Spermidine enhances chilling tolerance of kale seeds by modulating ROS and phytohormone metabolism.

Chilling stress is an important constraint for kale seed germination and seedlings establishment. It is vital to develop an effective approach to enhance kale seed germination ability under chilling stress. The present study reported that spermidine (Spd) could improve seed chilling tolerance in two kale cultivars 'Nagoya' (MGW) and 'Pigeon' (BB) during germination. The results showed that MGW was cold tolerant with a 90.67% germination percentage (GP) under chilling stress, while BB was cold sensitive with a 70.67% GP under chilling stress. Spd content in MGW and BB seeds during seed germination were up-regulated and down-regulated by chilling stress, respectively. Besides, chilling stress apparently decreased the gibberellin (GA) and ethylene (ET) contents, while increased the levels of abscisic acid (ABA) and reactive oxygen species (ROS) in MGW and BB seeds during germination. Exogenous Spd application increased GA, ET contents and decreased ABA content through regulating the gene expressions of metabolic-related enzymes, thus effectively alleviating the low temperature damage on kale seed germination. Besides, Spd significantly increased the activities of superoxide dismutase (SOD) and peroxidase (POD), and reduced the levels of hydrogen peroxide (H2O2) and superoxide anion (O2·-). The present study demonstrated that endogenous Spd metabolism plays an important role in kale seed germination under chilling stress. The effect of exogenous Spd on the metabolism of endogenous Spd, GA, ABA, ET and antioxidant enzymes might be the important reason for promoting the kale seed vigor at low temperature.