Competitive Al Inhibition of Net Mg Uptake by Intact Lolium multiflorum Roots : II. Plant Age Effects.

Rhizotoxicity of Al is more pronounced in younger plants. Effects of Al on nutrient uptake by plants of different age are poorly understood. The depletion technique was used to monitor net Mg(2+) uptake from nutrient solutions by intact 15- and 35-day-old plants of two ryegrass (Lolium multiflorum Lam.) cultivars. Lowering the pH from 6.0 to 4.2 decreased the maximum net ion influx without affecting K(m). Aluminum at 6.6 micromolar Al(3+) activity increased K(m) indicating competitive inhibition. The effects of pH and 6.6 micromolar Al(3+) on net Mg(2+) uptake were much larger in 15- than in 35-day-old plants. Aluminum at 26 micromolar Al(3+) activity competitively inhibited net Mg(2+) uptake by 35-day-old plants, while causing time- and external Mg(2+) activity-dependent net Mg(2+) efflux from 15-day-old plants. The equilibrium constant (K(i)) of a reversible combination of postulated plasmalemma Mg(2+) transporter and Al(3+) was calculated to be 2 and 5 micromolar Al(3+) activity for 15-day-old plants of Wilo and Gulf ryegrass, respectively, and 21 micromolar Al(3+) activity for 35-day-old plants of both cultivars. The Al(3+)-mediated increase in K(m) was larger for 15-day-old plants of the Al-sensitive cultivar ;Wilo' than of the more Al-tolerant cultivar ;Gulf,' while Al(3+) affected 35-day-old plants of both cultivars to the same extent.