Exogenous melatonin delays yellowing in broccoli based on hormone, nitrogen and sucrose metabolism regulation during postharvest

Abstract

Postharvest yellowing of broccoli has a serious impact on shelf life and consumers acceptance. The impact of exogenous melatonin (MEL) on superficial color, chlorophyll, carotenoid content, hormone metabolism, nitrogen (N) and sucrose content as well as the activities of enzymes related to N and sucrose metabolism were investigated in broccoli florets during postharvest. Results indicated that 200 μmol L−1 of MEL application was beneficial to suppressing yellowing along with maintenance of sensory quality of broccoli, enhanced the levels of endogenous MEL, salicylic acid (SA), abscisic acid (ABA) assocaited with a decrease in jasmonic acid (JA) content, and maintained higher N and lower sucrose content compared to control. Correlation analysis revealed that N was positively correlated to chlorophyll, and negatively correlated to carotenoid. Additionally, MEL treatment elevated the activities concerning nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), sucrose synthase synthesis (SS-s) and invertase (INV) in the florets, but reduced the activities of sucrose synthase cleavage (SS-c) and sucrose-phosphate synthase (SPS). Thus, MEL treatment retarded the yellowing of broccoli florets might be attributed to regulating endogenous hormone content, maintaining higher N and less sucrose content via the regulations of N and sucrose metabolism.