Cadmium (Cd) is among the most widespread and toxic pollutants in the surface soil layer. Its toxicity in soil is becoming a severe threat to organisms worldwide. It is the most dangerous carcinogen for the human body, and readily accumulates in kidneys and bones, leading to disruption of kidney function, osteomalacia and bone breakage. Wide areas of agricultural soil across China are heavily contaminated by Cd and thus it enters the food chain. In soil, Cd is distributed mainly in the topsoil and, without deposited by soil organic compounds, it is more easily assimilated by crop roots compared with other heavy metals. However, vegetable cultivars accumulate much more Cd in the shoot, the portion eaten by humans. In China, total vegetable production is 3.45 million tones, which ranks as the highest in the world, and most of the vegetable-farming land is located in suburban areas where highly intensive industries are located and thus the lands are widely contaminated by heavy metal pollutants. As a result, vegetables in most city markets contain Cd at levels two- to three-fold, or in extreme cases 5.2-fold, higher than the National Sanitary Criteria of Cd content in vegetable products. Economically, the heavy metal problem weakens the international competitiveness of the Chinese vegetable industry, therefore decontamination of heavy metals from polluted soils, as well as lowering of Cd residues in vegetables, is of great urgency both for human health and the national economy.
Hydroponic experiments were conducted in a greenhouse to study the effects of Cd on lipid peroxidation and membrane proton pump activity of Chinese kale (Brassica alboglabra Bailey) roots, and to explore the toxicity of Cd stress on plants. Plants were grown under controlled environmental conditions, and subjected to different Cd concentrations ranging from 0 to 8 mg/L. We determined the root activity, rate of O₂^- generation, H₂O₂ and malondialdehyde (MDA) contents as well as activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), H⁺-ATPase and Ca²⁺-ATPase of Chinese kale roots exposed to five different Cd concentrations for 10 days.
The root activity decreased with increasing Cd concentration, which resulted in a significant increase in O₂^- and H₂O₂ concentrations and MDA content, compared with the control (CK). Increasing Cd concentration enhanced activities of SOD, POD, and CAT (P < 0.05), which indicated that the Chinese kale plants were stressed by Cd. No significant difference in H₂O₂ content was observed among the CK, 1.0 and 2.0 mg/LCd treatments, while a similar trend was observed in rate of O₂^- generation between the CK and 1.0 mg/L Cd treatment. The activities of H⁺-ATPase and Ca²⁺-ATPase first increased and then decreased with the increase in Cd concentration. In addition, the activities of H⁺-ATPase and Ca²⁺-ATPase showed no obvious change (P > 0.05) in response to 1.0 mg/L Cd, but significantly decreased with stress induced by 2.0, 4.0 and 8.0 mg/L Cd (P < 0.05). A negative correlation existed between the level of membrane lipids and the two enzymes. We concluded that there was no noticeable change under a low Cd concentration of ATPase activity in the plasma membrane of Chinese kale roots, whereas under relatively high Cd stress activity dropped markedly and root growth was suppressed.