Magnesium ameliorates aluminum rhizotoxicity in soybean by increasing citric acid production and exudation by roots

Superior effectiveness of Mg over Ca in alleviating Al rhizotoxicity cannot be accounted for by predicted changes in plasma membrane Al3+ activity, Th e influence of Ca and Mg on the production and secretion of citrate and mal ate, and on Al accumulation by roots was investigated with soybean genotype s Young and PI 416937 which differ in Al tolerance. In the presence of a so lution Al3+ activity of 4.6 muM, citrate and malate concentrations of tap r oot tips of both genotypes increased with additions of either Ca up to 3 mM or Mg up to 50 muM. Citrate efflux rate from roots exposed to Al was only enhanced with Mg additions and exceeded malate efflux rates by as much as 5 0-fold, Maximum citrate release occurred within 12 h after adding Mg to sol ution treatments. Adding 50 muM Mg to 0.8 mM CaSO4 solutions containing Al3 + activities up to 4.6 muM increased citrate concentration of tap root tips by 3- to 5-fold and root exudation of citrate by 6- to 9-fold. Plants trea ted with either 50 muM Mg or 3 mM Ca had similar reductions in Al accumulat ion at tap root tips, which coincided with the respective ability of these ions to relieve Al rhizotoxicity, Amelioration of Al inhibition of soybean root elongation by low concentrations of Mg in solution involved Mg-stimula ted production and efflux of citrate by roots.