Caffeic acid pretreatment enhances dehydration tolerance in cucumber seedlings by increasing antioxidant enzyme activity and proline and soluble sugar contents

• Pretreating cucumber with 100 μM CA alleviates growth inhibition under dehydration.
• Pretreatment with CA enhances levels of proline and soluble sugars under dehydration.
• CA application elevates antioxidant enzyme activities in dehydration-stressed leaves.
• 100 μM CA enhances transcript levels of Cu/Zn-SOD, Mn-SOD and GPX under dehydration.
• Exogenous CA decreases ROS levels and increases dehydration tolerance of cucumber.

To determine the mitigative effects of exogenous caffeic acid (CA) on dehydration stress and the physiological mechanisms underlying these effects in plants, cucumber seedlings were pretreated with CA and exposed to dehydration conditions induced by 10% polyethylene glycol (PEG) 6000. Among the concentrations of CA examined, treatment with 100 μm CA caused the greatest reduction in the levels of superoxide radical (O2−), hydrogen peroxide (H2O2) and malonaldehyde under dehydration stress. Following 2 days of pretreatment with 100 μm CA, the activities of superoxide dismutase (EC 1.15.1.1), catalase (EC 1.11.1.6), 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) increased. When the CA-pretreated seedlings were subjected to dehydration, the activities of these antioxidant enzymes further increased and were higher than those observed under dehydration treatment alone, which was in accordance with the increased transcript levels of copper/zinc superoxide dismutase, manganese superoxide dismutase and guaiacol peroxidase genes and coincided with the increased contents of reduced glutathione and ascorbate. Meanwhile, CA + PEG treatment mitigated growth inhibition, reduced the osmotic potential and enhanced the increases in proline and soluble sugar levels observed in response to PEG treatment, and this treatment also increased the contents of endogenous CA and reduced the levels of O2−, H2O2 and malonaldehyde in the leaves. Therefore, pretreatment with 100 μm CA increases the deposition of endogenous CA, antioxidant enzyme activities and proline and soluble sugar contents in cucumber leaves, which may protect the seedlings from dehydration stress. This work lays the foundation for studies investigating the application of CA to seedlings as drought-resistance or water-retaining agents.