Genotypic Difference In Cadmium, Zinc Accumulation And Cadmium Tolerance Between Hybrid Rice Cultivars

In China,Cd concentration in croplands and subsequent accumulation in crops has been increased as a result of soil polltuion due to human activities such as mining,emission from industry and application of sewage sludges,which is posing potential public health risk for human through dietary intake.Among the major staple crops,rice(Oryza sativa L.) is the particular food crop with high Cd uptake and accumulation in grains.Rice is produced predominantly in South China where the soils are either high in Cd mobility or deficient in Zn or both.However,for meeting the demand by the increasing population with less available arable lands,high-yielding hybrid rice or super rice cultivars with high Cd affinity are under extension,which may raise serious potential health concerns for Chinese subsistence diet farmers.However,there had been much evidences of differences in Cd accumulation in plants and plant tolerance to between rice species and genotypes.For food security,it is imperative to breed cultivars with low Cd accumulation in grains.Meanwhile,Cd tolerance and low Cd accumulation in grains may be used as a strategy for low Cd rice production.However,the mechanisms involved in Cd tolerance and grain accumulation has been not clearly demonstrated.In the dissertation,an experiment of 110 cultivars including super rice and common hybrid rice cultivars was grown on a single paddy soil(Fluventic Stagnic Anthrosol) with a neutral reaction and low in total Cd in 2006 in order to depict the difference in grain Cd and Zn between rice cultivars.Furthermore,a pot experiment with 2 selected cultivars Zhongzheyou 1(high grain Cd accumulation) and J196(high grain Cd accumulation) of hybrid rice was conducted for the whole growing period using acidic red earth derived paddy soil,a Typical Hapludults which was collected from Yingtan City,Jiangxi Province.The up-take, accumulation and distribution of Cd and Zn in plant organs of hybrid rice and the Cd partitioning at sub-cell level were analysed for genotype tolerance of Cd using transmission electron microscopy observation and differential characterization of the antioxidative enzyme elimination,cell ultrastructure,subcellular structure and PCs chelating in response to Cd stress.The main results were as follows: 1.Genotypic difference on Cd,Zn absorption and accumulation of hybrid riceGrain Cd and Zn concentrations of the totally 110 cultivars in the field experiment were determined with GFAAS and AAS respectively.Wider variation of grain Cd was found in a range of 0.004-0.057 mg·kg^-1 than of Zn in a range of 10.25-30.06 mg·kg^-1 among the cultivars.Higher Cd but lower Zn concentration in grains of super rice cultivars was observed than of the common hybrid ones.Although a significantly negative linear correlation of grain Cd with grain Zn was found for both super rice and common hybrid cultivars,with much higher slope for the hybrid cultivars than the reported non-hybrid cultivars.Using the RNI value recommended by CFSG,calculated potential risk of food Cd exposure with 'Zn hungry' through subsistence diet intake was prominent with all the studied 110 hybrid rice cultivars,possessing high potential health problems for rice production in South China using the super rice cultivars.Thus,breeding of genotypes of rice cultivars with low grain Cd and low Cd/Zn ratio is needed for rice production in soils of South China where high Cd bioavailability is prevalent with acidic red soils.In the pot experiment of Cd and Zn uptake and portioning in organs in maturity with Red earth derived paddy soil was analysed.Cd was spiked at 0(as the control) and 2.5 mg·kg^-1 of soil,respectively.Two cultivars of hybrid rice,Zhongzheyou 1(high grain Cd cultivar) and J196(low grain Cd cultivar),were harvested and the biomass,Cd and Zn contents,of different organs were determined.Significant genotype differences in Cd,Zn uptake and grain partitioning was observed.A phenomenon of intense Cd accumulation in grains by super rice and,thus,imposing a very high Cd exposure and Zn depletion risk(as several times as the acceptable daily intake,ADI) to subsistence-diet farmers was demonstrated.The low Cd cultivar J196 tended to hamper the up-taken Cd in root and stem while the super rice Zhongzheyou 1 promoted higher partitioning to grain.Furthermore, stronger Cd but weaker Zn transfering to grains by the high grain Cd accumulation cultivars than the low one could be considered as a major plant factor controlling high Cd uptake and accumulation,and the high Cd/Zn ratio in rice grain.2.Genotypie difference in Cd toleranceThe genotypic difference in the changes of three antioxidative enzymes(SOD,POD and CAT) activities and the MDA contents under cd spiking were characterized.The results showed Cd stress altered malondialdehyde(MDA) content and activities of superoxide dismutase(SOD),calase(CAT) and peroxidase(POD) in rice plant.The response to Cd stress differed in terms of MDA contents and antioxidant enzyme activity.Generally,the activities of SOD,CAT firstly ascend then droped with increased Cd level,in contrast to that of POD.While MDA contents was found higher in cultivar J196,but SOD,CAT,POD tend to increase in Zhongzheyou 1.The higher MDA and smaller antioxidant enzyme activity were found in J196,indicating that J196 was more sensitive to Cd stress than Zhongzheyou 1.The effect of Cd-spiking treatment on Cd content and nonprotein thiols(NPT) production in rice was also studied using the pot experiment The study demonstrated a phenomenon that Cd stress had significant inhibitory effects on NPT overproduction.The Cd uptake of rice was affected by the Cd-spiking treatment and the difference between genotypes.Furthermore,the Cd content was enhanced under Cd spiking,coinciding with the enhancement of NPT level in root.Root NPT contents of J196 was higher than those of Zhongzheyou-1,suggested that J196 has the higher Cd transfer potential from root to stem compared to Zhongzheyou-1.In an effort to understand the process of Cd bioaccumulation,subcelluar partitioning of Cd were characterized with subcelluar distribution of Cd in roots and leaves of rice.The results showed that there was a genotypic difference in cell Cd concentrations between two cultivars.Cd concentrations in different plant parts were as follows:root＞leaf;Cd was mainly distributed in the cell wall,about 52.6-83.2%,and less distributed in the fractions of cell soluble component with distribution rate of 3.0-10.1%.The cell wall distribution of Cd by J196 was higher than Zhongzheyou-1,showing a stronger cell wall compartmentation of Cd in J196 than in Zhongzheyou-1;Under Cd-spiking,Cd transfer from root cell soluble component transfering to shoot via xylem by Zhongzheyou-1 was promoted.The increased Cd distribution in cell soluble component by Zhongzheyou-1 compared to by J196 may be account for the lower Cd in stem and more Cd traslocation from shoot to grain via phloem.To examine the rice(Oryza sativa L.) genotypic difference in the alternation of cell ultrastructure,comparative electronic microscopic observation was undertaken of cells from the two hybrid rice cultivars(Zhongzheyou 1,high grain Cd accumulation and J196,low grain Cd accumulation) under Cd spiked at 0 and 25 mg·kg^-1.Under Cd spiking, Cd was presnet in binding to cell wall and accumulating in vacuole in rice root cell of both cultivars.The leaf cell ultrastructure of two rice cultivars presented the similar symptom: Chloroplasts and mitochondria were swelling up;and thylakoids vacuolized apparently. However,Cd toxicity symptom existed in the rice cultivar-J196,making mitochondrion mesenchyme dissolve and mitochondria was on the verge of disorganization.Tolerance of cell ultrastructure of rice cultivar-Zhongzheyou 1 was stronger than that of J196.And it demonstrated a phenomenon of mitochondria and chloroplast of rice mesophyll cells,which were the most sensitive organelles.In summary,the significant genotypic difference in the Cd and Zn accumulation and distribution of hybrid rice and Cd tolerance mechanisms existed between the two cultivars. Therefore,it is possible to develop the cultivars with high tolerance and low grain Cd/Zn accumulation in rice breeding forh high yield.It is also suggested the practical measures should be taken such as rational soil-cultivar combination and optimization dietary structure for controling the human Cd food exposure and guarantee Zn nutrition intake by diet.