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Three Mechanisms for the Calcium Alleviation
of Mineral
Toxicities
Thomas B. Kinraide*
Appalachian Soil and Water Conservation Research Laboratory,
Agricultural Research Service, United States Department of Agriculture,
Beaver, West Virginia 25813-0400
Ca2+ in rooting medium is
essential for root elongation, even in the absence of added toxicants.
In the presence of rhizotoxic levels of Al3+,
H+, or Na+ (or other cationic toxicants),
supplementation of the medium with higher levels of Ca2+
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-Chapman-Stern 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 Ca2+ alleviation. Mechanism I is the displacement of
cell-surface toxicant by the Ca2+-induced reduction in
cell-surface negativity. Mechanism II is the restoration of
Ca2+ at the cell surface if the surface Ca2+
has been reduced by the toxicant to growth-limiting levels. Mechanism III is the collective ameliorative effect of Ca2+ beyond
mechanisms I and II, and may involve Ca2+-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 Al3+ toxicity.
*
E-mail kinraide{at}asrr.arsusda.gov; fax 1-304-256-2921.
Plant Physiol. (1998) 118: 513-520
Copyright Clearance Center: 0032-0889/98/118//08
© 1998 American Society of Plant Physiologists
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