COMPETITIVE AL-3+ INHIBITION OF NET MG-2+ UPTAKE BY INTACT LOLIUM-MULTIFLORUM ROOTS .2. 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 Mg2+ 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 Km. Aluminum at 6.6 micromolar Al3+ activity increased Km indicating competitive inhibition. The effects of pH and 6.6 micromolar Al3+ on net Mg2+ uptake were much larger in 15- than in 35-day-old plants. Aluminum at 26 micromolar Al3+ activity competitively inhibited net Mg2+ uptake by 35-day-old plants, while causing time- and external Mg2+ activity-dependent net Mg2+ efflux from 15-day-old plants. The equilibrium constant (Ki) of a reversible combination of postulated plasmalemma Mg2+ transporter and Al3+ was calculated to be 2 and 5 micromolar Al3+ activity for 15-day-old plants of Wilo and Gulf ryegrass, respectively, and 21 micromolar Al3+ activity for 35-day-old plants of both cultivars. The Al3+-mediated increase in Km was larger for 15-day-old plants of the Al-sensitive cultivar 'Wilo' than of the more Al-tolerant cultivar 'Gulf,' while Al3+ affected 35-day-old plants of both cultivars to the same extent.