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  Section: Plant Nutrition » Other Beneficial Elements » Aluminum
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Aluminum Concentrations

Aluminum-Accumulating Plants
Beneficial Effects of Aluminum in Plants
  Growth Stimulation
  Inhibition of Plant Pathogens
Aluminum Absorption and Transport within Plants
  Phytotoxic Species
  Aluminum Speciation in Symplasm
  Radial Transport
Aluminum Toxicity Symptoms in Plants
  Short-Term Effects
    - Inhibition of Root Elongation
    - Disruption of Root Cap Processes
    - Callose Formation
    - Lignin Deposition
    - Decline in Cell Division
  Long-Term Effects
    - Suppressed Root and Shoot Biomass
    - Abnormal Root Morphology
    - Suppressed Nutrient Uptake and Translocation
    - Restricted Water Uptake and Transport
    - Suppressed Photosynthesis
    - Inhibition of Symbiosis with Rhizobia
Mechanisms of Aluminum Toxicity in Plants
  Cell Wall
    - Modification of Synthesis or Deposition of Polysaccharides
  Plasma Membrane
    - Binding to Phospholipids
    - Interference with Proteins Involved in Transport
      - H+ -ATPases
      - Potassium Channels
      - Calcium Channel
      - Magnesium Transporters
      - Nitrate Uptake
      - Iron Uptake
      - Water Channels
    - Signal Transduction
      - Interference with Phosphoinositide Signal Transduction
      - Transduction of Aluminum Signal
    - Disruption of the Cytoskeleton
    - Disturbance of Calcium Homeostasis
    - Interaction with Phytohormones
      - Auxin
      - Cytokinin
    - Oxidative Stress
    - Binding to Internal Membranes in Chloroplasts
    - Binding to Nuclei
Genotypic Differences in Aluminum Response of Plants
  Screening Tests
Plant Mechanisms of Aluminum Avoidance or Tolerance
  Plant Mechanisms of Aluminum Avoidance
    - Avoidance Response of Roots
    - Organic Acid Release
    - Exudation of Phosphate
    - Exudation of Polypeptides
    - Exudation of Phenolics
    - Alkalinization of Rhizosphere
    - Binding to Mucilage
    - Binding to Cell Walls
    - Binding to External Face of Plasma Membrane
    - Interactions with Mycorrhizal Fungi
  Plant Mechanisms of Aluminum Tolerance
    - Complexation with Organic Acids
    - Complexation with Phenolics
    - Complexation with Silicon
    - Sequestration in Vacuole or in Other Organelles
    - Trapping of Aluminum in Cells
Aluminum in Soils
  Locations of Aluminum-Rich Soils
  Forms of Aluminum in Soils
  Detection or Diagnosis of Excess Aluminum in Soils
    - Extractable and Exchangeable Aluminum
    - Soil-Solution Aluminum
  Indicator Plants
Aluminum in Human and Animal Nutrition
  Aluminum as an Essential Nutrient
  Beneficial Effects of Aluminum
    - Beneficial Effects of Aluminum in Animal Agriculture
    - Beneficial Uses of Aluminum in Environmental Management and Water Treatment
  Toxicity of Aluminum to Animals and Humans
    - Toxicity to Wildlife
    - Toxicity to Agricultural Animals
      - Toxicity to Ruminants (Cattle and Sheep)
      - Toxicity to Poultry
    - Toxicity to Humans
      - Overview of Aluminum Metabolism
      - Overview of the Biochemical Mechanisms of Aluminum Toxicity
Aluminum Concentrations
  In Plant Tissues
    - Aluminum in Roots
    - Aluminum in Shoots
  Soil Analysis

In Plant Tissues

Aluminum in Roots

Increasing aluminum levels in the medium tended to result in increasing aluminum concentrations in roots of aluminum accumulators or aluminum excluders (Table 16.3). Concentrations of aluminum in roots were 2- to 250-fold higher than those in shoots (Table 16.3). In red spruce, root aluminum concentrations associated with a 20% decrease in root biomass ranged from 1700 to 6000 mg Al kg-1 (78). Aluminum in roots is present mostly as precipitated hydroxy or phosphate compounds outside the root cells (86). As a result, it is difficult to use aluminum concentrations in roots as a measure of aluminum toxicity unless an effort is made to remove or prevent extracellularly precipitated and adsorbed aluminum. Alternatively, it might be possible to analyze aluminum concentrations in root apices alone as a measure of toxicity (189-191).

Aluminum in Shoots
In accumulators, foliar aluminum concentrations of 65 tree species and 12 unidentified trees from an Indonesian rain forest ranged from 1 g kg-1 in delta tree (Aporusa spp. Blume, Euphorbiaceae) to 37 g kg-1 in Maschalocorymbosus corymbosus Bremek. (Rubiaceae) (13). Aluminum accumulators (Melastoma malabathricum L., Hydrangea macrophylla Ser., and Fagopyrum esculentum Moench.) exposed to increasing aluminum in solution showed increasing aluminum concentrations in leaves (22) (Figure 16.7). Facultative aluminum accumulators, jack pine (Pinus banksiana Lamb.) and loblolly pine (Pinus taeda L.), also had increasing foliar aluminum concentrations as solution aluminum increased (389) (Table 16.3).

Efforts to establish critical aluminum concentrations for toxicity in plants generally have been unsuccessful (78,82,390). For example, foliar concentrations in red spruce associated with a 20% decrease in foliar biomass ranged from 70 to 250 mg kg-1 (78). Similarly, foliar aluminum concentrations in red oak associated with a 20% decrease in leaf biomass ranged from 93 to 188 mg kg-1 (391). Within slash pine families, aluminum sensitivity was correlated positively with foliar aluminum concentration; however, no such correlation was found within loblolly pine families (392).

In accumulators, internal complexation of aluminum by organic anions, silicate, or other ligands resulted in poor correlations between foliar aluminum concentrations and restrictions in biomass growth. Raynal et al. (78) reported the absence of any significant correlation between biomass response and foliar aluminum levels in Pinus species. In the case of aluminum excluders, aluminum concentrations in shoots do not increase with increasing aluminum levels in the medium until a toxic threshold is exceeded (328), again resulting in poor correlation between foliar aluminum levels and biomass response. For example, in rice and barley, only trace amounts of aluminum were found in leaves at solution aluminum levels up to 111 µM, then foliar aluminum concentrations increased as aluminum levels in solution increased to 555 µM (22) (Figure 16.7). Similarly, increasing solution aluminum levels from 0 to 620 µM had no effect on biomass growth of Western hemlock (Tsuga heterophylla Sarg.), then foliar aluminum concentrations decreased from 300 to 250 mg kg-1 when biomass was affected adversely by solution aluminum (393). In sugar maple (Acer saccharum Marsh.), aluminum concentrations in leaves increased from 50 to 200 mg kg-1 as aluminum levels in solution increased from 0 to 600 µM, but then foliar aluminum concentration dropped to 150 mg kg-1 when shoot growth was restricted at 1000 µM aluminum in solution (394). Other examples of a lack of correlation between aluminum-induced growth inhibition and foliar aluminum concentrations can be found in Table 16.3 (395-399).

Aluminum Concentrations in Roots and Leaves

The pattern of increasing foliar aluminum concentrations with increasing solution aluminum differs in aluminum accumulator species (top) and aluminum excluder species
FIGURE 16.7 The pattern of increasing foliar aluminum concentrations with increasing solution aluminum differs in aluminum accumulator species (top) and aluminum excluder species (bottom) (From M. Osaki, T. Watanabe, T. Tadano. Beneficial effect of aluminum on growth of plants adapted to low pH soils. Soil Sci. Plant Nutr. 43:551-563, 1997.)

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