pH值和Fe、Cd处理对水稻根际及根表Fe、Cd吸附行为的影响

通过营养液-蛭石联合培养试验,设置系列pH值(4.5—7.5)和Fe、Cd处理,研究不同pH值及Fe、Cd浓度对水稻和蛭石表面Fe、Cd吸附的影响。结果表明,不同pH值处理下的根际氧化还原电位和酸度不同,0.9 mg/L Cd处理下的根际氧化势低于0.5 mg/L Cd,50 mg/L Fe处理下的根际酸度高于30 mg/L Fe处理。根表吸附Fe、Cd组分和数量都受根际Eh、pH值制约,根表Fe、Cd吸附量在处理pH值6.0时最低,并分别在处理pH值7.5和处理pH值4.5达到最高。但根系表面对Fe、Cd的吸附机制与蛭石表面不同,蛭石吸附Fe主要为晶态Fe,占到总沉积Fe的73%—87%;水稻根表沉积Fe以非晶态Fe为主,占总沉积Fe的91%—95%;与处理pH值和根际Eh间有显著的相关性(蛭石晶态Fe:ppH=0.011、pEh=0.042;水稻根表非晶态Fe:ppH=0.050、pEh=0.004)。蛭石表面交换态Fe及交换态Cd与处理pH值和Eh间存在显著的相关性(pH值:pFe<0.001、pCd=0.009;Eh:pFe=0.016、pCd=0.002),而根表交换态Fe及交换态Cd仅与处理pH值间有显著的相关性(pFe=0.007,pCd=0.048)。不同Fe、Cd浓度处理对根际Eh、pH值的升降和根表Fe、Cd吸附均有影响。与对照相比,增Cd处理可以降低根际Eh和升高pH值,减少溶液Cd浓度并增加根表Cd吸附量;增Fe处理则可以升高根际Eh和降低pH值,增加溶液Fe、Cd浓度并减少根表Fe、Cd吸附量。这是水稻应对Fe、Cd浓度胁迫的生理反应之一。

Effects of pH, Fe and Cd concentrations on the Fe and Cd adsorption in the rhizosphere and on the root surfaces of rice

Cadmium (Cd) pollution in soils has become a serious environmental problem as this toxic metal may be easily absorbed by plants, leading to inhibition of plant growth and development.Rice is a global staple food crop, however, its quality may be threatened by the accumulation of Cd in grain.The risk to human health associated with Cd-tainted rice may be reduced by minimizing the Cd mobility from soil to rice root.So far, a large number of studies investigated the effects of soil pH, Eh and root iron plaque on Cd bioavailability.These factors may increase, decrease or have no effect on the Cd uptake by rice.However, the underlying mechanisms for these various effects still need further clarification.Thus, in this study, effects of pH (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 in rice rhizosphere differed among various treatments, and the oxidizing potential was lower at 0.9mg/L Cd than at 0.5mg/L Cd, while the acidity in the rhizosphere at 50 mg/L Fe was higher than at 30 mg/L Fe.Besides, the composition and amount of Fe/Cd on the rice root were controlled by the pH and Eh in the rhizosphere.The adsorbed Fe and Cd were lowest at pH 6.0 and respectively reached the maximum values at pH 4.5 and 7.5.However, the mechanisms of Fe/Cd adsorption on the rice root surface were different from those on the surfaces of vermiculite.On the vermiculite surfaces, crystallized Fe was the major form, accounting for 73% to 78% of total Fe adsorption, while on the rice root, noncrystalline Fe was the major form, accounting for 91% to 95%, both of which separately had significant relations with the treatment pH and Eh (crystallized Fe on the vermiculite surfaces: p_pH=0.011, p_Eh =0.042; noncrystalline Fe on the root surfaces: p_pH=0.050, p_Eh =0.004).Either EXC-Fe or EXC-Cd on vermiculite surfaces had significant relations with treatment pH and Eh (pH: p_Fe<0.001, p_Cd=0.009; Eh: p_Fe=0.016, p_Cd=0.002), while EXC-Fe and EXC-Cd on the surfaces of rice root just significantly correlated with pH (p_Fe=0.007, p_Cd=0.048).In addition, Fe and Cd treatments affected the adsorption of these two elements on the rice root.Compared with control, Cd addition decreased Eh and increased pH in the rhizosphere, leading to a decline of soluble Cd and a rise of Cd adsorbed on the root surfaces, while Fe application increased Eh and decreased pH in the rhizosphere, resulting in more soluble Fe/Cd and less adsorption of the two elements on the rice root surface.These were physiological reactions of rice to the stresses caused by excessive Fe and Cd.