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

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Abstract
Aluminum impairs uptake of Mg2+, but the mechanisms of this inhibition are not understood. The depletion technique was used to monitor net Mg2+ uptake from nutrient solution by intact, 23-day-old plants of ryegrass (Lolium multiflorum Lam., cv Gulf and Wilo). Activities of Mg2+ and monomeric Al species in nutrient solution were calculated and used as the basis for expressing the results. The kinetics of net Mg2+ absorption was resolved into (a) a transpiration-dependent uptake component, (b) a metabolically mediated, discontinuous saturable component that is Al3+ sensitive and p-chloromercuribenzene sulfonic acid (PCMBS) resistant, and (c) a linear, carbonyl cyanide m-chlorophenylhydrazone resistant, Al3+ 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 Mg2+ uptake, while aluminum at 6.6 micromolar Al3+ activity exerted competitive inhibition of net Mg2+ uptake at pH 4.2. The Al3+-induced effect was obvious after 30 minutes. Cultivar-specific ability to retain a higher affinity for Mg2+ by postulated transport proteins in the presence of Al3+ might be one of the mechanisms of differential Al tolerance among ryegrass cultivars.