COMPARISON OF PLANT UPTAKE AND PLANT TOXICITY OF VARIOUS IONS IN WHEAT

The effects of varying solution concentrations of manganese (Mn), zinc (Zn), copper (Cu), boron (B), iron (Fe), gallium (Gal and lanthanum ( La) on plant chemical concentrations, plant uptake and plant toxicity were determined in wheat (Triticum aestivum L.) grown in a low ionic s trength (2.7 x 10(-3) M solution culture). Increasing the solution con centration of Mn, Zn, Cu, B, Fe, Ga and La increased plant concentrati ons 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 a verage, 3 times higher in the roots than the tops for Mn and Zn, 7 tim es for Cu, 9 times for Fe, 12 times for Ga and 15 times for La. In con trast, B concentrations were higher in the tops than the roots by, on average, 2 times. The estimated toxicity threshold (plant concentratio n 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 root s 0.2 mg g DM(-1) for B, 5 for Zn, 0.3 for Cu and 3 for La. Plant upta ke rates of the ions (as estimated by the slope of the relationship be tween solution ion concentrations and plant ion concentrations) was in the order B < Fe < Mn < La < Zn < Ga < Cu for the tops and B < Mn < F e < 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) mu M(-1)). P lant toxicity was estimated as the reciprocal of the plant concentrati on that reduced yield by 50% (change in relative yield per mg ion kg D M(-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 tox icity was high for a plant trace nutrient.