Extracellular processes, particularly the adsorption of aluminium (Al)
by pectate in the cell wall, have been proposed as important in the e
xpression of Al toxicity to plant roots. In vitro studies were conduct
ed on the effects of Al concentration (generally less-than-or-equal-to
32 muM), calcium (Ca) concentration (0.05 to 10 mM) and PH (3.2 to 5.
4) on Al sorption by Ca pectate. There was a rapid reaction between Al
and Ca pectate, there being no difference in Al remaining in solution
after reaction times of 1 to 16 min, and only a slight decrease after
24 h. Increased Al concentration in solution increased linearly the s
orption of Al by Ca pectate, with 70 to 84% of the Al originally in so
lution sorbed with less-than-or-equal-to 32 muM Al. In contrast, Al so
rption decreased with increased Ca concentration in solution, and as p
H decreased from 5.4 to 3.2. Only less-than-or-equal-to 30% of the sor
bed Al was desorbed after 1 h by 1 mM CaCl2, 10 mM CaCl2 or 1 mM HCl.
The amount of Al desorbed increased with a desorption period of 5 h, p
articularly with 1 mM HCl. These studies suggest that Al sorbed by Ca
pectate in root cell walls is in equilibrium with Al in solution, and
that Al toxicity is associated with the strong binding between Al and
Ca pectate external to the cytoplasm.