Aluminum Toxicity and Tolerance in Plants

Abstract

Aluminum (Al) is the most abundant metal in the earths crust, comprising about 7% of its mass. Since many plant species are sensitive to micromolar concentrations of Al, the potential for soils to be A1 toxic is considerable. Fortunately, most of the A1 is bound by ligands or occurs in other nonphytotoxic forms such as aluminosilicates and precipitates. However, solubilization of this A1 is enhanced by low pH and A1 toxicity is a major factor limiting plant production on acid soils. Soil acidification can develop naturally when basic cations are leached from soils, but it can be accelerated by some farming practices and by acid rain (Kennedy, 1986). Strategies to maintain production on these soils include the application of lime to raise the soil pH and the use of plants that are tolerant of acid soils. Although A1 toxicity has been identified as a problem of acid soils for over 70 years, our knowledge about the primary sites of toxicity and the chain of events that finally affects plant growth remains largely speculative. In this paper we review recent progress that has been made in our understanding of A1 toxicity and the mechanisms of A1 tolerance in plants. ALUMINUM TOXlClTYThe most easily recognized symptom of A1 toxicity is the inhibition of root growth, and this has become a widely accepted measure of A1 stress in plants. In simple nutrient solutions micromolar concentrations of A1 can begin to inhibit root growth within 60 min. However, the inhibition of growth per se offers little information about the causes of stress that will either precede or coincide with changes in growth. To understand the mechanisms of A1 toxicity, it is essential to identify the primary sites involved, both anatomical and metabolic, being mindful that A1 could have diverse effects and act differently in different species. Severa1 reviews on AI toxicity are available (see Haug, 1984;Taylor, 1988; Rengel, 1992a); here we limit our discussion to the sites of A1 toxicity in higher plants and to the possible role of Ca in the primary mechanism of A1 toxicity.The Phytotoxic Form of AI Part of the difficulty of studying Al-related processes in plants can be attributed to the complex chemistry of A1 * Corresponding author; e-mail manny@pican.pi.csiro.au; fax 61-6-246-5000. 315 (Martin, 1988;Kinraide, 1991). A1 hydrolyzes in solution such that the trivalent A1 species, A13+, dominates in acid conditions (pH < 5), whereas the A1(OH)2+ and Al(OH)2+ species form as the pH increases. At near-neutra1 pH the solid phase Al(OH),, or gibbsite, occurs, whereas Al(OH),-, or aluminate, dominates in alkaline conditions. Many of these monomeric A1 cations bind to various organic and inorganic ligands such as POb-, S02-, F-, organic acids, proteins, and lipids. Equilibrium constants are available for many of these reactions and these can be used to predict the relative concentrations of the monomeric A1 species and other A1 compounds in solution. A very toxic polynuclear A1 species, Al,,, can also form when A1 solutions are partially neutralized with...

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