Effect of Al-treatment duration
The concentration of citrate, malate and oxalate secreted
by the roots into the treatment media after 24 h is shown
in Fig. 4a. All three organic acids were detected in media
with and without Al. The amount of citrate in the
treatment medium remained low regardless of the
external Al concentration. Similarly, the secretion of
malate was unaffected by the addition of Al, although the
malate concentration was higher than that of the other
organic acids in the 0 and 0.1mM Al treatments. The
concentration of oxalate in the medium increased in a
dose-dependent manner with Al concentration, reaching
approximately 0.8 mmol h1 root (dry weight) in the
0.8mM Al treatment.
Oxalate secretion from the roots of the tea seedlings
over the duration of the 120-min treatment period is
shown in Fig. 4b. The cumulative oxalate efflux from tea
roots cultured in the absence of Al increased slightly but
steadily up to 120 min, suggesting that this efflux has
some function. The secretion of oxalate from roots