Physiological mechanism of plant roots exposed to cadmium Y.X. Chen a, * , Y.F. He a , Y.M. Luo b , Y.L. Yu a , Q. Lin a , M.H. Wong c a Department of Environmental Engineering, Zhejiang University, Huajiachi, Hangzhou 310029, China b Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing 210008, China c Institute for Natural Resources and Waste Management, Department of Biology, Hong Kong Baptist University, Kowloon, PR China Abstract Physiological experiments on plant roots exposed to cadmium were conducted on carrot and radish using a liquid culture and a pot experiment with a series of cadmium applications. Activities of four enzymes (catalase, peroxidase, polyphenol oxidase, superoxide dismutase), and concentrations of free proline and malonaldehyde in the roots of both plants were investigated. Results showed that the germination rate and growth of roots of both plants were inhibited at the concentration of 20 mg Cd/l, and the inhibition was increased with the increasing concentrations of cadmium, both in the liquid culture and in the pot experiment; activities of the four enzymes declined similarly in both species. The concentration of proline in roots reached the maximum when the application of cadmium was at the level of 20 mg/l in the liquid culture (or 20 mg/kg in soil), and then it declined slowly with the increasing concentration of cadmium. However, the reverse trend was observed for the concentration of malonaldehyde. All of bio-indicators measured here was quite sensitive to the addition of cadmium. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cadmium; Plant roots; Physiological mechanism; Enzyme activities 1. Introduction Cadmium is a non-essential element that can be highly phytotoxic. Influences of cadmium on plant growth and the mechanism of its accumulation in plants are major aspects of the studies on the eco-environ- mental effects of cadmium. Many report on its toxic mechanisms, and its absorption and accumulation in plants (Yang, 1995; Marchiol et al., 1996). Reduction of biomass production and nutritional quality has been observed in crops grown on soils contaminated with moderate levels of heavy metal (Cottonie et al., 1976). Photosynthesis (Baszynski et al., 1980), transpiration (Bazzaz et al., 1974), carbohydrate metabolism (Moya et al., 1993), and other metabolic activities (Huang et al., 1974; Van Assche et al., 1988; Sharma et al., 1998) were inhibited by cadmium. Proline accumulation, accepted as an indicator of environmental stresses, is also con- sidered to have important protective roles (Alia-Saradhi, 1991; Sharma et al., 1998). However, the reported studies are concerned mostly with the influences of cadmium on the upper part of plants. Little is known of cadmium toxicity to the root system, which is the main interface of material exchange between plants and their environment. The germination of seeds and the root development of seedlings are important stages of whole plant growth, it is also the most sensitive stage of plants to changes of their surrounding environment. Therefore, to study the inhibition of plants exposed to contaminants in this stage is a useful way to understand the toxic mecha- nisms of environmental contaminants to plants. Here we report a physiological study of cadmium toxicity to the germination and root development of carrot and radish. Chemosphere 50 (2003) 789–793 www.elsevier.com/locate/chemosphere * Corresponding author. Tel.: +86-571-86971159; fax: +86- 571-869-71411/71898. E-mail address: yxchen@zju.edu.cn (Y.X. Chen). 0045-6535/03/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0045-6535(02)00220-5