A Biotic Model Predicting Acute Copper Toxicity for Barley (Hordeum vulgare):influence of calcium, magnesium, sodium, potassium and PH

Abstract

The effects of selected cations and pH on the acute toxicity of copper (Cu) to barley root elongation were investigated to develop an appropriate biotic ligand model (BLM). The results showed that increasing activities of Mg²⁺ and Ca²⁺, but not Na⁺ and K⁺, linearly increased the EC50 (as Cu²⁺ activity). Unchanged EC50 at solution pH less than 6.5 and sharply decreased EC50 with increasing of solution pH when greater than 6.5 can be explained by toxicity of the CuHCO⁺₃, CuCO₃(aq) and CuOH⁺ complexes. Conditional binding constants were obtained for the binding of Cu²⁺, CuHCO₃⁺, CuCO₃(aq), CuOH⁺,Mg²⁺ and Ca²⁺ with biotic ligand: logK_CuBL 6.33, logK_CuHCO3BL 5.71, logK_CuCO3BL 5.70, logK_CuOHBL 6.39, logK_MgBL2.92 and logK_CaBL1.96. Using the estimated constants, a BLM was successfully developed to predict Cu toxicity to barley root elongation as a function of solution characteristics.

Introduction

Copper (Cu) is an essential element for plants and animals, but when present in excess, it can exert toxic effects. Copper concentration may be elevated in soils because of (i) widespread use of pesticides, (ii) land application of sewage sludge and Cu-rich animal manures, and (iii) mining and smelting activities (Baker and Senft, 1995). Therefore, environmental quality criteria are developed for metals such as Cu. Current soil quality criteria and risk assessment of metals in soils are based on total or soluble metal concentrations. However, a large body of evidence indicates that both total and soluble metals are unrelated directly to ecotoxicity (Smolders et al., 2004). Toxicity is affected by the bioavailability of the metal in the soil and concentrations of other elements which moderate toxicity responses.

Recently, a biotic ligand model (BLM) has been proposed as a tool to evaluate quantitatively how water chemistry affects the speciation and biological availability of metals in aquatic systems (Di Toro et al., 2001). The most important assumption of the BLM is that metal toxicity is caused by free metal ions reacting with bio-logical binding sites. The cations of H⁺,Ca2⁺,Mg²⁺,K⁺and Na⁺ might compete with metal ions for these binding sites and decrease the toxicity of the free metal ions. The BLM concept, developed originally for aquatic metal toxicity (Di Toro et al.,2001; De Schamphelaere and Janssen, 2002), has recently attracted increasing attention for predicting metal toxicity in terrestrial systems (Antunes et al., 2006; Thakali et al., 2006a,b; Lock et al.,2007a,b,c; Li et al., 2009; Wang et al., 2010). In most BLMs developed for metals, the linear relationship between H⁺ and median effective concentration (EC50) suggests that competition exists between H⁺ and free ions of the metals on binding sites of biotic ligands (Erickson et al., 1996; Meyer et al., 1999; Thakali et al., 2006a, b). However, it has been reported that H⁺ activity had no significant effect on metal toxicity (De Schamphelaere and Janssen, 2002; Lock et al., 2007b, c), therefore, it was unjustified to incorporate H⁺ competition into the BLM. For example, proton competition with Cu²⁺ bound to biotic ligand (BL) was used to describe the effects of pH on Cu toxicity to barley in the terrestrial BLM for soils with pH≤7 (Thakali et al., 2006a,b), but it was unable to explain effects of pH on toxicity of Cu to barley in a wide range of pH (Lock et al., 2007a). The pH effects on toxicity of Cu to barley may be due to the change of species of Cu in solutions. In addition to Cu²⁺,other inorganic species (such as CuCO³(aq) and CuOH⁺) are probably toxic to organisms, especially in a medium with relatively high pH (Niyogi and Wood, 2004). The BLMs developed recently for both aquatic and terrestrial ecosystems assumed that the effect of pH on metal toxicity at relatively high pH was a speciation effect, but without any significant competition between protons and metal ions (De Schamphelaere and Janssen, 2002; Markich et al., 2003; Li et al., 2009; Wang et al., 2010). To our knowledge, no data are available to assess the possible toxicity of other inorganic species of Cu to plants in aquatic culture, except for CuOH⁺ cation (Wang et al., 2009). The present study therefore aimed to investigate the effect of H⁺ competition on the toxicity of Cu²⁺ to barley root elongation across a wide range of pH values and to determine if other Cu species are implicated in toxicity responses. Further, the effects of Ca²⁺,Mg²⁺,Na⁺ and K⁺ on Cu toxicity to barley root elongation were determined across a wide range of ion levels in order to obtain conditional binding constants for Cu²⁺ as well as other cations with BLs. Finally, a BLM was established that can be used to predict Cu toxicity to barley for a broad range of solution characteristics.