Cadmium-induced inhibition of apoplastic ascorbate oxidase in barley roots

The effect of excess cadmium (Cd: 0.0, 0.25, 0.5, 1.0 and 2.0 mM) on growth and ascorbate oxidase (AO) activity was investigated in barley (Hordeum vulgare L. cv. Jubilant) roots. The study employed a filter-paper technique to germinate and grow the germinating seeds following imbibition with respective Cd treatments for 4 h at 25 degrees C in darkness. Cd was required at 1.0 mM to affect 50% root growth inhibition 72 h after the treatment. This Cd-induced root growth inhibition was accompanied by a corresponding loss of plasma membrane integrity in root cells as evaluated by Evans blue uptake. Excess Cd (1.0 and 2.0 mM) significantly inhibited the AO activity in all the analysed fractions of barley roots such as extracellular, soluble, cell wall (CW)- and membrane-bound fractions. AO was localized in the apoplast, and its highest specific activity was detected in the CW II fraction obtained by extraction with 1.0 M NaCl from purified cell walls. The analysis of AO isozyme profile showed that besides the reduced activity of two anionic and two cationic isozymes, one cationic AO isozyme was activated during excess Cd treatment, which could be detected in cell wall fractions CW II, III and IV.