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Environmental and Stress Physiology

Competitive Al³⁺ Inhibition of Net Mg²⁺ Uptake by Intact Lolium multiflorum Roots ¹

I. Kinetics

Department of Agronomy, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge, Louisiana 70803

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.

¹ Approved for publication by the Director of the Louisiana Agricultural Experiment Station as manuscript No. 88-09-2498 (Hatch Project No. 1579).

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