This document summarizes the toxicity of aluminum and cadmium in acid soils. It discusses how aluminum becomes soluble in acid soils below pH 5 and inhibits root growth. Aluminum binds to cell walls and membranes, disrupting nutrient transport. Plants have developed mechanisms to exclude or tolerate aluminum, such as releasing organic acids. Cadmium also inhibits plant growth and is mobile in soils. It accumulates more in roots than shoots and can cause oxidative stress. Plants detoxify cadmium by producing phytochelatins that bind it and transport it to vacuoles.

1. Aluminium and Cadmium
toxicity in acid soils
Tundup Namgial
(J-16-M-436)
SKUAST -Jammu

2. Aluminium Toxicity in Acid soils
 Aluminium in soils is present as insoluble alumino-silicates and oxides.
 Phytotoxic form of aluminium are relatively insoluble at alkaline, neutral or mildly acidic pH.
 As the soil pH drops below 5, the hexahydrate Al(H2O)6
3+, more commonly referred to as Al3+, is
solubilized into the soil solution.
 Soluble Al3+ is the major factor limiting growth because it inhibits root growth at very low
concentrations.
 Indeed the inhibition of root growth is the primary symptom of plant stress in acid soils.
 Root apices are the most sensitive part of the root and Al3+ must contact the apices directly for
growth to be affected.

3. • Most of the Al3+ absorbed by roots localises to the
apoplast. The fixed negative charges on the membrane
surfaces and pectin in the cell walls attract and bind Al3+.
• By binding to pectin in the cell walls Al3+ can rigidify the
walls and restrict solute flow through the apoplast (Horst
et al. 2010, Sivaguru et al. 2006).
• High concentrations of Al3+ in the apoplast can induce
callose production (1,3 beta D-glucan) and affect
membrane function by binding with lipids and proteins or
by displacing calcium from critical sites on membranes
(Foy et al. 1978).
• Al3+ can also directly inhibit nutrient uptake by blocking the
function of ion channels involved in Ca2+ and K+ influx .
• Al3+ can out-compete other cations like Mg2+ and Ca2+ for
important binding sites and even bind with DNA (Martin
1992).
Root apices of near-isogenic wheat
plants, ET8 and ES8, that differ in
Al3+ resistance. The resistant line
(ET8, on the left) is unaffected by the
treatment whereas the sensitive line
(ES8, on the right) shows
considerable damage to its tissues.
(Delhaize and Ryan 1995)

4. Mechanisms of Al3+ resistance
1. Mechanisms of Al3+ exclusion
• The exclusion mechanism for which most supporting evidence is available is the release of
organic anions from roots (Delhaize et al. 2007, Ma et al. 2001, Ryan et al. 2001).
• Malate and citrate are the two anions most commonly reported but oxalate efflux occurs from a
few species.
• Once these anions are released from root cells they bind the Al3+ and prevent it from
accumulating in the apoplast, damaging the cells and being absorbed by the roots.
• Efflux is largely restricted to the root apices and in nearly all cases it does not occur
continuously but is activated by exposure to Al3+.
• Reported in species from the Poaceae (e.g. wheat, barley, sorghum, maize and rye), the Fabaceae
(e.g. soybean, snapbean, common bean).

5. Al3+-activated organic anion efflux.
The Type I occurs in wheat where the anion channel is constitutively expressed. Al3+ is able to
rapidly activate efflux by interacting directly with the pre-existing proteins (red arrows).
The Type II response occurs in maize and rye and shows a delay between the addition of Al3+
and the start of organic anion efflux. This delay is interpreted as Al3+ first inducing the
expression of the transport protein via a signal transduction pathway possibly involving a
specific receptor (“R”)(blue arrows). Once synthesized and inserted in the plasma membrane,
Al3+ is thought to interact with the protein to activate efflux of organic anion (OA).

6. 2. Mechanisms of Al3+ tolerance
• Tolerance mechanisms allow plants to safely take-up and
accumulate Al3+ within their cells.
• Eg. Tea , Hydrangea sp and buckwheat (Fagopyrum esculentum).
• Most of the aluminium in tea leaves resides in the apoplast (Tolra et
al. 2011) whereas in the leaves of Hydrangea and buckwheat the
aluminium is bound in vacuoles by citrate and oxalate anions,
respectively.
• Hydrangea is an ornamental plant that changes the colour of its
flowers from pink to blue when grown in acid soils with high Al3+
availability (Ma et al. 1997).
• High shoot accumulation of aluminium implies soluble aluminium
is transported through the xylem and then stored safely in leaf
vacuoles or in the apoplast.

7. • Cadmium is a mobile element due to its weak affinity for soil colloids so it is easily
absorbed and transported to the shoots .
• The degree to which higher plants are able to uptake Cd depends on its concentration in
soil and its bioavailability, organic matter present in the soil, pH, redox potential,
temperature and concentration of competing elements.
• Generally, Cd enters first the roots, which are the first to experience Cd damage .Through
the cortex it penetrates the root and is translocated to above ground tissues.
• Only a small amount of Cd is transported to shoots as roots retain these ions.
• Generally the order of Cd accumulation in plants is: roots ˃ stems ˃ leaves ˃ fruits˃ seeds .
Cadmium toxicity in Acid soils

8. TOXIC EFFECTS:
• Several plant physiological processes like Nitrogen-
metabolism and oxidative reactions are inhibited by
Cadmium .
• Causes necrosis, leaf chlorosis, leaf roll, reduction in plant
growth.
• Uptake and transport of mineral nutrients is also affected
by affecting availability of nutrients or reduction in
population of soil microbes.
• cadmium classified as an element of intermediate toxicity,
but the mechanisms of cadmium toxicity are not
completely understood yet.
• Several researches have suggested that an oxidative stress
could be involved in cadmium toxicity, by either inducing
oxygen free radical production, or by decreasing enzymatic
and non-enzymatic antioxidants (Sandalio et al., 2001). ROS Generation by Heavy Metal
(Pinto et al 2003)

9. Mechanism of Cd detoxification
• Phytochelatins(PC) are synthesised on exposure to Cd.
• PC and LMW phytochelatins form PC-Cd complex in
cytosol and enter the vacules by way of ABC
transporter.
• Cd ions enter vacuole by way of antiport in exchange
for proton.
• Within vacuoles they form LMW complex and HMW
complex.
• CdS crystallite core coated with PCs is formed
The incorporation of sulfide into the HMW complexes
increases the stability of the complex.