Although total Zn concentration in the modelled rhizosphere solution was more than an order of magnitude higher compared to Cd (Table 22.2), modelling results show that a free Zn2+ pool was
lower by several %s compared with that of Cd2+ under the same conditions, dominating in acidic pHs (£4.5) similarly to Cd (Figs. 22.4 and 22.5). With pH rising, Zn2+ (and Cd2+) activity decreased, whereas the activities of ZnSO4 and Zn-oxalate stayed rather stable up to pH 7.5, and thereafter got depleted (Fig. 22.5b). Inorganic complex-ation of Zn, mostly with SO42- under low pHs, and CO42- under high pHs, dominated in basic conditions (pH > 8.0). LMW organo-complex-ation prevailed in slightly acidic to neutral pH (5.0-7.5) and that with HMW was negligible over the whole tested conditions (Fig. 22.5a).
However, compared to Cd, chloro-complexation with Zn barely present (<1%; data not shown).
Numerous Zn-in/organic ligand complexes (>20; data not shown) exist in the rhizosphere solution and are strongly pH dependent (Fig. 22.5). In slightly acid pHs (5.5-6.5), even in Zn-contaminated conditions (i.e. 7.5 mM Zn) Zn2+ typically accounts for around 40% of the soluble Zn fraction (Fig. 22.5a). Nutrient solution studies commonly suggest that free ionic metal form is the one most easily absorbed by plants, and Zn2+ is recognised as dominant plant-available Zn fraction, although there is a possibility it can enter the root cells complexed with certain organic (Broadley et al. 2007) and/or inorganic ligands. Accordingly, Lorenz et al. (1997) reported that for Zn and Cd free ionic concentrations in the soil solution from ten different contaminated soils did not predict concentrations in tested radish leaves/ tubers better than total Zn and Cd concentration in solution. One of the possible explanations is that Zn2+ is not the only form able to enter plant root. In the pH range from 3.5 to 7.5 activity of Zn2+ lineally decreased and thereafter sharply dropped due to inorganic complexation, mostly with CO3 forms (e.g. ZnCO3(aq) data not shown and Zn(CO()22-, Fig. 22.5b). These results confirm strong potential of Zn for inorganic compl-exation with pH increasing, especially in relatively low dissolved OM, and explain why Zn is the most common crop micronutrient deficiency under alkaline pHs (e.g. Rengel et al. 1999).
Intake of Zn and most other TEs (e.g. Cd, Cu) via consuming the food crops is their main route into the human body. Given that substantial areas of cultivated land worldwide are alkaline/saline (high pH/Na in saturated soil extract; Ondrasek et al. ((n press) and also Zn-deficient soils (e.g. Broadley et al. 2007), deficiencies of Zn in human populations affect up to three billion people. One of the promising solutions for improving Zn levels in edible crop tissues is soil application of inorganic Zn salts such as ZnSO( (Rengel et al. 1999). Accordingly, Khoshgoftar et al. (2004) showed that ZnSO( application to alkaline (pH ~8), salinised (up to 180 mM NaCl) and Cd-contaminated soil solution (0.01 mg Cd/L) may be a successful strategy, not only for Zn enrichment in cereal shoots (up to 90%), but also in other ways. They observed enhanced salt tolerance and increased dry matter of wheat shoots, as well as reduced shoot Cd accumulation (down to less than 50%) with Zn salt application. Similar antagonistic relationship between Zn (or Cu) and Cd in soil solution and their shoot phytoaccumu-lation under salinity has been recently confirmed in wheat genotypes by Khoshgoftarmanesh et al. (2006) and in radish by Ondrasek et al. submitted), what authors mainly contribute to the Zn-Cd competition for ligands as well as soil adsorption and root uptake sites.
As confirmed by the model (Figs. 22.4 and 22.5) between Zn and Cd species distribution and particular activities curves (e.g. their free cationic and carbonate-complexed forms), exist a quite coincidence as a consequences of their explained similar (physical/chemical) properties (Sect. 5.1). Relatively high presence of their most bioavail-able free forms over the most tested pHs may induce competition effects among Zn2+ and Cd2+ for ligands and binding sites in soil matrix, and thus influence their uptake and potentially toxicity. Furthermore, although the Zn-inorganic pool over the most pHs represented a relatively small (<20%) contribution in Zn speciation (Fig. 22.5a), under certain circumstances (e.g. excessive rhizo-sphere salinity), it may be of great importance for TEs mobility/uptake (Ondrasek et al. submitted).
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