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

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Abstract
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 aAl mono) were determined in nutrient solutions at 40 µM 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 mM). 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 aAl 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 aAl mono, which effectively removes the ionic strength effect of Ca. However, at Σ aAl mono ≥ 18 µM, 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 aAl mono than on the concentration of monomeric Al in solution. The critical Σ aAl mono, with increasing Ca concentration from 0.5 to 15 mM, varied from 12 to 17 µM for soybean, M for sunflower, M for subterranean clover, and M for alfalfa.