| Daylily is a member of Hemerocallis in Liliaceae family,the edible part of this crop is its young buds,which is usually insensitivity to ethylene.Compared to dried daylily buds,fresh daylily buds maintain a better taste characteristic and nutrient composition of tissue,including chlorophyll,ascorbic acid,etc.However,the sale of fresh daylily bud in the market is less.This is because the harvest time of daylily buds is just in the season of high temperature,the tissues appear flower opening and senescence easily under ambient temperature,which affect the processing,sales radius,and shelf life of daylily buds.This problem also seriously restricts the sustainable development of fresh daylily buds industry.Therefore,it is important to understand and develop the fresh-keeping technology to control the senescence of daylily buds.Recent studies found that,hydrogen(H2)improves the antioxidant capacity of tissues in the process of regulating plant root development,stomatal closure,and abiotic stress.Similar to these biological processes,the redox homeostasis in cell collapse accompanying the aging process of fruit and vegetable tissues.This prompts we explore the effect of H2 on the senescence of fruit and vegetable.Unlike daylily bud,which is relatively insensitive to ethylene,kiwifruit is very sensitive to ethylene signals.It therefore provides a good material for the research.Based on this,the investigation analyzed the potential mechanism of H2 in regulating the senescence of daylily buds and kiwifruit.The specific research results are as follows:1.Effects of preharvest hydrogen gas treatment on the growth,development and postharvest senescence of daylily budsThe effect of different concentrations of hydrogen gas(0.8 and 1.6 ?mol L-1)on the growth,development and postharvest senescence of daylily buds were compared.The results showed that,compared with the control and 1.6 ?mol L-1 H2 treatment,0.8 ?mol L-1 H2 treatment improved the total yield of daylily buds during the investigation time.Therefore,the effect of 0.8?mol L-1 H2 on the senescence of daylily buds stored at low temperature was analyzed.The result showed that,browning was the key problem that affected the storage life of daylily buds.By contrast,H2 treatment slowed down the browning degree,which was correlated with the suppressed polyphenol oxidase activity.This treatment also maintained the redox balance,as indicated by the lower relative leakage rate,lipid peroxidation level,and reactive oxygen species accumulation.In addition,the decrease of the unsaturated:saturated fatty acid ratio during storage was also attenuated by H2.The determination result of endogenous H2 level displayed that,the content of endogenous H2 decreased during postharvest storage of daylily buds.By contrast,exogenous H2 treatment maintained the higher level of H2.These results suggest that the metabolism of endogenous H2 affects the development and senescence process of daylily buds,whereas pre-harvest H2 treatment can not only increase the total yield,but also slow down the browning process of daylily buds.2.NO participates in the process of H2-alleviated senescence in daylily budsIn the experiment,hydrogen-rich water(HRW)and sodium nitroprusside(SNP)were used as donors of H2 and NO,respectively,and nitric oxide specific scavenger(2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt,cPTIO)was used as a negative control.The effects of H2 and NO on the senescence physiology of daylily buds stored in low temperature were studied.The results showed that,compared with the control of distilled water,both H2 and NO maintained the lower respiration rate and lipid peroxidation level,and delayed the flowering process in daylily buds.On the contrary,cPTIO treatment aggravated the flowering.To further understand the regulation mechanism of H2 and NO on the senescence of daylily buds,the effect of H2 and NO on the protein expression was analyzed by proteomics.The results found that the differential protein regulated by H2 and NO were mainly involved in the generation of precursor metabolites and energy,the stress response and the biosynthesis of nucleoside three phosphoric acid.The most proteins regulated by H2 participated in the oxidative phosphorylation through the network analysis of GO and KEGG.Moreover,H2 might play a regulatory role in the ?-subunit protein of H-ATPase complex.Both H2 and NO increased the expression level of the energy synthesis protein.To verify this result,the effects of all treatments on the energy level were assessed.As expected,both H2 and NO preserved the higher energy status,while the treatment of cPTIO decreased the ATP content and energy charge level.According to these common characteristics of H2 and NO in regulating the senescence of daylily buds,the daylily buds were treated by alone or combination of H2,NO,cPTIO to further explore the interaction between H2 and NO.The experimental results again showed that H2 and NO could delay the flowering process.Moreover,the function of H2 and NO could be weakened by cPTIO,which implied that NO might be involved in H2-regulated pathway.The results of endogenous NO determination confirmed that,H2 treatment could improve the level of endogenous NO.Subsequent experiment of alone or combination of potassium tungstate and H2 proved that,H2 might induce the synthesis of NO by increasing the activity of nitrate reductase.These results demonstrated that NO participated in H2-alleviated senescence of postharvest daylily buds.3.Effects of H2 and NO on the metabolism of small molecular substances during the senescence of daylily budsThe experiment first analyzed the change of small molecular substances during harvest?cell expansion?flowering?senescence,through the technology of liquid chromatography four stage rod time-of-flight mass spectrometry.It was found that some biomarkers that affected the senescence of postharvest daylily buds based on variable importance plot>1,including citric acid,proline,quercetin,phenylethylamine,tryptophan,arginine,serine,threonine,tyrosine,dopamine,succinic acid,S-adenosylmethionine,2-phenylacetamide,L-malic acid,cis-aconitic acid,and L-phenylalanine.In addition to quercetin,other substances were associated with glyoxylic acid metabolism and three carboxylic acid cycle.Subsequently,the effects of H2 and NO on the metabolism of small molecular substances during senescence of daylily buds were investigated.The result indicated that the regulation of H2 and NO on the senescence of postharvest daylily buds were associated with the higher citric acid,L-phenylalanine,ascorbic acid,and oxalosuccinic acid contents in the tissues.In addition,unlike H2,NO treatment maintained higher content of shikimic acid and succinic acid in tissues.4.Effect of H2 on ethylene biosynthesis of kiwifruit during storage at room temperatureThe experiment further tested the effect of H2 fumigation on senescence of kiwifruit.The results showed that the production of endogenous H2 in kiwifruit decreases during ripening.Fumigation with H2 not only increased endogenous H2 concentrations but delayed the softening and cell wall disassembly in flesh.Ethylene(C2H4)production in kiwifruit was also inhibited by H2,which was supported by a decrease in 1-aminocyclopropene-1-carboxylate(ACC)concentration,ACC synthase,and ACC oxidase activities,and the downregulation of the corresponding gene transcripts.Further,ACC-induced C2H4 production and ripening were blocked by H2 and the incidence of natural decay and disease incidence were reduced.Overall,the results suggested that the H2-induced delay of kiwifruit ripening resulted from an inhibitory effect on C2H4 biosynthesis.In summary,this study revealed that the physiological mechanism of H2 regulating the senescence of daylily bud and kiwifruit.These results not only clarified the downstream signaling molecule which can response to H2,but also produced new insights into the knowledge structure of NO regulation of fruit and vegetable senescence,and more importantly,the results could provide new ideas and methods for regulating the ripening and senescence of fruits and vegetables. |
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