Sodium nitroprusside reduces chilling injury in hami melons by regulating postharvest proline metabolism and energy metabolism

• Alleviation of cold injury in postharvest Hami melons: Compared with the control group (treated with distilled water), 1.0 mmol/L sodium nitroprusside (SNP) treatment significantly reduced the cold injury index and cold injury rate of "Xizhoumi 25″ Hami melons. This effect was observed during the late cold storage period (28–42 days at 3 ± 1 °C). SNP treatment also alleviated the severity of cold injury symptoms, including the expansion and darkening of brown spots on the peel. • Regulation of proline metabolism to promote proline accumulation: SNP treatment significantly increased the free proline content of Hami melons. Levels were significantly higher than the control group at 14, 21, 35, and 42 days of storage. This increase was achieved by enhancing the activities of proline synthesis-related enzymes (Δ¹-pyrroline-5-carboxylic acid synthase, P5CS; ornithine transaminase, OAT) and the expression of their corresponding genes ( Cm P5CS, Cm OAT). SNP also inhibited the activity of the proline degradation-related enzyme (proline dehydrogenase, ProDH) and the expression of its gene ( Cm ProDH). • Stabilization of energy metabolism to maintain energy supply: SNP treatment delayed the decline in adenosine triphosphate (ATP) and adenosine diphosphate (ADP) contents. It also suppressed the increase in adenosine monophosphate (AMP) content, thus maintaining a higher energy charge (EC) level in Hami melons during cold storage. SNP significantly enhanced the activity of H⁺-ATPase and slowed the decrease in Ca²⁺-ATPase activity during the mid-storage period (21–35 days). Additionally, it increased succinate dehydrogenase (SDH) activity and promoted the expression of energy metabolism-related genes ( Cm H⁺-ATPase, Cm Ca²⁺-ATPase, Cm SDH, Cm CCO). • Mechanism of enhancing cold tolerance: SNP treatment enhances cold tolerance in postharvest Hami melons through the synergistic regulation of proline and energy metabolism. This maintains cellular osmotic balance and membrane stability. It also ensures energy supply for proline synthesis and normal cellular physiological activities. These findings provide a new theoretical basis for controlling cold injury in postharvest Hami melons. This experiment examined the effects of sodium nitroprusside (SNP; 1.0 mmol/L) versus a control (0 mmol/L) on proline and energy metabolism in cantaloupe during low-temperature storage.As storage time increased, SNP treatment during the late storage period (28–42 d) reduced the chilling injury index and rate in Hami melon fruits. SNP increased free proline content, significantly raised P5CS and OAT activities ( p < 0.05), and inhibited ProDH activity and production ( p < 0.05). It delayed the decline of ATP and ADP and significantly increased H⁺-ATPase activity ( p < 0.05). During mid-storage (21–35 d), SNP slowed Ca²⁺-ATPase activity decline ( p < 0.05), maintained higher EC levels, inhibited AMP content, increased SDH activity, and promoted expression of CmP5CS, CmOAT, CmH⁺-ATPase, CmCa²⁺-ATPase, CmSDH , and CmCCO genes ( p < 0.05). Overall, SNP treatment enhances Hami melon cold resistance by regulating osmoregulatory substances and energy metabolism, providing a theoreticalbasis for controlling chilling injury in Hami melon.