This study was undertaken in order to explore the effect of exogenous sucrose treatment on the peel coloration in postharvest peaches. Freshly harvested ‘Chunmei’ peaches were dipped in 200 mmol/L sucrose and then stored at (20 ± 1) ℃. Fruit quality traits, and the contents of anthocyanins and endogenous sugars and enzyme activities related to anthocyanidin metabolism were measured during postharvest storage. The results showed that sucrose treatment had no significant effects on the respiratory intensity, ethylene release rate, total soluble solids content or hardness of peach fruit (P > 0.05), but it increased the contents of anthocyanins and sucrose. In addition, sucrose treatment maintained relatively high activities of the enzymes related to anthocyanin synthesis in postharvest peach peel, such as chalcone synthase (CHS), flavonoid 3-hydroxylase (F3H), dihydroflavonol reductase (DFR) and UDP-glucose: flavonoid 3-O-glucosyltransferase (UFGT). Therefore, exogenous sucrose treatment can improve anthocyanin synthesis by regulating anthocyanidin metabolism-related enzyme activities in peach peel. This finding will provide a reference for research on the improvement of color in peach peel after harvest.

The objective of this work was to study the effects of different light treatments on the color development of peach skin during postharvest storage and to explore the regulatory mechanism of light on anthocyanin metabolism. Peach fruits, cv. Zhongtao 9, were subjected to different lights (red, green, blue and white) for 12 h and darkness for another 12 h at (22 ± 1) ℃, while those kept in darkness for 24 h were considered as a control. The variations in fruit skin color, anthocyanin content and enzymatic activities involved in the anthocyanin metabolic pathway, as well as the expression levels of the structural genes of anthocyanin biosynthesis and related transcription factors after light treatment were measured. The results showed that the color development of peach skin was not affected by red and green light, but was weakly promoted by white light and significantly induced by blue light. The anthocyanin content in blue light-treated peach skin was 27.26 mg/kg, which was 4.48 and 10.34 times as high as that in the white light treatment and control groups on the sixth day after harvest, respectively. Under blue light treatment, the enzymatic activities of the anthocyanin biosynthesis pathway and the expression levels of structural genes and transcription factors were significantly higher than those in the control and other light treatment groups at most of the time points tested (P < 0.05). The expression levels of the structural genes PAL, CHS, F3H, DFR, ANS and UFGT and the transcription factor MYB10.1 showed significantly positive correlation with the color parameters and anthocyanin content of peach skin. Collectively, this study indicated that blue light treatment promoted the synthesis and accumulation of anthocyanins in peach skin by up-regulating the anthocyanin synthesis pathway, which will deepen the understanding of the regulatory mechanism of anthocyanin metabolism by light in plants and provide a theoretical basis for the development of postharvest technologies to improve peach skin color.

Riboflavin (vitamin B₂), a safe and non-toxic food additive, plays an important physiological role in plant growth, development and stress resistance. In this study, we investigated the effects of exogenous riboflavin on the postharvest quality maintenance of pak choi (Brassica rapa subsp. chinensis). Pak choi was treated at postharvest with different concentrations of riboflavin (0, 200, 400, 600, and 800 μmol/L) and evaluated for color, chlorophyll content, chlorophyll metabolism-related gene expression and antioxidant enzyme activity. The results showed that 400 μmol/L riboflavin treatment effectively maintained the quality of postharvest pak choi, mainly by 1) reducing the expression of chlorophyll catabolism-related genes (BrNYC1, BrPPH, BrPAO, and BrSGR1/2) to delay chlorophyll degradation, 2) reducing the total bacterial count on the surface of pak choi leaves to reduce pathogenic spoilage, and 3) upregulating the expression of antioxidant enzyme genes (BrPOD, BrSOD and BrCAT) and enhancing the antioxidant enzyme activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) to increase antioxidant activity. In addition, riboflavin treatment maintained high levels of antioxidants (ascorbic acid and total phenolics), thereby increasing the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging capacity and reducing the production of malondialdehyde (MDA). These findings are helpful to understand how riboflavin affects the postharvest quality of leafy vegetables, which is of reference significant for improving the postharvest commercial value and quality of leafy vegetables.

This study aims to investigate the potential mechanism of melatonin (MT) to alleviate the browning of postharvest lotus seeds. 1) The samples were treated with the distilled water, nitric oxide (NO), MT, the inhibitor of nitric oxide synthase (NOS) nomega-nitro-L-arginine methyl ester (L-NAME), the inhibitor of nitrate reductase (NR) tungstate (TUN), and the scavenger of NO carboxy-PTIO (cPTIO), MT + L-NAME, MT + TUN and MT + cPTIO. A systematic analysis was implemented to explore the possible pathway of NO biosynthesis in the lotus seeds. Then, the levels of key enzymes and substances were determined to involve in the NO biosynthesis. The results showed that the endogenous NO content of the MT treated lotus seeds was significantly higher than that of the control. The NOS activity of lotus seeds treated with MT was promoted by 60.36%−71.08% before 3 days of storage. In addition, the MT treatment significantly increased the contents of L-arginine and citrulline (P <0.05), which were the key substances in the NO biosynthesis. It was notable that the citrulline content in the MT treated sample was 1.44~1.59 times than those of the controls. By contrast, there was the varying influence of MT treatment on the activity of NR in lotus seeds. There was the beneficial effect of MT on the lotus seeds and the endogenous NO content of the tissue, when the MT was combined with the inhibitor of NR (TUN). However, this beneficial effect disappeared, when the MT was combined with the inhibitor of NOS (L-NAME) for treating the lotus seeds. The endogenous NO content in this treatment was significantly lower than that in the control. Therefore, the exogenous MT treatment was induced the NO biosynthesis in lotus seeds through the pathway of NOS. 2) The potential mechanism of MT was clarified to regulate the browning of lotus seeds. The phenolic compounds were identified in the lotus seeds using a LC20 HPLC (Shimadzu, Japan) system coupled to a TripleTOF® 5600 + quadrupole time-of-flight (QTOF) mass spectrometer equipped with a DuoSpray™ ion source (Sciex, Ontario, Canada). It was found that there were six flavanols and seven phenolic acids in the lotus seeds, and the content of catechin, a type of flavanol, accounts for over 60% of the total phenol content, the following was ellagitannin, a type of phenolic acid, accounts for 7.45% of the total phenol content. It infers that the catechin was the major phenolic compound of lotus seeds. The flavan-3-ol was the most common direct natural substrate of polyphenol oxidase (PPO) in the plant, especially for the catechin and epicatechin. Thereby, the potential mechanism of MT was determined, where the catechin metabolism was regulated to induce the NO production in the following experiments. The results indicated that the activity of PPO in the MT treated lotus seeds was lower by 35.39%−57.36% than those in the control. What’s more, the MT treatment significantly increased the activities of key enzymes, including cinnamate-4-hydroxylase, dihydroflavonol reductase, chalcone synthase, chalcone isomerase and colorless anthocyanin reductase (P <0.05) in the catechin synthetic metabolism. As a result, the content of catechin in the MT treated sample was higher by 15.35%−47.86% than those in the control after 1 days of storage. Whereas, this positive effect was negated, when the MT was combined with L-NAME or cPTIO for treating lotus seeds. Therefore, it was concluded that the exogenous MT treatment was induced the biosynthesis of NO through NOS pathway, and then the accumulated NO acted on the catechin metabolism of lotus seeds. The MT treatment was used to inhibit the PPO activity, and then the catechin was suppressed to participate in the enzymatic browning; The biosynthesis of catechin was promoted in this treatment. Consequently, the lotus seeds browning was also alleviated by the MT treatment. These findings can provide the theoretical and technical support to preservation of lotus seeds and the signal transduction between MT and NO.

Melatonin (MT) is a low molecular weight compound with multiple biological functions in plants. It is known to delay leaf senescence in various species. However, no data are available on the MT signaling pathway in postharvest vegetables. This study demonstrates that MT increases cGMP concentration and the expression of the cGMP synthesis gene BcGC1 in pak choi. The cGMP inhibitor LY83583 destroys effect of MT delaying the leaf senescence. LY83583 also prevents MT treatment from reducing the expression of chlorophyll metabolism-related genes (BcNYC1, BcNOL, BcPPH1/2, BcSGR1/2, and BcPAO) and senescence genes (BcSAG12 and BcSAG21). It also inhibits MT from reducing the activity of the key chlorophyll catabolism enzymes Mg-dechelatase, pheophytinase, and pheide a oxygenase. Thus, the ability of MT to maintain high levels of chlorophyll metabolites is also destroyed. The Arabidopsis cGMP synthetic gene mutant atgc1 was used to confirm that delayed leaf senescence caused by MT is mediated, at least in part, by the second messenger cGMP.