Growth Stimulation

Not surprisingly, aluminum addition has a growth stimulatory effect on aluminum accumulators. In tea, addition of aluminum and phosphorus increased phosphorus absorption and translocation as well as root and shoot growth (20,21). Similarly, the aluminum-accumulating shrub, Melastoma malabathricum L., exhibited increased growth of leaf, stem, and roots as well as increased phosphorus accumulation when aluminum was added to culture solutions (22).

Low levels of aluminum sometimes stimulate root and shoot growth of nonaccumulators. Turnip (Brassica rapa L. subsp. campestris A.R. Clapham) root lengths were increased by increasing aluminum levels up to 1.2 µM at pH 4.6 (23). Soybean (Glycine max Merr.) root elongation and ¹⁵NO₃^- uptake increased with increasing aluminum concentrations up to 10 µM, but were reduced when aluminum levels increased further to 44 µM (24). Shoot and root growth of Douglas fir (Pseudotsuga menziesii Franco) seedlings were stimulated by increasing aluminum levels up to 150 µM but were reduced at higher aluminum levels (25). Root elongation of an aluminum-tolerant race of silver birch (Betula pendula Roth) increased as solution aluminum increased up to 930 µM Al but then decreased at 1300 µM Al (26). Several researchers (23–25,27,28) have hypothesized that low levels of Al³⁺ ameliorated the toxic effects of H⁺ on cell walls, membranes, or nutrient transport, but aluminum-toxic effects predominated at higher aluminum levels.