Root growth inhibition by aluminum is probably caused by cell death due to peroxidase-mediated hydrogen peroxide production.

The effect of aluminum on hydrogen peroxide production and peroxidase-catalyzed NADH oxidation was studied in barley roots germinated and grown between two layers of moistened filter paper. Guaiacol peroxidase activity significantly increased after 48 h and was approximately two times higher after 72 h in Al-treated roots. The oxidation of NADH was also significantly increased and, like guaiacol peroxidase activity, it was two times higher in A1-treated roots than in controls. Elevated H2O2 production was observed both 48 and 72 h after the onset of imbibition in the presence of A1. Separation on a cation exchange column allowed the detection of two peaks with NADH peroxidase and H2O2 production activity. However, a difference between control and Al-treated plants was found only in one fraction, in which four times higher guaiacol peroxidase activity and five times higher NADH peroxidase activity were expressed and about three times more H2O2 was produced. One anionic peroxidase and three cationic peroxidases were detected in this fraction by native polyacrylamide gel electrophoresis. The anionic peroxidase was activated in the Al-treated root tips and also oxidized NADH but was detectable only after a long incubation time. Two of the cationic peroxidases were capable of oxidizing NADH and producing a significant amount of H2O2, but only one of these was activated by A1 stress. The role of these peroxidases during A1 stress in barley root tips is discussed.