POD may play a key role in controlling the level of oxygen free radicals induced by stress conditions (Martínez-Téllez andLafuente, 1997). Moreover, because of natural senescence, ethylene accumulation induces the synthesis of new peroxidase isoenzymesand provokes an increase in total peroxidase activity, i.e. both PPO and POD (Abeles et al., 1998). The activities of POD and PPO increase in response to ethylene and to biotic and abiotic stresses.