Abstract: Two pakchoi[Brassica campestris L. ssp. chinensis（L.）Makino]cultivars：i.e. Hangyoudong（HYD），a Cd-tolerant cultivar and Shanghaiqing（SHQ），a Cd-susceptible cultivar，were selected among 12 pakchoi genotypes based on the root tolerance index，and then grown hydroponically to investigate the effects of Si on biomass，photosynthetic parameters and leaf stoma structure and chloroplast ultrastructure in two contrasting pakchoi cultivars grown under Cd stress. The results showed that shoot and root biomass of SHQ and HYD with Cd treatment decreased by 33.8%，30.9% and 28.2%，6.8% compared with the corresponding controls；Cd contents were 56.2，86.6 and 44.7，62.9 times as high in the Cd treatment alone as in the corresponding control treatment. Net photosynthetic rate（Pn），stomatal conductance（Gs），transpiration rate（Tr）and chlorophyll content of the two pakchoi cultivars tested were all decreased under Cd-stress，while intercellular CO2 concentration（Ci）was increased under Cd-stress. Microscopic observations showed that the stoma transmutated and chloroplast structure swelled observably with chloroplast granae being destroyed heavily under Cd-stress，which were all counteracted by addition of Si. Addition of Si to Cd treatment significantly increased shoot and root biomass of SHQ and HYD by 24.9%，41.8% and 32.0%，43.5%，decreased Cd contents in shoot of both cultivars by 15.1% and 26.3%，and increased Cd contents in root of both cultivars by 24.7% and 33.6%，compared with the corresponding Cd treatments alone. The results showed that addition of Si suppressed Cd uptake in shoots and root-to-shoot transport，improved the photosynthetic efficiency. However，the alleviation of Si on above-mentioned parameters was smaller in Cd-sensitive plant cultivar than in Cd-resistant plant.

Key words: pakchoi, Brassica campestris L. ssp. chinensis（L.）Makino, cadmium, silicon, photosynthesis, chloroplast, ultramicroscopic structure