Exogenous Spermidine (Spd) alleviates NaCl-induced injury effects by improving photosynthesis and oxidative stress tolerance in Brassica napus seedlings

Abstract

To better understand the mechanism of exogenous application of Spermidine (Spd) to enhance the salt tolerance of rapeseed (Brassica napus L.). The seedlings of rapeseed cultivar ‘Zheyou-18’ were treated with different concentrations of spermidine under 200 mM NaCl. The portable photosynthetic system flame, photometer and portable fluorometer were used to determine the gas-exchange parameters, ion content and Chlorophyll fluorescence. A spectrophotometer was used to determine the activities of SOD (Superoxide Dismutase), POD (Peroxidase), CAT (Catalase), and APX (Ascorbate Peroxidase). Our study revealed that different concentrations of exogenous Spd can alleviate the harmful effect caused by NaCl stress in rapeseed, and inhibition in rapeseed seedling growth was significantly alleviated with the application of 160 mg·L-1 spermidine under 200 mM NaCl, which is reflected in dry and fresh weight. The Spd treatment further enhanced the photosynthetic efficiency of rapeseed leaves, which is reflected in the changes in gas exchange parameters Pn (Photosynthetic rate), Ci (Intercellular CO2 concentration), Tr (Transpiration rate), Gs (Stomatal conductance), Ls (stomatal limitation), WUE (Water Use Efficiency) and chlorophyll fluorescence parameters (Fv/Fm, ΦPSII, qP, NPQ), and antioxidant enzymes activity (SOD, POD, CAT, APX) which in turn reduced the level of active oxygen (H2O2, O2-). This study also indicated the Spd treatment reduced the absorption of Na+ and increased the absoraption of K+ and Ca2+. The physiological experiments demonstrate that exogenous Spd enhances salt tolerance in rapeseed under NaCl stress through multiple mechanisms, including improved photosynthetic efficiency, maintenance of cellular ion homeostasis, and reduced reactive oxygen species (ROS) levels