Exogenous Na2SiO3 Mitigates the Adverse Effects of Drought Stress on Cucumber Seed Germination by Regulating the AsA-GSH Cycle

Silicon (Si) plays a crucial role in mitigating biotic and abiotic stress in crops, yet its effects on cucumber seed germination under drought stress remain unclear. This study investigated the impact of exogenous Si on the ascorbic acid-glutathione (AsA-GSH) cycle during cucumber seed germination under PEG-6000-induced drought stress. Seeds of the cucumber cultivar ‘Xinchun No. 4’ were used in this study. Na2SiO3 served as the silicon source, and drought stress was simulated using PEG-6000. The treatments included distilled water (CK), 10% polyethylene glycol (PEG), and PEG combined with five concentrations of silicon (1, 3, 5, 7, and 9 mM Si). Results showed that 10% PEG significantly inhibited seed germination and reduced antioxidant capacity. In contrast, 5 mM Si (5.0 Si + PEG) alleviated PEG-induced stress, reducing malondialdehyde (MDA) and proline (Pro) by 36.87% and 13.71%, respectively, and decreasing reactive oxygen species (ROS) accumulation. Specifically, H2O2 and O2·− contents declined by 20.00–41.76% and 14.29–27.27%, respectively. The 5.0 Si + PEG treatment also reduced soluble sugar content by 29.08% and 27.84% at 48 h and 72 h, respectively, while increasing soluble protein content by 9.97% and 10.30% at 6 h and 12 h. Additionally, it enhanced activities of dehydroascorbate reductase (DHAR), glutathione reductase (GR), and glutathione Stransferase (GST) by 15.00%, 17.48%, and 18.81%, respectively, and elevated ascorbic acid (AsA) content and the GSH/GSSG ratio. In conclusion, 5 mM Si alleviated drought stress by activating the AsA-GSH cycle and enhancing antioxidant defense, providing valuable insights for Si application in agriculture.