Comparison of plant uptake and plant toxicity of various ions in wheat

Abstract

The effects of varying solution concentrations of manganese (Mn), zinc (Zn), copper (Cu), boron (B), iron (Fe), gallium (Ga) and lanthanum (La) on plant chemical concentrations, plant uptake and plant toxicity were determined in wheat (Triticum aestivum L.) grown in a low ionic strength (2.7×10⁻³ M solution culture). Increasing the solution concentration of Mn, Zn, Cu, B, Fe, Ga and La increased plant concentrations of that ion. Asymptotic maximum plant concentrations were reached for Zn (10 mg kg DM⁻¹ in the roots), Ga (2 mg kg DM⁻¹ in the tops and 18 mg kg DM⁻¹ in the roots) and La (0.4 mg kg DM⁻¹ in the tops and 4 mg kg DM⁻¹ in the roots). Plant ion concentrations were, on average, 3 times higher in the roots than the tops for Mn and Zn, 7 times for Cu, 9 times for Fe, 12 times for Ga and 15 times for La. In contrast, B concentrations were higher in the tops than the roots by, on average, 2 times. The estimated toxicity threshold (plant concentration at which a rapid decrease in yield occurred) in the tops was 0.4 mg g DM⁻¹ for B, 2 for Zn, 0.075 for Cu and 0.09 for La and in the roots 0.2 mg g DM⁻¹ for B, 5 for Zn, 0.3 for Cu and 3 for La. Plant uptake rates of the ions (as estimated by the slope of the relationship between solution ion concentrations and plant ion concentrations) was in the order B<Fe<Mn<La<Zn<Ga<Cu for the tops and B<Mn<Fe<Zn<La<Cu<Ga for the roots. In the roots, the uptake rates of La, Cu and Ga was exceptionally high (> 250 mg kg DM⁻¹ μM ⁻¹). Plant toxicity was estimated as the reciprocal of the plant concentration that reduced yield by 50% (change in relative yield per mg ion kg DM⁻¹). The plant toxicity of the ions tested was in the order Mn<Zn<B<Fe=Ga<La<Cu in the tops and Mn<Ga<Zn<Fe=La<Cu<B in the roots. Copper was unusual in that plant uptake and plant toxicity was high for a plant trace nutrient.