Research On Promotive Effects Of Fusarium Wilt In Cucumis Sativus By Cinnamic Acid, An Autotoxin In Root Exudates Of Cucumis Sativus L., And Mitigation Mechanism By Grafting And Cinnamic Acid-degrading Microbial Strains.

Greenhouse agriculture has become one of the most active industry in agriculture. Cucumber (Cucumis sativus L.) is a favourable vegetable for Chinese and has become the main variety of greenhouse vegetables. However, the contradiction between limited land for vegetable production and year-round demand for the supply leads to monocropping as the popular cropping system, resulting in a reduction both in productivity and the food quality. Autotoxicity is one of the most important factors responsible for the continuous cropping obstacles. It was well documented that cucumber plant has autotoxic potential, and more than 10 kinds of growth inhibitory substances including benzoic and cinnamic acids from its root exudates have been isolated and identified. These substances could significantly inhibit plant growth by influencing physiological metabolism process such as ion absorption, water utilization, photosynthesis and respiration etc. Fusarium wilt by Fusarium oxysporum. f. sp. cucumerinum (FO) is a kind of typical rhizosphere soil-borne disease. It is extensively distributed, and causes a lot of loss but is difficult to prevent. The pathogens can survival in the soil for many years, and result in the reduction of yield and quality in cucumber production. However, research on continuous cropping obstacles mostly dealt with separated factor and almost no studies have been carried out to investigate the interaction between these factors. This study investigated the promotive effects of cinnamic acid, an autotoxin in root exudates, on the incidence of Fusarium wilt caused by Fusarium oxysporum. f. sp. cucumerinum in Cucumis sativus and associated physiological mechanism. Furthermore, approaches to alleviate or eliminate continuous cropping obstacles, such as grafting, screening and inoculation of beneficial microbial strains were also tried. The main results are as follows:1. To determine the short effects of autotoxic substances in Cucumis sativust, we investigated the effects of root exudates and aqueous root extracts and identified autotoxins and their analogs on photosynthesis and antioxidant system. The results indicated that these substances significantly reduced leaf stomatal conductance (Gs), photosynthesis (Pn) and transpiration (E), together with an increased activity of active oxygen scavenging enzymes such as peroxidase (POD) and superoxide dismutase (SOD) and an enhanced level of membrane peroxidation.2. The effects of F. oxysporum. f. sp. cucumerinum (FO), the pathogen for Fusarium wilt incucumber and cinnamic acid (CA), an autotoxin in root exudates on plant growth, photosynthesisand incidence of Fusarium wilt in cucumber were studied in order to elucidate the interaction of autotoxins and soil-borne pathogens in the soil sickness. The results showed that FO and CA at 0.1 or 0.25 mM significantly decreased net photosynthetic rate (A), stomatal conductance (Gs) and the quantum yield of photosystem II photochemistry , followed by a reduction of plant biomass production, but did not induce photoinhibition. Pretreatment with CA before inoculation with FO increased the effectiveness of FO, together with a slight photoinhibition. CA pretreatrnent significantly increased percentage of plants affected by Fusarium wilt, browning index of vascular bundle and Fusarium population in the nutrient solution. All these results indicate that CA enhanced Fusarium wilt by predisposing cucumber roots to infection by FO through a direct biochemical and physiological effect. It is likely that soil sickness results from an interaction of many factors.3. To ascertain if oxidative stress plays a role in the promoted incidence of Fusarium wilt of cucumber, the activities of several antioxidant enzymes, namely superoxide dismutase (SOD), guaiacol-dependent peroxidase (GPX), ascorbate peroxidase (APX) and catalase (CAT) together with contents of H2O2, MDA and O2 were determined in cucumber root after exposure to cinnamic acid (0.05-0.25 mM ) prior to the inoculation of Fusarium oxysporum. Results i...