Inhibition Of Photosynthesis In Cucumber Seedling Leaves Inoculation Bypseudoperonospora Cubensis

Cucumber is one kind of important vegetables in the world. However, downy mildew, caused by Pseudoperonospora cubensis, is the most destructive plant disease in cucumber production areas worldwide. To date, some researchers focused on resistance genetic and cultivar improvement. Nonetheless, very little is known about downy mildew infection influencing photosynthesis in the infected leaves. Therefore, hypersensitive reaction (HR) was observed during the inoculation of the resistant inbred line and susceptible inbred line, respectively. The investigation was performed on stomata, gas exchange, chlorophyll fluorescence, enzyme. The purposes of this research are to elucidate the mechanism of photosynthesis during HR, and provide a clue of valid application in cucumber culture, resistant mechanism and improvement. The main results are as follows:1. Under different inoculation methods, a few downy mildew lesions became visible in both resistant and susceptible lines, and the chlorosis developed in the center of infected spots resulted in a localized tissue death in the resistant line with little further lesion spread within vines. This phenomenon was proved in the field natural infection conditions. Oxidative burst, rapid and abundant callose and lignin accumulation were investigated at the cell wall of attacked inoculated resistant plant. The results demonstrated that the resistant genotype DR performed incompatible interaction with P. cubensis characterized by its hypersensitive response to the pathogen attack.2. Hyphae infiltrated into the mesophyll cells through the stomata post-inoculated, the harmed stomata were observed in inoculated leaves so that it could not open and close regularly.3. During the first stage of hypersensitive reaction, in the resistant line, the intercellular CO2 concentration (CI) decreased evidently compared with the control, while no differences were observed in the stomata conductance (GS) in leaves of inoculated and noninoculated plants. The reduction of CO2 assimilation in leaves of inoculated was driven by the harmed stomata constraints. So it indicated that the decrease of the net photosynthetic rate (Pn) was dominatingly dependent on stomatal factor. But the intercellular CO2 concentration (Ci) increased significantly at 4-7 dpi, suggesting that non-stomatal limitation leads to the reduction of Pn. The corelations between photosynthetic active radiation (PAR) and stomatal conductance (GS) were positive, indicating that the transpiration rate and photosynthetic active radiation were decreased by stoma close .4. The determination of chlorophyll fluorescence parameters revealed that as the hyphae infiltrated continuousl, quantum efficiency of PSII photochemistry (ΦPSII), the PSII maximal photochemical efficiency (Fv/Fm), and photochemical quenching coefficient (qP) decreased continuously, wheras, non-photochemical quenching coefficient (NPQ) increased concomitantly. It indicated that the photoinhibition has been induced significantly, as the disease progressed, quantum efficiency of PSII photochemistry (ΦPSII) decreased in the inoculated leaves. So that photosynthesis apparatus was damaged severely and photosynthesis capacity was weakened quickly.5. The decrease of ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBPCase) and glyceraldehydes-3-phosphate dehydrogenase (3-GAP) and the increases of glycolate oxidase (GO) activities were also major factors in determining the depression in the photosynthetic productivity of these infected plants.