Abstract

This research studies the role of spermidine (Spd) in conferring tolerance in onion seedlings under oxidative sress, caused by NaCl. Stress condition was applied on two months old onion seedlings by adding 10 gL^-1 of NaCl, where 100 µM of Spd was sprayed twice daily before counting the stress duration. Under salinity stress, seedlings were observed for 7 days, and data were measured on relative leaf water, proline, reactive oxygen species (ROS), lipid peroxidation (as malondialdehyde, MDA), amine oxidases, enzymatic and non-enzymatic antioxidants in leaves. Salinity stress decreased the relative water content (RWC), where Spd application delayed the loss of RWC. Contrariwise, Spd increased the proline content in salinity stressed seedlings up to five days. Salinity increased the contents of superoxide (O₂^•-), hydrogen peroxide (H₂O₂) and MDA continuously and significantly with stress duration. More importantly, application of Spd decreased the ROS and MDA contents in stressed seedlings more effectively, up to three days of stress. Spd maintained higher activities of polyamine oxidase (PAO) and diamine oxidase (DAO) under salinity. Higher activities of superoxide dismutase (SOD), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR) and glutathione reductase (GR) in presence of Spd over salinity during the study period, suggested their ROS scavenging role under salinity stress. Conversely, glutathione peroxidase (GPX) and dehydroascorbate reductase (DHAR) played important role in reducing the oxdative stress for 3 to 5 days. Spd also maintained higher reduced glutathione (GSH), ascorbic acid (ASA) and their redox homeostasis in leaves during the study period. Thus, Spd observably confirms better tolerance in short term salinity.

Key words: Spermidine, oxidative damage, salinity, antioxidants, onion seedlings.