Acid rain and soil acidity
Approximately 30% (3950 million ha) of the world's land area is covered by acidic soil¹, and most of these soils are naturally acidic. Soils may become acidic for a number of reasons, and examples of anthropomorphic factors include the use of ammonia and amide containing fertilizers and the input of acidic precipitation. Nitrous and sulphurous oxides are emitted via the combustion of fossil fuels (coal, oil or gas), and by vehicle emissions. When nitrous and sulphurous oxides mix with precipitation, this may result in "acid rain" with a pH of 4.0-4.5. Normal rain has a pH of 5.0-5.6, however, in extreme cases the pH of acid rain may be as low as 2.0. Acid rain does not usually have an immediate effect on soil pH, but over a longer period, soil acidification may occur, especially in soils that have a low buffering capacity. The buffering capacity of soil is a measure of its ability to resist pH changes that would otherwise be induced by the addition of acids or alkalis².

Aluminium
Aluminium (Al) compounds comprise 7-8% by mass of the earth's surface. After oxygen and silicon (Si), Al is the third most common element in the earth's crust and therefore the most common metallic element in the lithosphere³.

Aluminium phytotoxicity is a major problem in both naturally occurring acidic soils, and soils affected by acidic precipitation⁴. Aluminium becomes available for plant uptake due to physical and chemical weathering of the aluminosilicates present in soil minerals². At neutral and mildly acidic pH values, Al remains in the form of insoluble aluminosilicates or oxides. Lowering soil pH increases the solubility of aluminium ions, with aluminium becoming available to plants when the soil is at a pH of less vhan 5.0. For a review, refer to Browne and Driscoll (1992)⁵.

Silicon
Silicon (Si) is the second most abundant element in the earth's crust constituting approximately 20 atomic percent of the lithosphere. It is an important part of many mineral soils and is found mainly in the form of solid mineral matter⁶. Silicon becomes available for plant uptake as silicic acid (Si(OH)₄), via the weathering processes outlined above. Silicic acid is usually present at a concentration of 0.1-0.6 mM in soil solution^7,8, however, may reach up to 1.2 mM⁹. Some plant taxa such as the Poaceae can contain comparatively large quantities of silica¹⁰, and up to 20% of Poaceae inflorescence bract dry weight is made up of Si atoms¹¹. It is more usual for plants to contain similar quantities of Si to those of calcium⁸. It is thought that Si is not usually essential for plant growth, but that it is beneficial⁸. The presence of silica has been associated with increased disease resistance, improved canopy structure, resistance to grazing, and tolerance to metal toxicity¹².

Conifers typically accumulate low levels of silica in their shoots and needles^3,13, however, some taxa accumulate levels similar to those found in dry land grasses. Data in Hodson et al. (1993)¹³, showed that Picea spp. were the heaviest accumulators, confirming early work by Klein and Geis (1978)¹⁴. Literature on the physiological effects of silica on conifers is limited. However, Emadian and Newton (1989)¹⁵ have found that silica treatment of Pinus taeda (loblolly pine) seedlings increased growth. This appeared to be due to silica enhancing cell expansion as growth was found to be associated with higher water and osmotic potentials, a greater symplastic water volume, increased tissue elasticity and reduced turgor.

Amelioration of aluminium toxicity by silicon in plants
Historically the remedy for amelioration of acidic soils and lakes has been application of lime. Lime increases pH and also adds base forming cations. Recently a number of researchers), have become increasingly interested in potential ameliorative properties of silica on growth of plants in acidic soils that contain available aluminium. My supervisor Dr. Martin Hodson has also examined amelioration in plants^{4,16,17,18,19}. There are a number of mechanisms that may account for the occurrence of amelioration, these include: Solution effects (co-deposition or inactivation in solution), co-deposition (as solids inside plants), cytoplasmic and enzyme protection, and indirect (e.g. by beneficial effects on uptake and transport of other minerals⁴.

No physiological work has been done on silicon and aluminium interactions in gymnosperms, thus making this a good taxon for investigations that will make an original contribution to knowledge. There are 610 conifer species distributed world wide²⁰. Conifers are vital to the economies of many countries around the world because they thrive in a wide range of conditions including less favorable climates and soils. Conifers also have value because they are vital components of biosphere ecology.

There are differences between angiospermae and gymnospermae in terms of their physiological responses to aluminium and silica. For example, conifers allow more aluminium into their needles than angiosperms, and tend to accumulate less silica than angiosperm families¹⁷. These differences may indicate in planta differences in terms of their response to aluminium and silica. This project will contribute to knowledge by investigating Al/Si interactions in the conifers.

References