The rhizotoxicities of Al3" and of La3" to wheat (Triticum aestivum L.) were similarly ameliorated by cations in the following order of effectiveness: H -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 whose toxicity has not been established. It may be impossible to differentiate formally the two hypotheses for H+ amelioration of Al toxicity or to establish the rhizotoxicity of all of the Al species (12), but in this article we shall argue that Al" is toxic and that Al3" toxicity, specifically, is ameliorated by H+ and other cations. The argument will rest upon new data that support the hypothesis that the negatively charged cell surfaces in the root accumulate A13+ and other toxic cations, and that amelioration is effected by treatments that reduce the negativity of the cellsurface electrical potential.Negative charges, located in the cell wall and on the plasma membrane, are carried on the carboxylate groups of cell-wall pectins, the residues of acidic amino acids in membrane proteins, and the phosphate groups of membrane phospholipids (16,19,23). The charges on the cell surface create electrical potential gradients that interact with the distribution of ions (5,18,19). Salts in the bathing medium reduce the negative surface potential in two ways: by cation binding and by charge screening. Because of the binding of H+ with carboxylate, phosphate, and amino groups, charge reversal of the plasma membrane surface can occur as the pH there drops below 4 (22). Divalent and polyvalent cations can also bind, and the latter can cause charge reversals at the membrane surface (1, 21). Charge screening occurs because coulombic attractions concentrate cations around the cell-surface negative charges. The effectiveness of the cations in charge screening increases with cation valence according to basic electrostatic models (5,18,19).Amelioration of Al toxicity by H+ and other cations can be interpreted as evidence of the influence of cell surface charge on Al toxicity, but another line of evidence also suggests the same conclusion. Several reports indicate that within closely related taxa, higher varietal sensitivity to Al corresponds to higher varietal cation exchange capacity of whole roots (8, 27, and

The aluminium (III) released from soil minerals to the soil solution under acid conditions may appear as hexaaquaaluminium (Al(H20)36 + , or A13+ for convenience) or may react with available ligands to form additional chemical species. That one or more of these species is rhizotoxic (inhibitory to root elongation) has been known for many decades, but the identity of the toxic species remains problematical for the following reasons. 1. Several A1 species coexist in solution so individual species cannot be investigated in isolation, even in artificial culture media. 2. The activities of individual species must be calculated from equilibrium data that may be uncertain. 3. The unexpected or undetected appearance of the extremely toxic triskaidekaaluminium (AIO4AlIz(OH)z4(H20)~ 2 or A113 ) may cause misattribution of toxicity to other species, especially to mononuclear hydroxy-Al. 4. If H + ameliorates Al 3+ toxicity, or vice versa, then mononuclear hydroxy-Al may appear to be toxic when it is not. 5. The identity and activities of the AI species contacting the cell surfaces are uncertain because of the H + currents through the root surface and because of surface charges. This article considers the implications of these problems for good experimental designs and critically evaluates current information regarding the relative toxicities of selected A1 species. It is concluded that polycationic A1 (charge >2) is rhizotoxic as are other polyvalent cations.

Ca2؉ in rooting medium is essential for root elongation, even in the absence of added toxicants. In the presence of rhizotoxic levels of Al 3؉ , H ؉ , or Na ؉ (or other cationic toxicants), supplementation of the medium with higher levels of Ca 2؉ alleviates growth inhibition. Experiments to determine the mechanisms of alleviation entailed measurements of root elongation in wheat (Triticum aestivum L. cv Scout 66) seedlings in controlled medium. A Gouy-ChapmanStern model was used to compute the electrical potentials and the activities of ions at the root-cell plasma membrane surfaces. Analysis of root elongation relative to the computed surface activities of ions revealed three separate mechanisms of Ca 2؉ alleviation. Mechanism I is the displacement of cell-surface toxicant by the Ca 2؉ -induced reduction in cell-surface negativity. Mechanism II is the restoration of Ca 2؉ at the cell surface if the surface Ca 2؉ has been reduced by the toxicant to growth-limiting levels. Mechanism III is the collective ameliorative effect of Ca 2؉ beyond mechanisms I and II, and may involve Ca 2؉ -toxicant interactions at the cell surface other than the displacement interactions of mechanisms I and II. Mechanism I operated in the alleviation of all of the tested toxicities; mechanism II was generally a minor component of alleviation; and mechanism III was toxicant specific and operated strongly in the alleviation of Na ؉ toxicity, moderately in the alleviation of H ؉ toxicity, and not at all in the alleviation of Al 3؉ toxicity.Ca 2ϩ in rooting medium is essential for root elongation, even in the absence of added toxicants (Hanson, 1984; present study). In the presence of rhizotoxic levels of Al 3ϩ , H ϩ , or Na ϩ (or other cationic toxicants), supplementation of the medium with higher levels of Ca 2ϩ alleviates growth inhibition (LaHaye and Epstein, 1969; Hanson, 1984; Kinraide and Parker, 1987; Yan et al., 1992; Yermiyahu et al., 1997a; present study). Several separate mechanisms for Ca 2ϩ alleviation of mineral toxicity have been proposed. A commonly proposed, or at least implied, mechanism is the restoration of toxicant-displaced Ca 2ϩ (LaHaye and Epstein, 1969; Hanson, 1984; Cramer et al., 1985; Lynch et al., 1987; Shortle and Smith, 1988; Schulze, 1989; Läuchli, 1990; Rengel, 1992; Yan et al., 1992; Yermiyahu et al., 1997a). The present investigation attempts to identify the mechanisms by which Ca 2ϩ alleviates mineral rhizotoxicity and to determine the relative importance of these mechanisms.Several previous investigations have demonstrated the importance of 0 in root-mineral interactions (Wagatsuma and Akiba, 1989; Suhayda et al., 1990; Kinraide, 1994; Yermiyahu et al., 1997a). Because PM surfaces are usually negatively charged (Wagatsuma and Akiba, 1989), the ion concentrations at root PM surfaces can differ significantly from the concentrations in the rooting medium. Treatments that reduce PM surface negativity, such as increases in the ionic strength or decreases in the pH of the rooting medium, reduce the effe...

Aluminum is a major constituent of most soils and limits crop productivity in many regions. Amelioration is of theoretical as well as practal interest because understanding amelioration may contribute to an understanding of the mechanisms of toxicity. In the experiments reported here 2-day-old wheat (Triticum aestivum L. cv Tyler) seedlings with 15-millimeter roots were transferred to solutions containing OA millimolar CaC12 at pH 4.3 variously supplemented with AlG3 and additionl amounts of a chloride salt. Root lengths, measured after 2 days in the test solutions, were a function of both Al activity and the cation activity of the added salt. Percent inhibition = 100 JAl3'J/(JAV3J + K, + alCQ') where 1A31+ is the activity of Al' expressed in micromolar, IC) is the activity of the added cation expressed in millimolar, and K. (= 1.2 micromolar) is the JAI'3+ required for 50% inhibition in the absence of added salt. For Ca2+, Mg2, and Na the values of a were 2.4, 1.6, and 0.011, respectively, and the values for v were 1.5, 1. (3,5, 25) describe high correlations between growth reductions and the activity of A13+ or total mononuclear species, but reports supporting a direct ameliorative effect of cations are few.Ca is a well known ameliorant that has been observed many times to relieve Al toxicity (2,3,7,18,26), but in only a few cases have the investigators attempted to demonstrate that some or part of the amelioration can be attributed to one or the other of the effects enumerated above (3, 18). Mg is also an effective ameliorant (2,10,18,26), and a recent study demonstrated that both Ca and Mg salts were much more effective ameliorants of Al toxicity than K and Na salts at comparable ionic strengths (18). Thus, Ca and Mg relieved toxicity beyond that attributable to ionic-strength effects, and a physiological interaction between Al and the cations was indicated. The only report ofamelioration by monovalent cations is the dissertation of Ali (2) who concluded that K and Na were equally ameliorative but less effective than Ca and Mg which were equal to each other.We have undertaken a study of cation amelioration that employs a recently published assay for Al phytotoxicity (18). The advantages of the assay are speed, simplicity, sensitivity to low Al levels, the elimination of nutrient requirements other than Ca, and a good correspondence to other assay procedures. The main advantage is that the simplicity of the basal medium (0.4 mM CaCl2 adjusted to pH 4.3 with HCI) permits a more precise computation of Al speciation than do more complete nutrient media. The objectives of the current study were (a)