Effect of Exogenous Spermidine on Osmotic Adjustment, Antioxidant Enzymes Activity, and Gene Expression ofGladiolus gandavensisSeedlings Under Salt Stress

Salt stress is the main factor limiting plant growth. Researches on physiological and biochemical mechanisms of plant responses to salt stress are of great significance in exploring and improving plant salinity tolerance.Gladiolus gandavensisis an ornamental plant with beautiful flowers and good adversity adaptability that is native to the southeastern coast of China. The purpose of this study was to identify the salt tolerance level ofG. gandavensisand explore the mechanism by which exogenous spermidine (Spd) adjusts the growth ofG. gandavensisunder different NaCl conditions. A pot experiment was conducted to examine the chlorophyll content, photosynthetic parameters, proline, reactive oxygen species (ROS) concentrations, antioxidant activities, and relative expression of thechlorophyll a/b response protein, CAT1, POD, MnSODM, P5CS, BADHgenes andbZIP,DREBtranscription factors inG. gandavensisseedlings under 0.3%, 0.6%, and 0.9% NaCl conditions, with and without 0.1 mmol center dot L(-1)Spd.G. gandavensisseedlings exhibited reduced chlorophyll content; a decreased net photosynthetic rate (Pn); and increased relative electric conductivity (REC), malondialdehyde (MDA), proline and soluble protein contents, antioxidant activities, and relative expression of theCAT1, POD, MnSODM, P5CS,andBADHgenes under 0.3%, 0.6%, and 0.9% NaCl conditions. And Spd spraying under 0.3% and 0.6% salt treatment reduced the decrease in chla content by approximately 55.2% and 23.4%, while increased Pn, proline content and the expression ofCAT1, POD, MnSODM, P5CS,andBADHgenes. Exogenously applied Spd effectively alleviated the damage caused by salt stress (0.3% and 0.6%) by upregulating the REC, proline content, gas exchange, antioxidant enzyme activity, and expression ofCAT1,POD,MnSODM, P5CS, andBADHgenes inG. gandavensisseedlings. However, when the seedlings were grown under 0.9% NaCl, no significant differences were found in the physiological and molecular responses between Spd-treated and non-Spd-treated plants. Therefore, the present study suggests that exogenous Spd can efficiently counteract the adverse effect of low (0.3%) and moderate (0.6%) salt stress onG. gandavensisseedlings.