Exogenous Melatonin Enhances the Cold Resistance of Pepper ( <i>Capsicum annuum</i> L. var. <i>cerasiforme</i> Bailey) by Promoting the Synthesis of Long‐Chain Alkanes in the Epicuticular Wax

Pepper (Capsicum annuum L. var. cerasiforme Bailey) is highly sensitive to low temperatures, limiting its cultivation in cooler regions. While melatonin (MT) is known to enhance plant cold tolerance through antioxidant and hormonal pathways, its role in modulating cuticular wax composition remains unclear. In this study, we demonstrate that exogenous MT application significantly improves cold resistance in pepper seedlings by upregulating the biosynthesis of long-chain alkanes in epicuticular wax. Physiological assessments revealed that MT treatment (100 μM) increased photosynthetic rate (P_n) by 65.66%, enhanced antioxidant enzyme activities (CAT, POD, SOD), and reduced malondialdehyde (MDA) content by 20.88% after 8 h at 4°C. Transcriptomic and gas chromatography-mass spectrometry (GC-MS) analyses identified two key alkane biosynthesis genes, CaCER1-like and CaCER1-like1. The expression of CaCER1-like was specifically induced by exogenous melatonin, while CaCER1-like1 was primarily upregulated by low-temperature stress. This coordinated transcriptional activation enhanced the accumulation of C27, C29, and C31 alkanes. Beyond cold-induced alkane biosynthesis, melatonin treatment provided a further 25.5% increase in total alkane content (MT8 vs. CK8), with a marked 27.5% rise in the critical C31 alkane. These compounds constituted over 57.7% of the wax. Scanning electron microscopy (SEM) imaging further showed that MT-induced wax crystallization shifted from lamellar to aggregated structures, with a reduction in epidermal gaps, suggesting a potential reduction in nonstomatal water loss. These findings reveal a novel mechanism by which melatonin enhances cold tolerance via wax modification, providing a potential strategy for improving pepper cultivation in cold climates.