Effect of Harvest Date and Storage on Antioxidant Systems in Pears

In many fruits the ripening process is characterised by an increase in oxygen consumption related to the climateric rise. The enhanced oxidative metabolism could generate an excess in active oxygen species (AOS) that, if not effectively detoxified by the cell defence systems, leads to a peroxidation of biomembranes, to a damage of cell compartmentation and, ultimately, to tissue senescence and general disorders. Studies on some components of the system of enzymatic and non enzymatic antioxidants involved in the protection of cells from oxidative damage, are reported. Pears (Pyrus communis ‘Passa Crassana’) were picked at 3 different stages of ripening: immature, commercial ripe and fully ripe. Commercially ripe fruits were stored for 3 and 4 months at normal atmosphere (T: 1,5° C; R.H.: 95%). Ethylene production rates were measured and the activity of the scavenging enzyme ascorbate peroxidase (APX) was evaluated. The levels of the antioxidant ascorbic acid (AA) and of malondialdehyde (MDA), a marker of lipid peroxidation, were also determined. Ethylene levels were barely detectable at all 3 harvest dates and increase progressively during storage. APX activity was positively affected by the ripening stage, whereas decreased significantly during cold storage. AA reached the highest level in commercial ripe fruits. Furthermore, storage had a negative effect on AA content and caused a gradual, marked decrease. MDA did not change in fruits of different ripening stages, while after storage the levels were significantly higher. These results suggest that principally during cold storage defense mechanisms against AOS fail to provide adequate protection and then oxidative stress occurs.