Abstract: Pak choi(Brassica chinensis L.)has been a major diet food in South China.Cd uptake by the plant and Cd concentration in edible leaves of the plant,effect of soil conditions,and plant cultivar,and their interaction are poorly understood.A pot experiment was conducted to explore Cd and Zn uptake during the whole growing period of the plant with and without Cd treatment.Two cultivars of pak choi,Siyueman and Gaogengbai,were grown in two soils(a Wushantu,Gleyic Stagnic Anthrosols and Hongshanitian,Ultic Stagnic Anthrosols) contrasting in soil chemical properties and soil total Cd and Zn contents.Cd was added at 0 mg·kg^-1 of soil(as the control) and 2 mg·kg^-1,respectively.Pak choi plants were harvested at different growth stages and biomass and Cd and Zn contents of different parts were determined.Uptake of Cd and Zn by the plant was significantly affected by soil type,plant cultivar,and soil Cd dosage,jointly.The studied two cultivars are different in Cd uptake with Siyueman being a cultivar of high Cd affinity and Gaogengbai of low affinity.Under a positive interaction of soil and cultivar,Cd uptake was enhanced by over ten times but Cd/Zn ratio in above ground parts of the plant did not follow the trend of Cd/Zn ratio in total uptake by the plant,for Cd and Zn distribution in different parts varied with cultivar as well as growing stage.The distribution coefficient was 56.3%-71.1% for Cd and 77.6%-80.6% for Zn.The decreased Cd/Zn ratio under Cd treatment resulted in a decreased Cd/Zn ratio in above ground parts,thus reducing the exposure risk;The dynamics of Cd and Zn uptake varied with growing stage and from cultivar to cultivar.Cd accumulated in the plant steadily with the growing period,while Zn uptake and accumulation turned slightly downwards after the seedling stage,thus leading to higher Cd/Zn ratio in the plant at the harvesting stage and higher risk of Cd exposure.This phenomenon was especially significant in the acid soil.

Key words: pak choi, cadmium, zinc, uptake, distribution, soil type, plant cultivar, growing stage