High levels of aluminium in soils are a particular problem in the tropics (Sect. 10.2.4.1) but also globally develop severe adverse effects in agriculture and forestry. Therefore the literature on aluminium/plant interactions is immense. To develop a background here for the aluminium relations of plants in the tropics key points were extracted from ca. 300 references to illustrate potential damage (Sect. 10.2.4.2) and responses of defence (Sect. 10.2.4.3). (These references cannot be cited here; a few reviews are the following: Luttge and Clarkson 1992; Delhaize and Ryan 1995; Kochian 1995; Rengel 1996; Ma 2000; Ciamporova 2002.)
Fig. 10.24A-D Brocchinia reducta in a wet marshy savanna (A), with the typical bromeliad inflorescence (B), with the wax on the adaxial leaf surface that should prevent the escape of animals fallen into the bromeliad tank (C), and a tank cut open to show the putrefying mass of animals at the bottom (D). (Gran Sabana, Venezuela; February 1989)
10.2.4.1 The Aluminium Load in Tropical Soils
Clay minerals typical for savanna soils are:
They have low cation-exchange capacity (CEC) and low water storage capacity. Ferralization is a frequent process where bases and silicious acid are leached, leaving aluminium and iron oxides (Al2O3, Fe2O3). Thus, ferralitic soils always have very high concentrations of Al3+. A special formation is the "arecife" in the Llanos of Venezuela (see Sect. 10.1.2.2 and Fig. 10.7). Iron oxide is precipitated at high groundwater table level in the young alluvial sediments forming these soils, such that gravel, sand and clay are solidified to a hard crust of a thickness of 1 - 3 m. It generally lies at a depth of 30 - 80 cm but may also be lower or closer to the surface (Fig. 10.7). Soils of the Brazilian cerrados contain between 75 and 360 ppm Al3+ (Eiten 1972).
By comparison, high Al-load in acidifying soils has also been observed in the temperate zone. It is thought to be one of the possible reasons for forest decline, and here, the equilibrium soil solution contains up to 20-40 ppm Al.
Damage of plants by high aluminium levels in the substratum occurs at multiple levels and is due to both extracellular effects on the surface of roots and cells and intracellular effects after uptake and translocation of aluminium in the plants. The major interactions can be listed as follows.
• Ionic interactions:
- Phosphate. Al precipitates phosphate at surfaces in the apoplast in the form of the hardly soluble Al2 (PO4)3 salt and thus reduces P-availability.
- Iron. High Al levels in the medium are associated with high acidity, which at the same time leads to increased mobility of iron and thus may induce Fe-stress. Conversely Al may also induce Fe deficiency and chlorosis because it inhibits the biosynthesis of phytosiderophores, Fe-complexing agents functioning in iron uptake of plants.
- Divalent cations, Ca2+, Mg2+, Mn2+, Zn2+. Al occupies important cation-exchange sites in the apoplast and thus prevents access of essential divalent cations to these sites, which adversely reduces their availability to the plants.
- Various other nutrients. Al may inhibit nitrate uptake and induce boron deficiency.
• Cell wall interactions:
- Al modifies cell wall components making the cell wall thick and rigid, thus inhibiting growth.
- Al binds to pectins (depending on the degree of methylation of the pectins) occupying ion exchange sites.
• Membrane interactions:
- Al binds to proteins and phospholipids of membranes. Thus, Al affects structure and fluidity of membranes increasing their rigidity and reducing permeability and it occupies cation exchange sites.
- Al blocks K+ - and Ca2+-channels in membranes.
- Al inhibits proton-pumping ATPases of membranes (plasma membrane and tonoplast) and thereby reduces electric membrane polarization.
• Metabolism interactions:
- Al generally affects metabolism. It binds to ATP making it metabolically unavailable.
- Al elicits oxidative stress indirectly via enhancement of Fe-mediated peroxidation which affects membrane structure and can cause DNA-damage.
• Cytoskeleton interactions:
- Non-hydrolysable Al3+-ADP and Al3+-ATP complexes bind to actin/myosin and prevent cytoskeleton function.
• Interactions with intracellular messenger networks:
- Al-induced callose formation blocks apoplastic and symplastic transport routes, and thus, inhibits basipetal transport of the phytohormone indole acetic acid (IAA).
- Al may become involved in Ca2+/calmodulin interactions, which are important in intracellular regulatory processes and signalling at the molecular level.
- Binding to phosphorylated proteins associated with DNA Al may interfere with transcription.
10.2.4.3 Protective Plant Responses
There are multicomponent tolerance and resistance mechanisms, a summarising listing of which is as follows.
• Aluminium exclusion:
- Alkalinization of the rhizosphere.
- Secretion of Al-chelators, such as organic acids (malate, citrate, oxalate) and flavonoid-type phenolics (catechin), supports Al-exclusion from the cells.
- Excretion of phosphate diminishes Al-mobility.
- Al-elicited formation of reactive oxygen species (e.g. H2O2) may result in kind of a hypersensitive reaction killing small areas of tissue and excluding Al from the remaining tissue.
• Aluminium inclusion:
- Organic acid complexes (malate, citrate, oxalate) of Al are not toxic and serve as transport forms for sequestration, especially in the central cell sap vacuoles, and thus, provide internal tolerance.
- Complexes with inorganic phosphate bind Al, but the disadvantage is that this locks up significant amounts of phosphate.
- Al-silicon complexes can form in the shoot tissue.
Gene regulation is involved in protective plant responses, which affects various metabolic functions involved in organic acid secretion. We have seen above that the organic acid anions malate, citrate and oxalate are involved in both exclusion and inclusion and internal tolerance mechanisms of aluminium. Aluminium signalling elicits upregulation of de novo synthesis of organic acids and transport mechanisms and activation of anion channels and the proton transporting ATPase in the plasmamembrane.
Aluminium has different consequences for tropical forests as compared to savannas. In the forests, even when Al-concentrations in the soil are high the effect on plants is smaller, because the nutrient cycle is tightly coupled between the decomposing litter and the vegetation and tends not to involve the mineral soil very much. In savannas, however, plants take up minerals from the soil solution, which is in equilibrium with an Al-enriched exchange complex. Since most forest-tree species are more sensitive to aluminium than savanna plants, the Al-load of soils can in part explain the competition between forest, cerrados and savannas (Eiten 1972) and this may be one important determinant of the complex ecological regulation leading to the co-occurrence of forest and savanna in the tropics (Medina 1982). Under certain circumstances Al may even stimulate growth, e.g. by alleviating H+ -toxicity at low pH and by attenuation of excess phosphorus toxicity (Watanabe et al. 2006). For Miconia albicans (Melastomataceae) from the cerrados of Brazil, conditions which lead to a degree of aluminium accumulation, such as non-calcareous acid soils, are even favourable for growth (Haridasan 1988). In another tropical Melastomataceae, Melastoma malabathricum, growing on acid sulphate soils, Al is a nearly essential mineral reducing toxic iron accumulation in roots and shoots (Watanabe et al. 2006).
Among trees in a cloud forest of Northern Venezuela there are Al-accumulators and Al-excluders, reflected in the xylem sap concentration in Al. In the Al-accumu-lator Richeria grandis (Euphorbiaceae), Al-levels in the leaves increased with age to levels of about 15,000 ppm (Cuenca et al. 1990, 1991). The gallery-forest tree Vochysia venezolensis (Vochysiaceae) in South America also accumulates up to 25,000 ppm Al related to dry matter (Eiten 1972; Sarmiento 1984). In a savanna in Trinidad, Al-levels in the grass Panicum stenoides were on average 910 ppm with maximum levels over 4, 000 ppm, and the herbaceous Melastomataceae Acisan-
thera uniflora contained over 20,000ppm (Sarmiento 1984). Haridasan (1982) lists Al-levels between 4000 and 14,000 ppm for various Al-accumulating cerrado species of central Brazil, but the highest levels of Al in plants quoted from the literature are 66,100 ppm for the Melastomataceae Miconia acinodendron and 72,300 ppm for the Symplocaceae Symplocos spicata. For comparison, in areas of forest decline in the temperate zone, Al-levels in the root dry mass range from 20 to 14,000 ppm depending on sites and soil depths (Lüttge and Clark-son 1992).
10.3 The Fire Factor
10.3.1 The Causes of Fire: Anthropogenic and Natural
Fires play an important role in tropical biota (Goldammer 1990; Fig. 10.25). Dynamical global vegetation models (Bond et al. 2005) impressively illustrate the role of fires (Fig. 10.26). There would be more forest and less savanna coverage without fire (compare the South-American and the African continents with the actual tree cover in Fig. 10.26A, the tree cover simulated with fire on in Fig. 10.26B and the simulated tree cover with fire off in Fig. 10.26C). Fires can originate naturally (see below) but currently the major cause of fires is man. Alexander von Humboldt (1808/1982) (Humboldt 1982) recognized this in his "Journey to South America" and mentions benefits and even the pleasure of fires, but also suggests the drawbacks.
"The pastoral people burn the grassland to obtain fresher and finer grass by new growth ... Thus, if one relaxes on a magnificent tropical evening at the shore of the lake1 and enjoys the delightful coolness, one observes with pleasure the picture of the fires along the horizon, reflected in the waves beating the shore. ... The savanna is frequently burnt to improve the pasture ever since the Llanos were inhabited. Together with the grasses by chance the scattered groups of trees are also destroyed. No doubt, these plains in the 15th century were not as bare as now. Nevertheless, even the first conquerors coming from Coro describe the savannas, in which one sees nothing but sky and grass, widely tree-less and difficult to pass because of the heat reflected by the ground."2
Fires ignited by man have not only been used in slash-and-burn agriculture (Sects. 1.3 and 3.3.3) or in the management of pastures (Sect. 10.3.3) but also by very early hunter/gatherer societies to drive game out of forest thickets (Kern 1994). However, fires in savannas, as well as in other tropical and non-tropical ecosystems, may also be caused naturally (Overbeck and Pfadenhauer 2007). In certain dry savannas of Africa during storms there is often lightning with little rain or before the rain sets in. In Australia, fire has been long considered as a natural environmental stress factor (Walter and Breckle 1984). Palaeontological findings show that fires destroying vegetation must have occurred since the Devonian (376 x 106 years ago). The prerequisites for ignition of such fires are:
• a certain minimal atmospheric concentration of oxygen,
• the presence of combustible material.
The minimal O2-concentration required was shown experimentally to be 13%, i.e. slightly less than two-thirds of the present level. Since it is assumed that atmospheric oxygen has resulted from photosynthetic O2-evolution, we may conclude that 13% must have been reached by the time of the Devonian. Terrestrial vegetation had also developed to a stage that the second of the above criteria was fulfilled. As the possible causes we may list:
• sparks formed during rockfalls,
• self-ignition of fermenting material
(Walter and Breckle 1984; Jones and Chaloner 1991). Thus, plants have been exposed to fire for long enough to allow evolution of special adaptations with stress avoidance and resistance in an ecophysiologically defined group of plants called pyrophytes.
1 Lake of Valencia, Venezuela.
2 Südamerikanische Reise "... brennt das Landvolk die Weiden ab, um ein frischeres, feineres
Gras als Nachwuchs zu bekommen... Wenn man so an einem herrlichen tropischen Abend am Seeufer1 ausruht und die angenehme Kühle genießt, betrachtet man mit Lust in den Wellen, die an das Gestade schlagen, das Bild des roten Feuerrings am Horizont... Seit die Llanos bewohnt... sind, zündet man häufig die Savanne an, um die Weide zu verbessern. Mit den Gräsern werden dabei zufällig auch die zerstreuten Baumgruppen zerstört. Die Ebenen waren ohne Zweifel im 15.
Jahrhundert nicht so kahl wie gegenwärtig; indessen schon die ersten Eroberer, die von Coro herkamen, beschrieben die Savannen, in denen man nichts sieht als Himmel und Rasen, im allgemeinen baumlos und beschwerlich zu durchziehen wegen der Wärmestrahlung des Bodens."
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