Effects Of Rhizospheric Chemical Conditions On Iron And Cadmium Adsorption On Root Surface And Their Uptke By Rice

Rice (Oryza sativa L.) is one of the most important staple cereals in a large part of the world, especially in China. It’s the daily staple for over half of the global human beings and in China the ratio reached nearly60%. Rice is regarded as a type of high-accumulation cereals. It’s therefore important to investigate the mechanisms contributing to the process of Cd enrichment and transporting in the soil-rice root system, which is not only beneficial for seeking ways to eliminate Cd from contaminated paddy soils, but also can guarantee our daily staple consumption safe. Recently, the reports about how to limit Cd entry into rice roots and shoots are not rare. Nevertheless, there is an inconsistent agreement in the relationship between the dissolved Cd in the rhizosphere and Cd bioavailability. The dissolved Cd in the rhizosphere not always correlated positively with Cd accumulation in rice, sometimes may correlate negatively. The rhizosphere plays an important role in altering cadmium (Cd) solubility in paddy soils and Cd accumulation in rice. More studies are needed to elucidate the mechanism controlling rice Cd solubility and bioavailability under different rhizospheric chemical conditions to explain the discrepancy of previous studies. Two vermiculite-solution experiments were conducted to investigate Cd deposition in the rhizosphere, Cd adsorption on rice root surface, Cd accumulation in rice roots, shoots and low weight molecular organic acids exudated by rice roots under different rhizospheric conditions.1. Cd/Fe adsorption on the vermiculite surface and root surface under different rhizospheric chemical conditionsEffects of pH value (4.5-7.5), Fe and Cd concentrations on Fe and Cd adsorption in the rhizosphere and on the root surfaces of rice were studied using nutrient solution and vermiculite culture. The results showed that Eh and pH value in rice rhizosphere differed among various treatments, and the oxidizing potential was lower at0.9mg L-1Cd level than at0.5mg L-1Cd, while the acidity in the rhizosphere at50mg L"1Fe level was much higher than at30mg L-1level. Besides, the composition and amount of Fe and Cd on the rice roots were controlled by the pH and Eh in the rhizosphere. The adsorbed Fe and Cd were lowest at pH value6.0and respectively reached the maximum values at pH value4.5and7.5. However, the mechanisms of Fe and Cd adsorption on the rice root surface were different from those on the vermiculite surfaces. On the vermiculite surfaces, crystallized Fe was the major form accounting for73%to78%of total Fe adsorption, while on the rice roots, noncrystalline Fe was the major form accounting for91%to95%, both of which separately had significant relations with the treatment pH and Eh (ppH=0.011*, pEh=0.042*; ppH=0.050*, pEh=0.004**). Either EXC-Fe or EXC-Cd on the vermiculite surfaces had significant relations with treatment pH and Eh (pFe=0.000***,pCd=0.009; pFe=0.016*, pCd=0.002**),while EXC-Fe and EXC-Cd on the surfaces of rice roots just significantly correlated with pH (pFe=0.007*,pcd=0.048). In addition, different Fe and Cd treatments affected the adsorption of these two elements on the rice roots. Compared with control, Cd addition could not only decrease Eh and increase pH in the rhizosphere but also lead to decline of soluble Cd and upheaval of Cd adsorbed on the root surfaces, while Fe application could increase Eh and decrease pH in the rhizosphere, resulting in less soluble Fe and Cd and their adsorption on the rice roots. These were physiological reactions of rice to the stresses caused by excessive Fe and Cd.2. Uptake of Fe and Cd by rice under different rhizospheric conditionsThe same nutrient solution and vermiculite culture was conducted to assess the effects of pH value, Eh and Fe concentration on Cd and Fe fractions on the vermiculite/root surfaces and their uptake by rice. Solution pH value was set from4.5to7.5with additions of Fe (30and50mg L-1) and Cd (0.5and0.9mg L-1). Results showed that at pH value5.5, Eh in the rice rhizosphere was higher whereas transpiration, Cd and Fe adsorption on the vermiculite/root surfaces and accumulation in rice were lower than the other pH value treatments. Cadmium addition enhanced pH value and lowered Eh in rice rhizosphere while Fe had the opposite effects. Compared with control, both Cd and Fe addition resulted in the decline of rhizosphere Cd and Fe solubility and bioavailability. Higher redox potential in the rice rhizosphere resulted in the decline of transpiration and Cd and Fe accumulation in the rice tissues, suggesting that the transfer of two elements from the nonrhizosphere to rice roots was depressed when the rhizosphere was more oxidized.3. Effects of rhizospheric conditions under Fe deficiency on Cd/Fe adsorption and uptake by rice Effects of rhizospheric conditions under Fe deficiency on Cd and Fe adsorption on the vermiculite/root surface and their uptake by rice were investigated using nutrient solution and vermiculite culture. Results showed that compared with normal Fe nutrition level, acidity and reducing condition in the rice rhizosphere under Fe deficiency was stronger, which might increase the dissolved Cd and Fe concentration. Compared with normal Fe nutrition level, noncrystalline Fe and EXC-Cd concentrations on the vermiculite surface under Fe deficiency were lower and Fe adsorption on rice root Fe plaque decreased, resulting in less competition with Cd adsorption on the vermiculite and root surface. The noncrystalline Cd concentration on the vermiculite surface increased while DCB-Cd concentration decreased. Both Cd2+and Fe2+uptake by rice roots were higher under Fe deficiency, which might be due to the decreasing secretion of low weight molecule organic acids (LWMOAs). Compared with the chelated Fe and Cd, the soluble Fe and Cd in the rhizosphere were more easily untaken by rice roots.4. Effects of rhizospheric conditions under different Fe forms on Cd/Fe adsorption and uptake by riceEffects of the rhizospheric conditions under different treatment Fe form on Cd and Fe adsorption on the vermiculite/root surface and their uptake by rice were investigated using nutrient solution and vermiculite culture. Results showed that LWMOAs played an important role during the process of Cd and Fe adsorption on the rice roots and their accumulation in rice roots/shoots. Compared with ferrous sulfate(FeSO4) and ferrous oxalate(FeC2O4) addition, the total LWMOAs concentration under ferric citrate(FeC6H5O7) addition was higher, which increased Cd and Fe accumulation in the rice roots/shoots. Under FeC2O4addition, the dissolved Fe concentration was increased leading to the inhibition a strategy-I Fe acquisition system, which might decrease the expressing of Cd-transporter OsIRTl and OsIRT2. Thus the Cd and Fe accumulation under FeC2O4addition was the lowest. Under FeSO4addition, the dissolved Fe2+was easily oxidized to Fe3+, mainly deposited on the vermiculite suface in the form of noncrystalline Fe, which decreased Cd accumulation in the rice roots.