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A series of hydroponics experiments were performed to investigate roles of silicon (Si) in enhancing cadmium (Cd) tolerance in two pakchoi (Brassica chinensis L.) cultivars: i.e. cv. Shanghaiqing, a Cd-sensitive cultivar, and cv. Hangyoudong, a Cd-tolerant cultivar. Plants were grown under 0.5 and 5 mg Cd L(-1) Cd stress without or with 1.5 mM Si. Plant growth of the Cd-tolerant cultivar was stimulated at the lower Cd level, but was decreased at the higher Cd level when plants were treated with Cd for one week. However, Plant growth was severely inhibited at both Cd levels as stress duration lasted for up to three weeks. Plant growth of the Cd-sensitive cultivar was severely inhibited at both Cd levels irrespective of Cd stress duration. Addition of Si increased shoot and root biomass of both cultivars at both Cd levels and decreased Cd uptake and root-to-shoot transport. Superoxide dismutase, catalase and ascorbate peroxidase activities decreased, but malondialdehyde and H2O2 concentrations increased at the higher Cd level, which were counteracted by Si added. Ascorbic acid, glutathione and non-protein thiols concentrations increased at the higher Cd level, which were further intensified by addition of Si. The effects of Si and Cd on the antioxidant enzyme activity were further verified by isoenzyme analysis. Silicon was more effective in enhancing Cd tolerance in the Cd-tolerant cultivar than in the Cd-sensitive cultivar. It can be concluded that Si-enhanced Cd tolerance in B. chinensis is attributed mainly to Si-suppressed Cd uptake and root-to-shoot Cd transport and Si-enhanced antioxidant defense activity.

The main objectives of this study were to elucidate the roles of silicon (Si) in alleviating cadmium-induced oxidative stress in roots of pakchoi grown hydroponically using two contrasting pakchoi (Brassica chinensis L.) cultivars: cv. Shanghaiqing (SHQ), a Cd-sensitive cultivar and cv. Hangyoudong (HYD), a Cd-tolerant cultivars. Lipid peroxidation, enzymatic and non-enzymatic antioxidants and histochemical stainning were examined in the two pakchoi plants grown under lower (0.5 mg Cd L-1) and higher (5 mg Cd L-1) Cd stress without or with 1.5 mM Si. Root length was decreased by Cd stress, especially in the sensitive cultivar. Addition of Si significantly increased root length of both cultivars under both levels of Cd tested. The decreases in activities of superoxide dismutase (SOD), catalase (CAT), asorbate peroxidases (APX) and increases in concentrations of malondialdehyde (MDA) were observed in the Cd treatments, which were all counteracted by addition of Si. These alleviative effects of Si were more significant in the Cd-tolerant cultivar than in the Cd-sensitive cultivar. Such effects were confirmed by using histochemical staining methods. All these results suggest that Si-mediated alleviation of Cd toxicity be attributed mainly to Si-enhanced root length, antioxidant defense capacity and membrane integrity and to Si-suppressed oxidative damage.

Cadmium (Cd) is present in all soils, usually as a trace constituent, but it can reach higher levels in agricultural soils. Cd can then be absorbed by plants and become a potential risk to human health. Once taken up by a plant, there are mechanisms for heavy metal detoxification in the plant. Here, a cadmium-tolerant and a cadmium-sensitive rice cultivars were grown hydroponically to investigate the effects of cadmium (Cd) applied at low levels on uptake and transport, subcellular distribution and binding forms of Cd in rice plants. Our results showed that increasing the Cd treatment from 1.0 μM to 5.0 μM Cd increased the shoot Cd content by 55% in the cadmium-tolerant cultivar, and by 108% in the cadmium-sensitive cultivar. For the cadmium-tolerant cultivar, increasing Cd treatment from 1.0 μM to 5.0 μM increased the root Cd content by 116%, whereas for the cadmium-sensitive cultivar, increasing Cd treatment from 1.0 μM to 5.0 μM increased the root Cd content by 80%. Further, the ratio of Cd accumulation in shoots over roots decreased from 0.19 to 0.14 in the cadmium-tolerant cultivar, while it increased from 0.20 to 0.26 in the cadmium-sensitive cultivar, showing that the transportation ability for Cd was different between the two tested rice cultivars. At the higher Cd level of 5.0 μM, most of the Cd in the plants was localized in cell walls and vacuoles in both cultivars, whereas small portions of Cd were distributed in the cytoplasm, suggesting that the important metabolic and physiological processes were not impaired under Cd stress. Furthermore, the major portions of Cd in the cells were combined with organic acids, proteins and polysaccharide, and were consequently detoxified. The difference in the distribution of cadmium in rice plants resulted in the difference in Cd tolerance between the two rice cultivars used. It can be concluded that the retention of Cd in root cell walls, compartmentation of Cd into vacuoles and the suppressed transportation of Cd from roots to shoots are the most important mechanisms involved in the detoxification of Cd in rice plants. [ABSTRACT FROM AUTHOR]