Responses Of Brassica Chinensis L. To Cadmium Stress And Silicon-Alleviated Cadmium Toxicity

Soil contamination with heavy metals especially cadmium (Cd) in China has become a public concern because of repeated irrational application of organic manure, sewage sludge and rock phosphate, leading to accumulation of excess cadmium in pakchoi (Brassica chinensis L.) and high food safety risks. There are many varieties of pakchoi in China and distinct genotypical differences exist in responses to Cd toxicity. Two pakchoi(Brassica chinensis L.) 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 were then grown hydroponically to investigate the effects of Cd on plant growth, uptake, and transport, photosynthetic parameters, antioxidative defenses and subcellular distribution of Cd. By using transmission electron microscopy (TEM), we analyzed the alleviative effect of silicon (Si) on leaf and root ultrastructure and distribution pattern of cadmium in organs and organelles. In addition, we studied plasma membrane integrity and lipid peroxidation in roots using histochemical analysis method. The main results were presented as follows:1. Effect of Cd on plant growth, Cd uptake and distribution in two pakchoi cultivarsIn the present study, increased Cd level in the medium caused a significant increase of Cd concentration in shoots and roots of two pakchoi plants with most of Cd absorbed by plants remained in the roots. And significant differences existed in different pakchoi cultivars. Cd distribution rate was increased with Cd level in cell of two cultivars, but there existed genotypical difference. In different plant parts, Cd distribution rate was also increased with Cd level. In different parts of shoots, Cd was mainly distributed in the cytoplasm, followed by cell wall and organelles. In different parts of roots, a similar trend was found in the treatment with 0.5 mg L"1 Cd, but in the treatment with 5 mg L-1 Cd was mainly distributed in the cell wall, followed by cytoplasm and organelles.2. Effect of Cd on photosynthetic parameters and leaf soluble sugar in pakchoiNet photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), water use efficiency (WUE), stomatal limitation value (Ls) and chlorophyll cocntent of the two pakchoi cultivars tested were all decreased with increasing Cd concentrations, while intercellular CO2 concentration (Ci) and soluble sugar content were increased with increasing Cd concentrations. A reverse change trend of intercellular CO2 concentration (Ci) was observed for stomatal limitation value (Ls). The degree of increase or decrease of above-mentioned parameters was smaller in Cd-tolerant plant cultivar than in Cd-susceptible plant.3. Effect of Cd on antioxidant defense capacity in pakchoiThe MDA and H2O2 contents together with GSH and NPT contents were increased with increasing Cd concentration and exposure time, This effect was more significant in Cd-tolerant cultivar than in Cd-susceptible cultivar. SOD activities of both cultivars tested increased at Day 1 after Cd treatment, and then decreased with increasing Cd concentration and exposure time; CAT activities in both cultivars tested were increased with exposure time at the lower level of Cd, and then decreased with increasing Cd concentration.4. Effect of Si on plant growth, macronutrient and micronutrient concentration, and antioxidant defense capacity in pakchoi plants under Cd stressSignificant reduction in biomass was observed with distinct chlorosis in leaves of two pakchoi plants subjected to Cd stress, especially in the Cd-susceptible cultivar (SHQ). Cadmium stress dramatically increased Cd content, and disturbed the balance of P, K, Ca, Mg, Fe, Cu and Zn in shoots and roots of both cultivars tested. The activities of superoxide dismutase (SOD), catalase (CAT), asorbate peroxidases (APX) decreased, but the concentrations of H2O2, malondialdehyde (MDA), ascorbic acid (AsA), glutathione (GSH) and non-protein thiols (NPT) increased significantly. Under Cd stress, the toxicity symptoms of Cd were alleviated by addition of Si with the biomass being increased. Addition of Si significantly increased shoot and root biomass of the plants grown under the higher level of Cd. Furthermore, Si significantly decreased Cd uptake and root-to-shoot transport. The interactive effects of Si and Cd on the antioxidant defense activity were further verified by the isoenzyme profiles using gel electrophoresis. On the other hand, the concentrations of ascorbic acid, glutathione and non-protein thiols increased significantly under the higher level of Cd, which were all further intensified by addition of Si. The alleviative effects of Si on Cd toxicity were more significant in the Cd-tolerant cultivar than in the Cd-sensitive cultivar.5. Effect of Si on antioxidant defense capacity and oxidative damage of pakchoi in roots under Cd stressRoot length was decreased by Cd stress, especially in the suseptible cultivar. Addition of Si significantly increased root length of both cultivars under both levels of Cd tested. Superoxide dismutase (SOD), catalase (CAT) and asorbate peroxidases (APX) activities were decreased, but H2O2, malondialdehyde (MDA), ascorbic acid (AsA), glutathione (GSH) and non-protein thiols (NPT) concentrations, expect AsA, GSH and NPT contents were increased by 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. Furthermore, such effects were confirmed by using histochemical staining methods. Cd stress induced the loss of plasma membrane integrity and the damage of membranes due to lipid peroxidation, which was alleviated by Si. The alleviative effects of Si on Cd toxicity were more significant in the Cd-tolerant cultivar than in the Cd-sensitive cultivar.6. Effect of Si on ultras tructure of two pakchoi cultivars seedings under Cd stressMicroscopic observations indicated that the stoma transmutated and chloroplast structure swelled observably with chloroplast granae being destroyed heavily under Cd-stress, and lamella being illegible and disassemble. Transmission Electron Microscope (TEM) analysis showed that serious untrastructural damage was observed in both cultivar roots treated with Cd. Plasmolysis, concentrated cytoplasm, ambiguity of organelle were all symptoms of root cells under Cd stress. However, exogenous Si improved the stoma shape and chloroplast structure of the leaves of Cd-stressed pakchoi with more intergrated membranes, which were badly damaged in pakchoi under Cd stress. And exogenous Si alleviated the plasmolysis of root cell. The results show that the resistant cultivar HYD was much more adapative to Cd stress than the susceptible culticar (SHQ).