Calcium and magnesium enhance arsenate rhizotoxicity and uptake in Triticum aestivum.

Cations such as calcium (Ca) and magnesium (Mg) alleviate toxicities of cationic toxicants and increase those of anionic toxicants such as arsenic (As) present as arsenate under aerobic conditions. Increasing evidence exists that these phenomena are related to the outer surface electrical potential (ψ 0°) of the root cell plasma membrane (PM). Short-term (48-h) nutrient culture experiments with wheat (Triticum aestivum L.) seedlings investigated Ca and Mg effects on arsenate rhizotoxicity and uptake. Increased Ca and Mg in solution from 0.2 to 3.5 mM equally reduced the negativity of ψ 0° from -45 to -15 mV. This increased arsenate activity at the PM surface from 0.3 to 0.9 µM at the same bulk concentration of 2.0 µM NaH(2) AsO(4) consequently increased arsenate uptake and rhizotoxicity. However, increased Mg was more effective than Ca in enhancing As uptake by roots, especially with the activity of As in the range of 0.5 to 1.5 µM in the bulk medium. This was explained by a modified electrostatic uptake model which indicated a mechanism other than through an effect on ψ 0°. Subcellular fractionation of roots indicated that > 95% of As was associated with cellular debris and heat-stable protein (HSP). The results of the present study show that Ca and Mg reduce the negativity of ψ 0°, increasing arsenate uptake and rhizotoxicity, and suggest that Mg is more efficient than Ca in enhancing the uptake of As due, in large part, to inducing greater As binding by peptides in the HSP fraction.