Exogenous melatonin mitigates chilling injury in zucchini fruit by enhancing antioxidant system activity, promoting endogenous proline and GABA accumulation, and preserving cell wall stability

Zucchini fruit are sensitive to chilling injury (CI) during postharvest low temperature storage which makes their cold stored life short. In this work, zucchini fruit were treated with melatonin (200 µmol L−1) and stored at 5 ºC for 15 d. The pre-storage melatonin treatment reduced weight loss (6.51%) and suppressed symptoms of CI (1.5 Score) along with lower malondialdehyde (MDA), relative ion leakage (RIL), superoxide anion and hydrogen peroxide (H2O2) in contrast with control. The treated zucchini exhibited higher L* (51.89) and lower a* (−10.17) and b* (29.15) values of exocarp in comparison with control. Melatonin treated zucchini showed enhanced γ-aminobutyric acid (GABA) concentration due to higher glutamate content and stimulated glutamate decarboxylase (GAD) and suppressed γ-aminobutyric acid transaminase (GABA-T) activities. In addition, melatonin application increased proline accumulation owing to stimulation of Δ-1-pyrroline-5-carboxylate synthetase (P5CS) and ornithine δ-aminotransferase (OAT) enzymes and reduced proline dehydrogenase (ProDH) activity as compared with control. The activity of antioxidative such as ascorbate peroxidase (APX), peroxidase (POD), catalase (CAT) and superoxide dismutase (SOD) enzymes remained higher in melatonin treated zucchini. The fruit treated with melatonin exhibited reduced water soluble pectin (WSPN) along with higher sodium carbonate soluble pectin (SCSP), protopectin (PRPN), chelate soluble pectin (CSPN), cellulose (CLS) and hemicellulose (HLS) in comparison with control. The exogenous melatonin suppressed the activities of polygalacturonase (PG), β-galactosidase (β-Gal), cellulase (CX), and pectin methylesterase (PME) along with higher firmness. In conclusion, exogenous melatonin at 200 µmol L−1 concentration could be used for zucchini fruit CI mitigation under cold storage.