The forms and distribution of aluminum adsorbed onto maize and soybean roots

Abstract

Purpose The Al forms on maize and soybean roots were investigated to determine the main factors affecting the distribution of Al forms and its relationship with Al plant toxicity. Materials and methods Solution culture experiments were conducted to obtain the fresh roots of maize and soybean. KNO 3 , citric acid, and HCl were used to extract the exchangeable, complexed, and precipitated forms of Al on the roots. Results and discussion The complexed Al was higher than the exchangeable and precipitated Al. Root CECs of soybean and maize were 77 and 55 cmol kg −1 , and functional groups on the soybean roots (262.4 cmol kg −1 ) were greater than on maize roots (210.8 cmol kg −1 ), which resulted in more exchangeable and complexed Al on soybean roots than on maize roots, and was one of the reasons for the increased Al toxicity to soybean. The total and exchangeable Al were the highest on the plant root tips and decreased gradually with increasing distance from the tips. Ca 2+ , Mg 2+ , and NH 4 + cations reduced the exchangeable Al on the roots. Oxalate and malate also reduced the adsorption and absorption of Al by roots, and the effect of oxalate was greater than malate. Conclusions Higher exchangeable and complexed Al on plant roots led to increased Al plant toxicity. Ca 2+ , Mg 2+ , and NH 4 + and oxalate and malate can effectively alleviate Al plant toxicity.

Determination of critical pH and Al concentration of acidic Ultisols for wheat and canola crops

“…Thus, at the same exchangeable Al level, the Al saturation (percentage of exchangeable Al in CEC) was lower at the Ultisol from Anhui than that at the Ultisol from Hunan, while the base cation saturation (percentage of exchangeable base cation in CEC) was higher at the Ultisol from Anhui than that at the Ultisol from Hunan. Base cations can alleviate Al toxicity to plants (Meriño-Gergichevich et al, 2010;Liu and Xu, 2015). Therefore, the higher CEC and greater base cation saturation of the Ultisol from Anhui led to the lower critical values of soil pH and the higher exchangeable Al in the Ultisol from Anhui compared with that from Hunan.…”

Relative abundance of chemical forms of Cu(II) and Cd(II) on soybean roots as influenced by pH, cations and organic acids

“…The deprotonation of these acidic groups gives plant roots a negative charge and enables roots to adsorb heavy metals through electrostatic interaction. In addition, these acidic groups have a strong complexing ability and thus can adsorb heavy metals specifically through the formation of surface complexes with metal cations on the plant root surfaces33. The degree of dissociation of weak acids increases with increasing system pH, so that pH not only affects the charge characteristics of a root surface, but can also affect the hydrolysis of heavy metal cations in solution, thereby affecting their reaction with functional groups on the root surface.…”

“…Exogenous cations of Ca 2+ , Mg 2+ and NH 4 + effectively inhibit absorption of Mn(II) by barley and thus reduce Mn(II) toxicity to the plant35. Cations such as Ca 2+ , Mg 2+ and NH 4 + compete with Al 3+ for adsorption sites and inhibit adsorption of Al 3+ by the roots of soybean and maize33. Exogenous cations may also inhibit the adsorption of heavy metal cations by plant roots and affect the relative abundance of the chemical forms of heavy metals on plant roots.…”

“…Neutral salts were previously used to extract exchangeable heavy metals adsorbed on soils and exchangeable Al from plant roots1033. EDTA was used to extract heavy metal cations adsorbed specifically by soil organic matter and crop straw biochars in previous studies1213.…”

“…The mechanism for specific adsorption of Cu(II) and Cd(II) on soybean roots was formation of surface complexes of the metals with functional groups on the roots, similar to the specific adsorption of heavy metals by organic matter and biochars. HCl was used to extract the precipitates of Al hydroxides from plant roots in previous studies3336. Cu(II) and Cd(II) mainly formed the precipitates of hydroxides on soybean roots in present study.…”