This review focuses on the mechanisms underlying aluminum (Al) toxicity in trees. The major topics discussed include the uptake and localization of Al, effects of Al on growth and composition, factors determining the response to Al, proposed mechanisms of Al resistance, and the occurrence of Al phytotoxicity under field conditions.

The effect of aluminum (Al) on the growth of sugar maple (Acersaccharum Marsh.) was studied in two experiments in nutrient solution containing from 0 to 2000 μM Al at pH 4. Visible symptoms of injury to shoots or roots were not observed in either experiment. Reductions in seedling shoot size at the 1000, 1500, and 2000 μM concentrations were readily apparent; however, root biomass for these same concentrations did not differ significantly from the pH 4 controls. A plot of the relative total dry matter accumulation versus Al in solution showed that above 600 μM Al total seedling biomass declined exponentially. Of the mineral elements examined in these experiments, Ca was the most severely affected by increasing Al in solution. Even at 100 μM Al reductions in leaf, stem, and root Ca content were 17, 13, and 50% of the pH 4 controls, respectively. Some stimulation of growth and slightly higher P and K tissue concentrations were observed at low (100–500 μM) Al concentrations. At higher Al levels, P, K, and Mg were also reduced below the tissue content of the controls. The Al critical toxicity level for a 20% reduction in total seedling biomass was found to be 137 μg Al g dry weight−1 for newly expanded leaves.

No abstract

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Unexplained declines in the growth of both red spruce (Picea rubens Sarg.) and Ioblolly pine (Pinus taeda L.) have been reported within their natural ranges recently. The possible role of AI phytotoxicity as a causal agent in these declines has been studied extensively. Results of experiments with seedlings grown in solution, sand, and soil indicate minimal involvement of Al in the decline of loblolly pine. However, both controlled studies and field data indicate that reductions in tissue Ca and Mg may occur at AI concentrations well below those causing direct injury. Thus, AI may be involved in reductions in pine growth through interference with nutrient uptake and translocation. Several independent studies demonstrate that AI does directly affect growth of red spruce seedlings at AI concentrations of <0.25 raM, a concentration that might approach the range of soil solution AI concentrations measured in native red spruce stands in the Appalachian Mountains (0.1-0.28 raM). In these seedling studies, concentration of Ca and Mg in red spruce roots and foliage were also markedly reduced by relatively low solution AI concentrations. Thus, a contributing role for AI phytotoxicity in red spruce decline, through both direct biomass reduction and indirect effects due to interference with nutrient uptake, appears plausible. D.