Aluminium/silicon interactions in conifers

It is widely accepted that acidic precipitation has been responsible for the die-back of trees in Western Europe and North America. The mechanisms responsible for the die-back are, however, not very well understood. It is known that free aluminium (Al) is mobilised in soils that are affected by acidic precipitation, and Al toxicity is probably a major factor in the die-back. Al toxicity in conifers has been attributed to the induction of calcium (Ca) or magnesium deficiency. However, the potential role of silicon (Si) in alleviating Al toxicity has received little attention. Here we review the literature concerning Al and Si in conifers, and present some new data documenting mineral deposition in their needles. Needles of Picea glauca, Pinus strobus, Larix laricina and Abies balsamea were analysed using X-ray microanalysis to determine mineral distribution in the leaf tissues. The distribution of Al, Ca and Si varied with the species, but Al was invariably codeposited with Si. This may be a mechanism for sequestering toxic Al and thus could represent a defence against Al toxicity. (C) 1999 Elsevier Science Inc. All rights reserved.