Aluminum-Accumulating Plants

Relative to aluminum accumulation, there appears to be two groups of plant species: aluminum excluders and aluminum accumulators (8). Most plant species, particularly crop plants, are aluminum excluders. Aluminum contents in most herbaceous plants averaged 200 mg kg^-1 in leaves (Hutchinson, cited in [9]). Chenery (10,11) analyzed leaves of various species of monocots and dicots for aluminum content, and defined aluminum accumulators as those plants with 1000 mg Al kg^-1 or greater in leaves. Aluminum accumulation appears to be a primitive character, found frequently among perennial, woody species in tropical rain forests (9,12).

Masunaga et al. (13) studied 65 tree species and 12 unidentified species considered to be aluminum accumulators in a tropical rain forest in West Sumatra and suggested that aluminum accumulators be divided further into two groups: (a) those with aluminum concentrations lower than 3000 mg kg^-1; and (b) those with higher aluminum concentrations. For trees with foliar aluminum concentrations greater than 3000 mg kg^-1, positive correlations were noted between aluminum concentrations and phosphorus or silicon concentrations in leaves.

Although Chenery (11) did not consider gymnosperms to be aluminum accumulators, Truman et al. (14) proposed that most Pinus species are facultative aluminum accumulators. In Australia, values of foliar aluminum ranged from 321 to 1412 mg kg^-1 for Monterey pine (Pinus radiata D. Don), 51 to 1251 mg kg^-1 for slash pine (Pinus elliotii Engelm.), and 643 to 2173 mg kg^-1 for loblolly pine (Pinus taeda L.) (15). In addition, foliar aluminum concentrations≥1000 mg kg^-1 were reported in Monterey pine and black pine (Pinus nigra J.F. Arnold) grown in nutrient solutions containing aluminum (14,16,17).

Tea (Camellia sinensis Kuntze) is one crop plant considered to be an aluminum accumulator, with aluminum concentrations of 30,700 mg kg^-1 in mature leaves, but much lower concentrations of only 600 mg kg^-1 in young leaves (18). Most of the aluminum was localized in the cell walls of the epidermis of mature leaves (18).

Another well-known aluminum-accumulating plant is hydrangea (Hydrangea macrophylla Ser.), which has blue-colored sepals when the plant is grown in acidic soils and red-colored sepals when grown in alkaline soils. The blue color of hydrangea sepals is due to aluminum complexing with the anthocyanin, delphinidin 3-glucoside, and the copigment, 3-caffeoylquinic acid (19). Two excellent reviews of aluminum accumulators are by Jansen et al. (9) and Watanabe and Osaki (8). Possible mechanisms of aluminum tolerance will be discussed in later sections.