Long-term effects of exogenous silicon on cadmium translocation and toxicity in rice (Oryza sativa L.)
Introduction
Background cadmium (Cd) levels in agricultural soils are normally low than 1 mg kg−1. However, because of long-term use of phosphate fertilizers and sewage sludge, higher values have been observed in many agricultural soils (Adriano, 2001). Cadmium-contaminated foods resulting from contaminated soils through the food chain could threaten human health. Rice is the staple food crop for about half of the world's population. It has been reported that rice consumers ingest about half or more of their daily cadmium intake from rice (Iwao, 1977). A survey of the Cd content of rice samples from 22 countries by Masironi et al. (1977) revealed that Cd concentrations ranged from 2 ng g−1 in rice grains from Brazil to 65 ng g−1 in those from Japan. Therefore, methods to decrease Cd content in rice grains are important for human health.
It was suggested that the chemical amendments with steel sludge or furnace slag which was abundant of Si alleviated the heavy metal contamination and improved plant growth (Chen et al., 2000). Liang et al. (2005) reported that the effects of chemical amendment should lead to silicate-induced pH rise in the soils and decrease soil Cd availability. However, this should not be the only reason for Si alleviating ion's stress. It is reported that the formation of hydroxyaluminumsilicates in the apoplast of the root apex detoxified Al (Hodson and Evans, 1995, Ricardo et al., 2002, Wang et al., 2004). In the freshwater snail, a silicon-specific mechanism existed for the in vivo detoxification of aluminum which provided regulatory evidence for Si in a multicellular organism (Desouky et al., 2002). Similarly, in spring sandwort (Minuartia verna), Si was directly involved in the detoxification of zinc by forming Zn-silicate in the epidermal cell walls (Neumann et al., 1997, Neumann and zur Nieden, 2001). Silicon also plays important roles in alleviating biotic stresses brought on by fungal spore infection such as rice blast as reported by Kim et al. (2002) or powdery mildew of wheat as reported by Fawe et al. (1998) and phytophagous insects (Salim and Saxena, 1992) or mammals (Gali-Muhtasib and Smith, 1992). Therefore, Si is considered to be a beneficial element for higher plants (Epstein, 1999), but is essential for rice growth and development (Ma and Takahashi, 2002a). Limited studies have indicated that Si could alleviate Cd toxicity in plants (Horst and Marshner, 1978, Horiguchi and Morita, 1987, Wang et al., 2000, Rogalla and Römheld, 2003). The Cd level in the cell-wall fraction of Cd-treated maize was suggested to be much higher than that of Cd-treated peas which accumulated more Cd in the soluble fraction (Kahn et al., 1984, Lonzano-Rodríguez et al., 1997). On the other hand, Si alleviated Cd stress in plants by depositing high concentrations of Si in cell walls of the endodermis and epidermis to restrict transport of the metal in bypass flow from roots to shoots (Shi et al., 2005).
However, nutrient solution studies on Si enhancement of plant resistance are usually conducted over the short term and therefore interactions between Si and Cd in the long term may have been underestimated. During the growth period rice plants continually absorbed Si and then accumulated Si in both shoots and roots (Sangster, 1978). Silicon in the roots can change their plasticity and plays an important role in plant's response to environmental stress (Hodge, 2004). Therefore, when we consider long-term effects of Si on alleviation of Cd toxicity there are two unresolved questions. Firstly, what is the nature of the interaction between Si and Cd and secondly, does Si translocation in shoots and roots change with different conditions of stress. To answer these questions the present paper describes the results of a long-term experiment in which rice plants were grown in nutrient solution under a range of Cd stress levels with or without exogenous Si.
Section snippets
Plant culture
Rice seeds (Oryza sativa L., cultivar “Yuefu”) were surface-sterilized in 0.3% hydrogen peroxide for 20 min, rinsed with double distilled water, and then placed on a tray to germinate under controlled conditions at 25 °C in the dark. Seventeen days after germination, uniform seedlings were transferred to half-strength complete nutrient solution in 2 L plastic pot, and 2 days later the nutrient solution was changed to full-strength solution, which was followed the formulation introduced by the
Effect of Si on rice growth with or without Cd stress
Increasing Si supply from 0 to 2 mmol L−1 significantly increased the biomass of shoots and roots, but further increase in Si did not further increase the biomass (Table 1). In comparison with zero-Si rice, the biomass of +Si rice increased noticeably with time: 0.11–6.91 g plant−1 in shoots and 0.01–1.42 g plant−1 in roots, respectively. In addition, the effect of Si on dry matter was more prominent at later growth stages than at early ones and on shoots than roots. In different growth stages, the
Discussions
The main question about how to alleviate heavy metal toxicity, above all, is how plants resist heavy metal stress in the long term and decrease metal translocation to the shoots. Rice is reported to be a Si-accumulator (Epstein, 1999, Ma and Takahashi, 2002a) and Si is beneficial to the growth of rice. The present study indicated that Si supply increased the biomass of both shoots and roots (Table 1). It is well known that Si deficiency markedly reduces the grain yield of rice (Korndörfer and
Acknowledgements
This work was supported by the National Natural Science Foundation of China (G30170550), the ‘948’ Program of the Chinese Ministry of Agriculture and “the Program for Changjiang Scholar and Innovative Research Team in University” (IRT0511). We are also grateful for Dr. Z.X. Dou for her comments on the paper and Dr C.X. Tang and P. Christie for linguistic corrections.
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