| The objective of this paper was to study the physiological and biochemical mechanisms and to identify molecular marker of resistance to downy mildew in non-heading Chinese cabbage. The results were as follows:1. To identify whether the resistance of downy mildew converted or not in different growth periods, two parent lines, F1 and F2 populations were inoculated in cotyledon and seedling stages using an artificial inoculation method. The results indicated that the phenotype of two parents were fit to those genotype, and F1 populations expressed the resistance to the downy mildew in both cotyledon and seedling stages. All these explained that the resistance of downy mildew did not convert in different growing period in this experiment. In addition, the F2 population segregated a clear 3:1 of resistance to susceptibility in both cotyledon and seedling stages, which suggested the resistance was controlled by a single dominant locus.2. To find the relationship between resistance of downy mildew and the activity of protective enzyme, the activities of PPO, POD, PAL and β -1,3-glucanase were studied after the different resistant plants were infected with Peronospora parasitica for 120 hours. The results showed that the activities of four enzymes increased after infection. But they increased faster and higher in resistant plants than those in susceptible variety. The peak values of the activities of PPO, POD, PAL and β -1,3-glucanase appeared 72, 96, 72 and 96 hours after infection and the high activities of the four enzymes in resistant variety lasted longer than in susceptible variety.3. To optimize the AFLP system for non-heading Chinese cabbage, some different treatments of main factors were studied. First, through using different treatments on the concentration of DNA during the course of digestion of genomic DNA and ligation of adapters, we identified that the concentration of DNA between 25 and 250 ng/μl were fit for...
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