| Broccoli (Brassica oleracea L. ssp. italica) is known to have high nutritional value with significant contents of vitamins, glucosinolates and phenolic compounds. The inflorescences are harvested while the floral heads, branchlets, and are totally immature, with the sepals completely surrounding the flower. However, broccoli deteriorates very quickly after harvest at ambient temperature due to the loss of green color and the consequent yellowing of florets, which decreases the commercial acceptance of the broccoli florets. Therefore, it is crucial to study the regulation and mechanism of quality deterioration of broccoli florets after harvest for transportation and storage. Basing on the change of color and yellowing of broccoli florets upon various temperatures storage, the present study was designed to investigate the effect of6-benzylaminopurine (6-BA),1-methylcyclopropene (1-MCP) and ethanol on inhibiting yellowing of postharvest broccoli florets and the possible mechanisms involved. The effect of1-MCP in combination with6-BA in delaying senescence on broccoli florets was also assessed. Furthermore, the effect of1-MCP treatment on regulation sugar metabolism and involvement of enzymes in broccoli florets was investigated, which laid a theoretical basis for sugars acting as signaling molecules in regulating senescence of broccoli florets. The results were as follows:(1) The effect of different storage temperatures (273,278,283,293and303K) on color and chlorophyll content of postharvest broccoli florets was investigated. The results showed that-a/b value and chlorophyll content of broccoli florets markedly decreased during stored at283,293and303K. The increase of storage temperature, change scope of-a/b value and chlorophyll content was also promoted. There were no significant differences in color and chlorophyll content of broccoli florets during stored at273and278K, which suggested that low temperature can delay the yellowing and chlorophyll degradation of broccoli. The kinetic models of-a/b value and chlorophyll content with respect to storage time and temperature were established based on the Arrhenius equation, respectively. Then, a prediction model for chlorophyll content based on-a/b value was developed according to the linear relationship between chlorophyll content and-a/b value. Finally, the chlorophyll content of broccoli can be accurately predicted at the storage temperature from273to305K by the prediction model, which confirms the feasibility of applying the objective color parameter measurement for assessment of broccoli quality.(2) The effect of6-BA on color, antioxidant activity, contents of total phenols, glucosinolate and sulforaphane in broccoli florets was investigated. The results showed that200mg/L6-BA treatment markedly inhibited the increase of L value and malondialdehyde (MDA) content, and retarded the decrease of H value.6-BA treatment reduced rate of chlorophyll degradation by regulating the activities of chlorophyllase and Mg-dechelatase. When compared to control florets, the1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity and the activities of superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) were enhanced in florets treated with6-BA, whereas the activity of peroxidase (POD) was reduced. The contents of total phenols, glucosinolate and sulforaphane were increased after treated with6-BA. These results indicated that6-BA could maintain quality, delay senescence and improve the nutritional value of broccoli.(3) The effect of cthanol treatment on visual quality, antioxidant enzymes and bioactive compounds in broccoli florets was investigated.500μ1/1ethanol treatment markedly extended shelf life, inhibited the increase of L*value and the decrease of H value. It reduced the rate of chlorophyll degradation by inhibiting the activities of chlorophyllase and Mg-dechelatase. In addition, the DPPH radical scavenging activity and the activities of SOD, APX and CAT were enhanced in florets treated with ethanol. The contents of total phenolic compounds, glucosinolates and sulforaphane were also profoundly increased after treated with ethanol. These results indicated that a application of ethanol can prolong shelf life, maintain visual quality and the nutritional value of broccoli florets.(4)2.5μl/11-MCP treatment significantly extended shelf life, inhibited the increase of L value and the decrease of H value.1-MCP might delay chlorophyll degradation and florets yellowing by inhibiting the activities of chlorophyllase, Mg-dechelatase and POD. The enhancement in antioxidant activity by1-MCP may be due to increased phenolic content and higher activities of antioxidant enzymes including CAT, SOD and APX. Thus1-MCP has a potential application in postharvest treatment for prolonging shelf life, maintaining quality and improving the health benefit of broccoli florets.(5) In vitro incubation of broccoli florets showed that various soluble sugars can retard the chlorophyll degradation in broccoli florets. The effect of1-methylcyclopene (1-MCP) on chlorophyll content, sugar metabolism and involvement of enzymes in broccoli florets was investigated. The results showed that1-MCP treatment maintained higher chlorophyll a and b contents compared with the control.1-MCP treatment reduced the rate of decrease of sucrose, fructose and glucose, and maintained higher levels of sugars. When compare to control florets, the activities of sucrose phosphate synthase (SPS) and sucrose synthase-synthesis (SS-S) were enhanced in florets treated with1-MCP, whereas the activities of sucrose synthase-cleavage (SS-C), glucokinase (GK) and UDP-glucose pyrophosphorylase (UGPase) were reduced. Our results indicate that sugar may play an important role in broccoli senescence, and1-MCP may delay senescence of broccoli florets through regulating sugar metabolism and maintaining higher sugar content.(6) The effect of a combined treatment of2.5μl/11-MCP and200mg/L6-BA on surface color, antioxidant enzymes and health-promoting compounds in broccoli florets stored at15℃was investigated. The combined treatment of1-MCP and6-BA significantly delayed senescence and maintained visual quality of broccoli florets compared with1-MCP or6-BA treatment alone during storage. The combined treatment profoundly reduced the rate of increase of L*value, and retained high level of H and chlorophyll content. The combined treatment enhanced the biosynthesis of glucosinolate and the formation of the anticarcinogen sulforaphane, which maintained the health benefit of broccoli.(7)6-BA,1-MCP and ethanol treatments can inhibite chlorophyll degradation. When compared to control, there were no significant differences in BoCHL1, BoCHL2and BoCHL3expression in6-BA-treated broccoli florets. Ethanol treatment suppressed BoCHL2and BoCHL3expression, while1-MCP treatment induced BoCHLl, BoCHL2and BoCHL3expression. These results indicated that BoCHL1, BoCHL2and BoCHL3could be unimportant in chlorophyll degration of broccoli florets because their expression patters did not correlate with degradation of chlorophyll. The up-regulation of BoPAO was repressed by ethanol and induced by1-MCP in senescence broccoli florets. It suggest that the expression of chlorophyll degradation enzymes genes could be regulated by various postharvest treatments, leading to the suppression of florets yellowing in broccoli. |
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