Effects of Phosphorus/Aluminum Molar Ratio and Calcium Concentration on Plant Response to Aluminum Toxicity

Total aluminum (Al) in solution often comprises both monomeric and polymeric Al species. Concentrations of total and monomeric Al and Σ activities of monomeric Al species (Σ a Al mono ) were determined in nutrient solutions at 40 µ M Al with varying P/Al molar ratios (0, 0.02, 0.2, 2.0 and 4.0) and calcium (Ca) concentrations (0.5, 1.5, 5.0, 15 m M ). Effects of these treatments on root elongation of soybean [ Glycine max (L.) Merr.], sunflower ( Helianthus annuus L.), subterranean clover ( Trifolium subterraneum L.), and alfalfa ( Medicago sativa L.) were determined. Concentrations of total and monomeric Al in solution decreased with increasing P/Al molar ratio. Ca concentration had no significant effect on concentration of total or monomeric Al, but significantly affected the Σ a Al mono in solution. Root elongation of the plant species increased with increasing P/Al molar ratio and Ca concentration. The increase in root elongation with increase in P/Al molar ratio was associated with a decrease in monomeric Al in solution. The beneficial effect of Ca on root elongation was evident even when Al in the root environment was expressed as Σ a Al mono , which effectively removes the ionic strength effect of Ca. However, at Σ a Al mono ≥ 18 µ M , increasing Ca concentration failed to improve root elongation. The range of critical Al values for a 50% reduction in root elongation with varying Ca concentration was much narrower if based on Σ a Al mono than on the concentration of monomeric Al in solution. The critical Σ a Al mono , with increasing Ca concentration from 0.5 to 15 m M , varied from 12 to 17 µ M for soybean, <8 to 16 µ M for sunflower, <7 to 15 µ M for subterranean clover, and <5 to 10 µ M for alfalfa.