| Pakchoi ( Brassica campestris ssp. chinensis L. ) is a very important green leafy vegetables in China, and has abundant genotypes from long time natural and artificial selection. In the production, pakchoi is easily polluted by Cadmium (Cd), which is one of the most toxic heavy metals to both plants and animals. It is one of best cost-effective and efficient approaches to choose cultivars of pakchoi with low Cd accumulation in edible parts. Meanwhile, it has been demonstrated that growth environmental factors may influence Cd absorption and accumulation in plants. It would thus be beneficial to reduce Cd uptake and accumulation in pakchoi by agronomic practices. The mechanism about the plant genotypic difference in Cd absorption and accumulation still be disputed. These experiments will be carried out to study the physiological mechanism of genotypic difference in effects of Cd on growth of pakchoi, Cd-uptake and accumulation in pakchoi plants, based on the research of genotypic difference of Cd contents in shoots of pakchoi and Cd subcellular distribution in plants of pakchoi. Meanwhile, the possibility to reduce Cd uptake and accumulation in pakchoi plants by application of Silicon (Si) or Selenium (Se) will be also studied. The major results were summarized as fellows:1. Investigation of genotypic difference of Cd contents in shoots of pakchoiDifference of Cadmium (Cd) contents in shoots of 87 cultivars of pakchoi ( Brassica campestris ssp. chinensis 1.) were studied in a Cd-contaminated soil (6 mg Cd kg-1 ) in two growth seasons. The results showed that significant differences of shoot Cd contents were found among the genotypes (cultivars). Cd contents of pakchoi in spring and summer were higher than in autumn and winter. The results of ANOVA showed highly significant differences of genotype (cultivar), growth season and cultivar growth season interaction for shoot Cd contents. The results indicated the significance to choose suitable cultivars, growth conditions and growth seasons to produce safe pakchoi.2. Cd subcellular distribution in plants of pakchoiCd contents in different pakchoi plant parts were as follows: root > blade > petiole. Cd was mainly distributed in fractions of soluble component, cell wall and chloroplast (or plastid in root) and less distributed in fractions of nucleolus, mitochondrion and ribosome. In four important organelles (plastid(or chloroplast), nucleolus, mitochondrion and ribosome) of the root and blade cells, Cd distribution rates were increased with increasing of Cd treated concentrations, especially more significantly in Huqingyihao. In the petioles, Cd distribution rates were increased with Cd concentration in mitochondrion and ribosome, but decreased in chloroplasts and nucleolus. Cd distribution rates in four most important organelles (chloroplast, nucleolus, mitochondrion and ribosome) of Huqingyihao were higher than in Suzhouqing in petiole. BecauseCd increasing with the increasing of Cd concentration in the important organelles in Huqingyihao was much higher than in Suzhouqing, the damage on cellular life activities by increasing Cd concentration in Huqingyihao would be stronger than in Suzhouqing.3. Influence of Cd on photosynthesis of pakchoiLow concentrations of Cd (0. 01, 0. 1 mg L"1) promoted plant growth, especially more considerably on Suzhouqing. High concentrations of Cd (10 mg L"1) significantly inhibited the shoot growth, however, didn' t affect the root growth. Cd decreased chlorophyll content, especially on Suzhouqing. High concentrations of Cd (1-, 10 mg L"1) considerably reduced net photosynthesis rate. The reduction on stomatal conductance and increase on intracellular Cft (Ci) under high concentrations of Cd indicated that a non-stomatal restriction resulted in an decrease of net photosynthesis. The determination of chlorophyll fluorescence showed that Cd didn' t influence photochemical efficiency of PSH(Fv/Fm). High concentrations of Cd reduced actual quantum yield of PSIl(PSII), photochemical quenching (qP) and elec...
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