Abstract:

This study was conceived to explore the relationship between exogenous spermidine (Spd) and the activities
of antioxidant enzymes, namely catalase (CAT), peroxidases (POD) and superoxide dismutase (SOD) and investigate the
changes in these antioxidant enzyme activities during the ripening process of oranges. The leaves and fruits of five-yearold
‘Huangguogan’ orange trees were sprayed with spermidine at three concentrations to examine the effect of exogenous
Spd on the antioxidant enzyme activities during the later developmental stage. The results showed that exogenous Spd
could significantly improve the contents of Spd and spermine (Spm) and significantly decrease the content of Put. During
the early color-turning period, exogenous Spd treatment caused significant increases in the activities of CAT, POD and SOD
and a significant decrease in malondialdehyde (MDA) content in the ripe fruits, and retarded the rapid accumulation of
MDA. These findings suggest that exogenous Spd can improve the antioxidant defense system in ‘Huangguogan’ fruits by
enhancing the activities of CAT, POD and SOD.

Key words: Spd, ‘Huangguogan&rsquo, orange, antioxidant enzymes, malondialdehyde