Aluminium Toxicity and Other Metals

Aluminium Toxicity in Acidic Soils

  • Aluminium toxicity is a primary concern in acidic soils.
  • Understanding the distribution of aluminium species relative to pH is crucial.

Distribution of Aluminium Species by pH

  • At very low pH (acidic conditions), Al^{3+} is the predominant form.
    • The proportion of Al^{3+} decreases as pH increases.
    • At pH around 4.7, Al^{3+} is approximately 50%.
    • At pH 6, Al^{3+} is nearly zero.
  • As hydroxyls (OH-) are added, the charge decreases (e.g., from 3+ to 2+ to 1+), shifting the maximum proportion toward higher pH values.
    • AlOH^{2+} is maximized around pH 5.
    • Al(OH)_2^+ is predominant at pH values higher than 5.5.
  • Aluminate ion (AlO_2^−) predominates in alkaline pH.

Toxicity Concerns by Aluminium Species

  • In acidic soils, Al^{3+} is the most damaging due to its high proportion.
  • AlOH^{2+} also has the potential to cause damage.
  • Other forms present at low proportions in acidic conditions are less of a concern.

Impact on Root Growth

  • Aluminium toxicity primarily affects root tip growth.
  • Healthy roots are typically long and thin, whereas roots affected by aluminium toxicity are short and stubby (coralloid appearance).
  • Lateral roots particularly are affected, ceasing growth when exposed to aluminium.

Nutrient Uptake Disturbances

  • Aluminium toxicity significantly influences nutrient uptake.
  • Calcium uptake decreases substantially with increasing aluminium concentrations.
  • Ammonium and potassium uptake are less affected.
  • Phosphate uptake may slightly increase.

Effects on Cation Uptake

  • Divalent cations (like calcium) are significantly impacted by aluminium toxicity.
  • Monovalent cations are less affected.
  • Anions (like phosphate) may experience a slight stimulating effect.

Calcium Uptake and Aluminium Toxicity

  • Increasing aluminium concentrations lead to a decline in calcium uptake.
  • Wheat genotypes (Atlas - resistant, Scout - sensitive) illustrate the difference in response.

Michaelis-Menten Kinetics

  • V{max} (Imax) and Km (Kilometers) are used to analyze calcium uptake in the presence of aluminium.
  • Resistant genotypes (Atlas) show relatively small changes in V{max} and Km with increasing aluminium.
  • Sensitive genotypes (Scout) exhibit a substantial increase in Km with little change in V{max}.

Competitive Inhibition

  • The observed changes (increase in Km, little change in V{max}) indicate competitive inhibition.
  • Calcium and aluminium compete for the same transporters.
  • Increased aluminium concentration depresses calcium uptake.
  • Higher calcium concentrations can potentially outcompete aluminium.

Ameliorative Effects of Divalent Cations

  • Magnesium and calcium can ameliorate aluminium toxicity.
  • Increasing calcium concentrations slightly alleviate the initial hit of aluminium toxicity.
  • Magnesium provides more significant improvement in growth under medium stress levels.
  • Under severe stress, both calcium and magnesium offer limited help.

Plant Resistance Mechanisms: Tolerance vs. Avoidance

  • Two main resistance mechanisms:
    • Avoidance: Preventing aluminium uptake.
    • Tolerance: Coping with aluminium within plant tissues.
  • Avoidance is preferable; tolerance is necessary when avoidance is incomplete.

Tolerance Mechanisms

  • Aluminium can be taken up by plants, and some plants are very tolerant.
  • Tea plants are highly tolerant, binding aluminium to organic acid anions, rendering it non-toxic.
  • Drinking tea results in aluminium ingestion, but it's not a major concern due to the bound form.

Avoidance Mechanisms

  • Two primary types:
    • Increasing pH in the glycosphere.
    • Exuding organic acid anions.

Increasing pH in the Glycosphere

  • Raising soil pH shifts the proportion of Al^{3+} to less toxic forms.

Exuding Organic Acid Anions

  • Organic acid anions (e.g., citrate, malate, oxalate) bind aluminium, detoxifying it.
  • This results in less aluminium uptake.

Root Tip pH

  • Maintaining a slightly higher pH at the root tip is crucial for aluminium resistance.
  • Atlas maintains a higher pH at the root tip compared to Scout when aluminium is present.
  • The difference is small (less than 0.1 pH units).

Citrate Exudation in Maize

  • Maize uses citrate exudation for aluminium resistance.
  • Without aluminium, there is minimal citrate efflux.
  • Under aluminium stress, the resistant genotype significantly increases citrate exudation.
  • The sensitive genotype does not effectively increase citrate exudation.

Malate Exudation in Wheat

  • Wheat uses malate exudation for aluminium resistance.
  • Exogenous malate addition helps sensitive genotypes exposed to aluminium.
  • Tolerant genotypes increase malate exudation upon aluminium exposure.
  • Resistance has limits; higher aluminium concentrations eventually inhibit even resistant genotypes.

Genetic Transfer and Backcrossing

  • Genes for aluminium tolerance from Brazilian wheat varieties were introduced into Australian varieties (e.g., Egret).
  • Backcrossing is used to eliminate unwanted genes from the donor while retaining the aluminium tolerance gene.

Malate Content and Exudation

  • Malate content varies in different root segments.
  • Exudation is highest at the root tip, despite relatively lower malate content there.

Root Growth in Soil and Subsoil

  • Sensitive genotypes show limited root growth in acidic subsoil with high exchangeable aluminium, compared to tolerant genotypes.
  • Shoot growth differences are secondary, resulting from the primary effect on root growth.

Relationship Between Malate Exudation and Root Growth

  • A positive relationship exists between malate exudation and root growth.
  • Increased malate efflux leads to higher relative root growth, indicating improved resistance to aluminium toxicity.

Genetic Engineering

  • Genes for citrate biosynthesis have been transferred into potatoes.
  • Citrate exudation is regulated by external aluminium concentration.
  • Response diminishes upon removal of aluminium stress.

Exclusion of Aluminium

  • Exclusion of aluminium via organic acid anions is a key resistance mechanism.
  • Citrate, malate, and oxalate bind aluminium, rendering it non-toxic.

Toxicity of Other Metals

  • Aside from aluminium, other metals and metalloids can be toxic.
  • Cadmium is a significant non-nutrient toxic metal.
  • Other metals (e.g., manganese, iron, copper, zinc, molybdenum) are micronutrients but can be toxic at high concentrations.
  • Arsenic and selenium are metalloids, with arsenic being a common toxicant.

Plant Responses to Metal Toxicity

  • Plants employ various mechanisms to deal with metal toxicity.
    • Root avoidance of contaminated areas.
    • Exudation of organic acid anions and amino acids.
    • Cell wall binding.
    • Compartmentalization in roots and vacuoles.
    • Enzyme resistance and metal tolerance.

Metal Hyperaccumulation

  • Hyperaccumulators are plants that accumulate very high concentrations of toxic metals (at least 10 grams per kilo or 1%).

Advantages of Hyperaccumulation

  • Potential defense against herbivory due to unpalatability.

Examples of Hyperaccumulators

  • Many plants in the Brassicaceae family (e.g., Alyssum) hyperaccumulate nickel.
  • Concentrations can be extremely high (e.g., 40% nickel in tissue).

Characteristics of Hyperaccumulators

  • Often found in Mediterranean regions.
  • Amino acids like histidine play a role in complexing metals like nickel and zinc.

Extreme Examples

  • Some trees accumulate over 25% nickel in their sap.

Phytomining

  • Hyperaccumulators can be used to extract metals from the soil.
  • Commercial viability is still limited.