Exogenous sucrose increases chilling tolerance in cucumber seedlings by modulating antioxidant enzyme activity and regulating proline and soluble sugar contents

• Pretreating cucumber with 50 mM sucrose reduces growth inhibition under chilling.
• The pretreatment enhances antioxidant enzyme activities in chilling-stressed leaves.
• 50 mM sucrose elevates transcript levels of Cu/Zn-SOD, GSH-Px and APX under chilling.
• Sucrose also induces SAI, NI, proline and soluble sugars in chilling-stressed leaves.
• Exogenous sucrose decreases ROS levels and enhances chilling tolerance of cucumber.

To determine the alleviative effects of exogenous sucrose on chilling stress, cucumber seedlings were pretreated with sucrose for 1 day and exposed to 15/8 °C for 3 days. Among the sucrose concentrations, pretreatment with 50 mM sucrose led to the highest levels of plant growth and caused the lowest levels of superoxide radical, hydrogen peroxide and malonaldehyde under chilling stress. The combined effects of 50 mM sucrose pretreatment and chilling stress increased the activities of superoxide dismutase (EC 1.15.1.1), guaiacol peroxidase (EC 1.11.1.7), glutathione peroxidase (EC 1.11.1.9), ascorbate peroxidase (EC 1.11.1.11), glutathione reductase (EC 1.6.4.2), dehydroascorbate reductase (EC 1.8.5.1) and monodehydroascorbate reductase (EC 1.6.5.4) in leaves. This coincided with the increased transcript levels of copper/zinc superoxide dismutase, glutathione peroxidase and ascorbate peroxidase genes and correlated with the elevated contents of reduced glutathione and ascorbate. The combination of the sucrose pretreatment and chilling increased the contents of proline, soluble sugars, fructose, glucose and endogenous sucrose and enhanced the activities of soluble acid invertase (EC 3.2.1.26) and neutral invertase (EC 3.2.1.27) as well. We propose pretreatment with 50 mM sucrose leads to higher contents of endogenous sucrose, thereby activating antioxidant enzymes and enhancing chilling tolerance in cucumber; the induction of proline, soluble sugars, soluble acid invertase and neutral invertase are also related to chilling stress mitigation by the sucrose pretreatment. Exogenous sucrose may have application possibility in nutrient solutions for a future trial of chilling reduction.