Exogenously applied spermidine improves drought tolerance in creeping bentgrass associated with changes in antioxidant defense, endogenous polyamines and phytohormones

Improvement of endogenous polyamine (PA) levels by exogenous spermidine (Spd) application in leaves may promote drought tolerance of plants. In this study, leaves of creeping bentgrass (cv. 'Penn-A4') were pretreated with 0.2 mmol Spd every another day for three cycles before water was limited for 15 days in a growth chamber. The application of exogenous Spd elevated the accumulation of endogenous PA, including putrescine, Spd and spermine in leaves of creeping bentgrass under drought stress. Exogenous Spd effectively alleviated the damage effects from drought stress, as demonstrated by lower O-2 (center dot-) generation rate, H2O2 and malondialdehyde content, higher relative water content, chlorophyll content and antioxidant enzyme activities (superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase) as compared to untreated plants. Additionally, the pretreatment with Spd resulted in a lower accumulation of indole-3-acetic acid (IAA) and gibberellin A(3) (GA(3)) after 15 days of drought stress, but an unchanged accumulation of abscisic acid compared to the untreated ones. These results suggest that ameliorating drought stress through exogenously applied Spd may be associated with increased antioxidant protection capability and after a longer stress treatment with changed IAA and GA(3) accumulation level on account of improved endogenous PA content. In addition, drought tolerance mechanisms related to PA regulation of phytohormones are very complex and deserve further investigation.