Multiple functions of exogenous melatonin in cucumber seed germination, seedling establishment, and alkali stress resistance.

Background: Exogenous melatonin plays a crucial role in various plant developmental processes and stress responses and has considerable potential for future agricultural applications. However, its effects on early cucumber seedling growth and resistance to alkaline stress have not been adequately explored. This study investigated the role of exogenous melatonin during the early growth stages of cucumber, specifically focusing on seed germination, post-germination seedling growth, and 1-leaf stage seedling growth, with particular emphasis on its influence on alkali stress resistance. These findings are intended to enhance the application of melatonin in cucumber seedling cultivation and provide a theoretical basis for promoting growth and improving stress tolerance in agricultural production.

Results: Exogenous melatonin enhanced cucumber seed germination and early seedling growth with promoting and inhibitory effects at low and high concentrations, respectively. However, the effects of exogenous melatonin on cucumber growth varied at different developmental stages. Additionally, alkali stress significantly hampered the growth of cucumber seedlings; however, the external application of melatonin mitigated the damage caused by this stress. This protective effect was evidenced by a marked increase in the survival rate, stem diameter, and biomass of cucumber seedlings, along with a significant reduction in malondialdehyde content and electrolyte leakage rate. Further investigation revealed that exogenous melatonin promotes the accumulation of osmoregulatory substances, specifically soluble sugars, and proline, under alkaline stress. It also enhances the activities of antioxidant enzymes, including peroxidase, superoxide dismutase, catalase, and dehydroascorbate reductase, while significantly decreasing the accumulation of reactive oxygen species such as H₂O₂ and O₂⋅-. Furthermore, exogenous melatonin increased the activities of PM-H⁺-ATPase and V-H⁺-ATPase and stimulated the expression of stress-related genes, thereby regulating Na⁺ and K⁺ homeostasis under alkali stress. Additionally, exogenous melatonin promoted the synthesis of endogenous melatonin in cucumbers subjected to alkaline stress by inducing the expression of melatonin synthase genes, namely, CsASMT, CsCOMT, CsTDC, and CsSNAT.

Conclusions: Exogenous melatonin promoted cucumber seed germination and seedling establishment and enhanced cucumber alkali stress tolerance by mediating osmotic adjustment, reactive oxygen species scavenging, ion homeostasis maintenance, endogenous melatonin synthesis, and expression of stress-related genes.