treated plants was dramatically decreased. Howerer, the increased leaf electrolyte leakage, red ink absorption and E/gs ratio showed the leaf membrane was damaged by fusaric acid. Also, the crude toxin extracted from the pathogen culture medium had the similar pathogenesis mechanism with fusaric acid. Taken together, fusaric acid play a major role in pathogenesis process of pathogen and disturb the water balance of infected plants.3. The leaf relative conductiry and MDA content were dramatically increased in the lower leaves of infected plants, but Fv/Fm of leaf chloroplast was significantly decreased. Furthermore, transmission electron microscopy of the lower infected leaf showed that the leaf membrane and chloroplast were seriously damaged. However, at the same time, the leaf membrane and chloroplast of infected upper leaves were still intact. Through extracting the fusaric acid of different leaf positions of infected plant, it was showed that fusaric acid was only detected in infected lower leaves. The lower leaves was damaged seriously much more than upper leaves after using syringe injecting both microcondia suspension and FA into the upper leaf. Moreover, the water soluble substances of lower leaves could increase the FA production in pathogen culture medium. Taken together, it was showed that FA could firstly accumulate in the lower leaves after pathogen infection and probably inducing the specific symptom of banana wilt disease.4. During the early infection stage, FOC could induce stomata closure and decresed photosynthetic rate. During the later stages of FOC infection, CE and Fv/Fm were dramatically reduced. At the same time, leaf photorespiration rate was started increased. Through measuremt of metabolic intermediates, the results showed that malate and H2O2 were also increased at this infection stage. Furthermore, both phenolic substances and lignin content were dramatically enhanced at this time, which were showed that increased photorespiration rate was closely related with resistance of infected plant. Afer the weak light treatment, the infected plants showing disease symptom was much earlier than mormal light treated plants. It was concluded that pathogen infection could incrase the photorespiration rate of infected plants, which further play a important role in defense of plants.5. Contents of amino acids in leaves were significantly increased whereas they were decreased in roots after the F. oxysporum infection. Through analysis the change of amino acids type, it was showed that the contents of serine, valine, histidine, isoleucine and leucine were changed more significantly in leaves, and serine, glycine, alanine and isoleucine were decreased signicicantly in roots. Throuh HPLC analysis, it was showed that 5 kinds of organic acids were detected in both infected leaves and roots. They were oxalic acid, citric acid, malic acid, succinic acid and fumaric acid, respectively. After the pathogen infection, the organic aicd content were increased in both leaf and root. Furthermore, through analysis the phenolic acids change, it was showed that ferulic acid, cinnamic acid and salicylic acid were dected in banana leaf, while hydroxybenzoic acid and eugenol acid were detected in roots. The results above were showed that both contents and kinds of amino acids, organic acids and phenolic acids could change after the FOC infection, but the interaction among them remains unclear.6. Compared with ammonium treatment, nitrate treatment could dramatically decrease the disease incidence. After the pathogen infection, the biomass, leaf chlorophyll content and net photosynthetic rate were significantly reduced of ammonium treated plants. The number of pathogen in infected nitrate treated banana plants were decreased compared with ammonium treatment. Through measuring the soluble sugar content, it was showed that leaf total sobuble content in both different nitrogen treatement did not differ after the pathogen infection. Howerer, in roots, soluble sugar content of nitrate treated plants was decreased. Meanwhile, afthe the FOC infection, lignin conten did not change in ammonium treatment, but its content in nitrate treatment was increased after pathogen infection. In conclusion, nitrate nutrition is favored by bananan seedlings and superior for banana plants to resist the banana wilt disease. So in field production, it was much better to increase the nitrate fertilizer input in banana seedlings to reduce the occurrence of banana wilt.7. High concentration of iron and boron combinations could reduce the disease index of banana wilt disease, the number of pathogen and increase the biomass accumulation. FA was detected in all iron and boron concentrations treated plants, but high concentration of iron and boron could reduce the FA content in infected plants. Furthermore, high concentration of iron and boron could dcrease both spore germination rate and mycelium growth compared with other treatments. Through analysis mannitol content, it was.showed that high concentration of iron and boron could increase its content, also increased mannitol ratio could derease the FA production in pathogen culture medium. In conclusion, high concentration of iron and boron could decrease the occurrence of banana wilt disease. Therefore, in field production, increase the iron and boron fertilizer input were much better to control the banana wilt disease. |
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