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 living organism worldwide. It is one of the main pollutants in paddy fields near industrial areas and highly toxic to rice growth and development, and can be easily taken up by rice plants and enter the food chain. Thus, it is necessary to reduce Cd accumulation in rice, which is one of the most frequently consumed crops not only in China but also in the world. Therefore, precautionary measurements should be done to reduce accumulation of Cd in rice to alleviate the risk of health hazards in response to Cd-polluted soils.
Several strategies have been proposed for the successful management of the Cd-contaminated agricultural soils. One approach, applicable on slightly contaminated soils, is aiming to screen and use low Cd-accumulating genotypes of crops. Secondly, the toxic effect of Cd can be decreased by application of proper chemicals and rate of essential nutrients such as Zn, S and N in Cd contaminated soil. Therefore, the selection of plant genotypes with high ability to repress root uptake and shoot transport of Cd is a reasonable approach to alleviate adverse effects of Cd toxicity in crop plants.
Hydroponics experiments were conducted in green house of Zhejiang University, Hang Zhou, China during 2002-2004, to study the genotypic variation in tolerance to Cd toxicity and its interaction with Zn, S and N based on the measurement of plant growth parameters, photosynthetic rate and chlorophyll content, biomass accumulation, interacting effect of Zn, S and N on Cd uptake, changes in antioxidative defense system both in roots as well as shoots and lipid peroxidation. Plants were grown under controlled environmental conditions, and subjected to different Cd concentrations ranging from 0 to 5 uM.
In the first experiment the effects of Cd toxicity on growth, lipid peroxidation, antioxidant enzymes and Cd accumulation in different plant organs were studied in two rice cultivars, Bing 97252, relatively Cd-tolerant cultivar and Xiushui 63, relatively sensitive one. Plants were exposed to 0, 0.1, 1 and 5 uM Cd. Cd stress
inhibited plant height and chlorophyll content, altered melondialdehyde (MDA) content and activities of superoxide dismutase (SOD), cataiase (CAT) and peroxidase (POD). Roots and shoots responded differently to Cd in terms of antioxidant enzyme activity. Generally the activities of SOD, POD and CAT decreased with increase in Cd level, while the activity of MDA increased with increase in Cd level. With the increase in Cd concentration in medium, MDA content in shoots and roots of Xiushui 63 increased at much higher rate than Bing 97252 at both growth stages. A significant decrease in chlorophyll content, plant height and photosynthetic rate has been noted in higher Cd treatment (1 and 5 uM). The accumulation of Cd in roots and shoots increased with Cd levels in medium, Xiushui 63 being higher than Bing 97252.
The second experiment was conducted to investigate the alleviating effect of Zn on growth inhibition and oxidative stress caused Cd toxicity by using the same rice cultivars. The treatments were consisted of three Zn (0, 0.2 and 1 umol) and three Cd levels (0, 0.1 and 1 uMI). The results showed that addition of Zn into medium solution demonstrated the distinct effect of alleviating Cd toxicity, which may be reflected by the significant increase in growth parameters, including plant height and biomass, chlorophyll content and photosynthetic rate, and the marked decrease in MDA content and the activities of anti-oxidative enzymes relative to the control (without Zn) when the plants were exposed to Cd stress. However it is noted that Zn increases shoot Cd concentration at higher external Cd level, probably due to the enhancement of Cd translocation from roots to shoots. Therefore further study is necessary to determine the effect of external Zn level on Cd translocation from shoot to grains or grain Cd accumulation before Zn fertilizer is applied