Abstract:
Aluminum (Al), the third most abundant element
overall, after silicon and oxygen, is found virtually in all food, air,
soil and water. Under acidic conditions, Al is solubilized into
[Al(H2O)6]
3+, which is highly toxic to many plant species. Present
paper reviewed Al toxicity and tolerance mechanism in agricultural
plants with special reference to cereals and legumes. Even at
micromolar concentrations, cell division in the root tip meristem
in sensitive plants is quickly inhibited by Al3+. Expressing the
species-dependent manner of response, growth enhancement at
low concentrations of Al3+ is also reported from some plants such
as soybean. Plasma membrane can be identified as the primary
target of Al toxicity, where production of higher reactive oxygen
species and higher fatty acid peroxidation has been observed due
to alteration of plasma membrane integrity. Though, toxicity and
the mechanisms attributed to Al-resistance are extremely complex
phenomena, exclusion is widely accepted as the key mechanism
involved in detoxifying Al3+. Exudation of chelating ligands,
formation of pH barrier at the rhizosphere or at root apoplasm, cell
wall immobilization, selective permeability of the plasma membrane,
and Al efflux have been proposed as the possible mechanisms for
Al exclusion. Al-induced exudation in cereals and legumes is
dominated by citrate, malate, and oxalate in varying degrees
depending on the species and/or cultivar. Apart from sensitive
cultivars, moderately tolerant or tolerant cultivars can also be
distinguished from various kinds of cereals and legumes. However,
reliable techniques for screening such resistant genotypes have not
been developed for any economically important crops.
