Abstract
The ascorbate-glutathione cycle has been shown to be of great importance in multiple stress reactions. In the present study, alterations in the antioxidant pools combined with the evolution of the capacities of enzymes involved in defence against oxidative stress was investigated in roots and primary leaves of Phaseolus vulgaris L. after a 50-μM Zn application to roots. A difference has to be made between both plant organs. In roots, an overall oxidation of ascorbate occurred immediately after the start of the zinc treatment. This was accompanied by a decrease in ascorbate peroxidase (APOD; EC 1.11.1.11) and glutathione reductase (GR; EC 1.6.4.2) capacities, the latter resulting in an increase of glutathione disulphide (GSSG). In addition, in the primary leaves ascorbate was strongly affected by zinc toxicity and an immediate increase in the total amount could be observed. Whereas enhancement of the capacities constituting the ascorbate-glutathione pathway was observed, it only limited the effects of oxidative stress in primary leaves. At the end of the experiment, a strong increase in ascorbate oxidation was noticed suggesting that an excess of zinc can cause oxidative stress.
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