Low-Dose Foliar Melatonin Enhances Chilling Stress Tolerance in Cucumber

The study evaluated the potential of low-dose foliar melatonin (MT) treatments to alleviate chilling injury in cucumber plants, with a particular focus on mimicking sudden spring frosts in open-field cultivation. Cucumber seedlings were sprayed with MT solutions at 0.1, 1, and 10 mg L−1 (0.4, 4, 40 µM respectively), then subjected to a controlled chilling regime (4 °C for 72 h). A set of biochemical and physiological parameters was assessed, including stem elongation, leaf area, tissue water content, photosynthetic pigment concentrations, malondialdehyde (MDA) and hydrogen peroxide (H2O2) accumulation, antioxidant enzyme activities, and soluble carbohydrate profiles. In addition, the thermostable fraction of chlorophylls a and b was determined as an indicator of photosynthetic system integrity; this parameter substantially increased under MT treatment, particularly at 1 and 10 mg L−1, despite an overall reduction in total pigment content. The results demonstrated that MT pretreatment, especially at 1 mg L−1, significantly mitigated chilling-induced growth inhibition, preserved chloroplast integrity, and reduced membrane lipid peroxidation. Moreover, MT promoted the accumulation of osmo- and cryoprotective sugars, contributing to improved cellular homeostasis under low temperature. These key findings indicate that low-dose MT priming is a promising, nature-derived strategy to enhance the tolerance of cucumber to acute chilling events, supporting the development of sustainable and organic-friendly approaches for spring frost protection.