INTERACTIVE EFFECTS OF AL-3+, H+, AND OTHER CATIONS ON ROOT ELONGATION CONSIDERED IN TERMS OF CELL-SURFACE ELECTRICAL POTENTIAL

The rhizotoxicities of Al3+ and of La3+ to wheat (Triticum aestivum L.) were similarly ameliorated by cations in the following order of effectiveness: H+ almost-equal-to C3+ > C2+ > C1+. Among tested cations of a given charge, ameliorative effectiveness was similar except that Ca2+ was slightly more effective than other divalent cations and H+ was much more effective than other monovalent cations. H+ rhizotoxicity was also ameliorated by cations in the order C3+ > C2+ > C1+. These results suggest a role for cell-surface electrical potential in the rhizotoxicity of Al3+, La3+, H+, and other toxic cations: negatively charged cell surfaces of the root accumulate the toxic cations, and amelioration is effected by treatments that reduce the negativity of the cell-surface electrical potential by charge screening or cation binding. Membrane-surface activities of free Al3+ or La3+ computed according to a Gouy-Chapman-Stern model correlated well with growth inhibition, which correlated only poorly with Al3+ or La3+ activities in the external medium. The similar responses of Al-intoxicated and La-intoxicated roots to ameliorative treatments provide evidence that Al3+, rather than AlOH2+ or Al(OH)2+, is the principal toxic species of mononuclear Al. Comparisons of the responses of Al-sensitive and Al-tolerant wheats to Al3+ and to La3+ did not support the hypothesis that varietal sensitivity to Al3+ is based upon differences in cell-surface electrical potential.