Abstract: A hydroponic experiment was carried out about cucumber seedlings of two different cultivars in salinity tolerance to explore the effects of exogenous spermidine on polyamine content and antioxidant system in roots of the seedlings under salinity stress.Results showed that under salinity stress,exogenous spermidine increased the contents of free spermidine and spermine,perchloric acid soluble conjugated(PS-conjugated) and perchloric acid insoluble conjugated(PIS-conjugated) putrescine,spermidine and spermine,and decreased the content of free putrescine in roots of cucumber.Besides,it also increased SOD,POD and CAT activities,decreased O₂^- production rate,H₂O₂ and MDA contents and electrolyte leakage,thus significantly promoting root growth.Furthermore,positive correlation was observed of the content of PIS-conjugated spermidine in seedlings roots with antioxidant enzyme activity.The findings suggest that high levels of free spermidine and spermine,PS-conjugated and PIS-conjugated polyamines,particularly PIS-conjugated spermidine,may enhance antioxidant enzyme activity,decrease O₂^- production rate and membrane damage,thus strengthening salinity tolerance of the plant.

Key words: polyamine, spermidine, antioxidant enzyme, reactive oxygen, salinity stress, cucumber(Cucumis sativus)