Comparative toxicity of Al3+, Yb3+, and La3+ to root-tip cells differing in tolerance to high Al3+ in terms of ionic potentials of dehydrated trivalent cations

In order to analyze the binding pattern of Al3+ to and the mechanism(s) of Al3+ toxicity in root-tips, we compared the responses to trivalent metal ions (Al3+, Yb3+, and La3+) of root-tip cells differing in Al tolerance. Results were as follows: 1) The retardation of net root elongation for rice (Al-tolerant plant species) and pea (Al-sensitive plant species) was in the order of Yb3+ > La3+ > Al3+. Tolerance to Yb3+ or La3+ was the same both in rice and pea. 2) The uptake of each metal ion by the root-tip portion (1 cm) was in the order of La3+ > Yb3+ much greater than Al3+ for both plant species. The primary sites of localization were as follows:. epidermis and outer cortex cells for Al3+, endodermis, xylem, and to a lesser extent whole cortex cells for Yb3+, xylem and moderately in whole root-tip cells for La3+. By Al3+ treatment, the phosphate concentration in root-tip cells of pea considerably increased in the apoplast. 3) By Al3+ treatment, the normal permeability of the plasma membrane was mostly retained in the tip portion of rice root, while it was significantly impaired in the epidermis and outer cortex cells in the tip portion of pea root, By Yb3+ and La3+ treatments, the impairment of the permeability of the plasma membrane involved almost whole cells in both plant species. 4) The decreasing order of the calculated value of the ionic potential of the dehydrated ions was as follows: Al3+(56.1) > Yb3+(30.5) > La3+(24.7) > Ca2+(20.0). The conclusions were as follows: 1) The trivalent metal ions were bound to the negatively charged sites of the root-tip cells in a dehydrated form. The value of the ionic potential was negatively or positively correlated with the absorbability by whole roots or the localization in the outer cells, respectively, and finally determined the severity and the primary sites of the toxicity in root tips. 2) The outer localization of sites for the binding and the impairment by Al3+ treatment was ascribed to the largest value of the ionic potential for Al3+ among the trivalent metal ions tested. 3) In Al-sensitive plants, phosphate may be supplied from the proximal cortex cells and/or the above-ground tissues by phloem translocation accompanied by the uptake of Al3+.