Abstract: The microbial population dynamics in the rhizospheres of cucumber was studied by cultivation and culti- vation-independent analysis based on directly extracted DNA. Rhizosphere samples were taken at 2nd, 4th, 7th and 10th week after the seeds were planted, which was positively related to the corresponding date of cucumber growth stages. Results showed that the total number of bacteria, fungi and actinomyces began to rise at planting and quickly reached its peak at seedling or flowering period, but decreased slightly later. Bacterial population in rhizosphere was higher than that of corresponding control except certain periods, while both showed the similar change trend at all stages. Nitrogen fixing bacteria, nitrogen-fixing bacteris and ammonifiers showed the same change tendency in population as bacteria and actinomyces did while cellulose decomposing microbes had the contrary rhizosphere effects as cucumber developed. There was a positive correlation between the rhizosphere microbe population and cucumber development. PCR was employed to amplify V3 region of 16SrDNA, then the products were subjected to denaturing gradient gel electrophoresis (DGGE). DGGE profile indicated that a few of microbe species lived stably in the farmland, but some were affected in population due to cucumber growth. Significant difference was observed in rhizosphere and control soils especially for the seedling and flowering samples. Few prominent bands in DGGE patterns which displayed stronger or less illumination means the bacteria represented had great population change in that period. This phenomenon indicated that flowering cucumber heavily affected rhizosphere bacteria, or the bacteria, most probably uncultured bacteria, functioned specially for cucumber flowering. Most detected bands with no illumination change in DGGE were likely to represent the indigenous microbles that were essential for constructing and stabilizing farmland microecological environment.

Key words: Soil, Cucumber, Rhizosphere microbe, PCR-DGGE