The aim of this study was to quantify the long-term effects
of Al in solution on mineral ion uptake and associated
H+ release by the roots of maize cultivated in conditions
compatible with field data. Two maize (Zea mays L.) cvs
of differing sensitivity to Al were compared. The culture was
conducted in hydroponic conditions for culture
periods of 2–5 d in the presence of 0–30 μM Al.
Proton fluxes were localized along the roots using a
videodensitometry method with bromocresol green on agarose gel.
Aluminium reduced the uptake of anions, particularly
NO3−, whereas the uptake of K+
and NH4+ was
unaffected. These effects were accompanied by an increase in net
H+ release which was quantitatively comparable
with the reduction in NO3−
uptake. The reduction in NO3−
uptake and the concomitant increase in H+ release
increased with the quantity of Al accumulated in the roots, which
was itself dependent on the concentration of Al
in solution. The Al-sensitive cultivar bound more Al to its roots
than the Al-tolerant one; the reduction in NO3−
uptake and the increase in H+ release were also greater in
the Al-sensitive cv. Dye indicator videodensitometry
showed that the increased H+ release was general all
along the roots.
These results led us to specify the relation between the Al
deposition in roots, the reduction in NO3−
uptake and the stimulation in rhizosphere acidification by the plants.
They show that, with maize, Al induces an inhibition
in NO3− uptake and an increase in net
H+ release in a 1[ratio ]1 ratio. These concomitant
effects suggest that inhibition in
NO3−/OH−
exchange unmasks active H+ excretion which is otherwise
insensitive to Al. Both these effects are
closely related to the Al content in roots expressed per unit root length.