Competitive Al³⁺ Inhibition of Net Mg²⁺ Uptake by Intact Lolium multiflorum Roots

Aluminum impairs uptake of Mg(2+), but the mechanisms of this inhibition are not understood. The depletion technique was used to monitor net Mg(2+) uptake from nutrient solution by intact, 23-day-old plants of ryegrass (Lolium multiflorum Lam., cv Gulf and Wilo). Activities of Mg(2+) and monomeric Al species in nutrient solution were calculated and used as the basis for expressing the results. The kinetics of net Mg(2+) absorption was resolved into (a) a transpiration-dependent uptake component, (b) a metabolically mediated, discontinuous saturable component that is Al(3+) sensitive and p-chloromercuribenzene sulfonic acid (PCMBS) resistant, and (c) a linear, carbonyl cyanide m-chlorophenylhydrazone resistant, Al(3+) sensitive component that might be a type of facilitated diffusion. Lowering the pH from 6.0 to 4.2 exerted a noncompetitive inhibition of net Mg(2+) uptake, while aluminum at 6.6 micromolar Al(3+) activity exerted competitive inhibition of net Mg(2+) uptake at pH 4.2. The Al(3+)-induced effect was obvious after 30 minutes. Cultivar-specific ability to retain a higher affinity for Mg(2+) by postulated transport proteins in the presence of Al(3+) might be one of the mechanisms of differential Al tolerance among ryegrass cultivars.