Genotypic And Environmental Effect Of Toxic Heavy Metal Concentrations In Rice Grains

At present, a huge area of arable land in China has been contaminated by toxic heavy metals, which not only inhibits the growth and development of crops, leading to reduced yield and quality, but also poses a great threat to human health via food chain due to uptake by crops. It is, therefore, imperative to reduce toxic heavy metal concentrations in edible parts of crops, particularly in rice, which is one of the most frequently consumed cereals worldwide. The uptake, accumulation of and tolerance to some heavy metals vary greatly among plant species and among genotypes within a species. In rice, a significant difference also exists among genotypes in heavy metal concentrations in grains and their tolerance, thus providing a possibility of developing rice cultivars with high tolerance and low grain accumulation of toxic heavy metals.It is considered the most economic and efficient approach to reduce heavy metal concentrations in rice grains planted on the slightly or moderately contaminated soils. Meanwhile, heavy metal uptake and accumulation by crops is a function of the complex interaction of plant, soil, environmental and cultivated factors that affect bioavailability of heavy metals in soils. Up to date, however, the genotypic and environmental variation of toxic heavy metal concentrations in rice grains and its mechanism remain poorly understood, thus hindering the progress of breeding and agronomic improvement in heavy metal tolerance and accumulation. On basis the previous survey of rice cultivars for heavy metal tolerance and accumulation in grains, the present experiments were carried out to study the genotypic and environmental variation in toxic heavy metal concentrations of rice grains and to screen out some rice genotypes with low heavy metal concentrations in grains, focusing on the most widespread and severe toxic heavy metals in Southeast of China, including Cd, Pb, As and Cr. Meanwhile, the approaches of identifying the rice genotype with distinct Cd-tolerance, and accumulation, and the effects of Cd on rice growth and nutrition metabolism were also studied. The main results are as follows:1. The relationship between heavy metal concentrations in rice grains and DTPA-extracted concentrations in soilIn this study, a huge of rice and soil samples were taken from diverse locations of Zhejiang, China, and heavy metal concentrations were analyzed in two successive years, 2002 and 2003. The results showed that there was a great difference in heavy metal concentrations in soil (DTPA-extractable) and rice grains. Approximately 33.3%, 5.0% and 21.6% of rice samples in this study had Cd, Cr and Pb concentrations above0.1, 1.0 and 0.2 mg/kg, the AMVs, respectively, while no sample had Ni, As, Cu and Zn concentrations higher than the corresponding AMVs, indicating that the major issue in safe rice production locally is the risk posed by Cd, Cr and Pb contamination. The lower pH of soil (soil acidification) will lead to an increased phyto-availability of As, Cd, Ni and Fe. There were highly significant and positive correlation between heavy metal concentration in rice grains and that in soil (DTPA-extracted). However, there was a substantial difference among heavy metals in the predicting accuracy by using the regression equations, characterized by great different in coefficient of determination (R2). It was shown that the obtained regression models based on multiple-cultivars is plausible for the estimation of the potential levels of heavy metals in rice grains. Based on the established linear regression models, heavy metal concentrations in rice grains can be estimated according to the DTPA-extracted soil heavy metal levels. Thus, the critical Cd, Cr and Pb values in soil for safe rice production are 0.067 mg/kg, 1.474 mg/kg and 4.374 mg/kg, implying that the rice grown in the soil with Cd, Cr and Pb level higher than the critical values has a risk with grain Cd, Cr and Pb concentration beyond AMVs.2. Screening of rice genotypes with low Cd, Cr, Pb and As concentration in grainsSev...