Effect Of LED Light Quality On Growth And Metabolism Of Broccoli Microgreens

Microgreens have some traits with crisp taste,rich nutrition,and easy to cultivate,which are a new type of vegetables with development prospects.Light Emitting Diode(LED)is widely used in plant factories.The precise control of light to achieve high yield and quality of vegetables has become the frontier and hotspot of environmentally controlled agriculture.In order to evaluate the effect of different LED light quality ratios on the growth index,edible quality and glucosinolates synthesis of broccoli microgreens(Brassica oleracea L.var.italica).In this paper,broccoli microgreens were used as experimental materials.By setting 4 kinds LED ratio of LED red(R,660 nm)and blue(B,455 nm)as R,R5B1,R5B3,R5B5,in addition LED white light(W)as a control.After 7 days light treatments the broccoli microgreens were harvested.Systematic measurement of growth indexes such as hypocotyls,fresh weight,edible rate,and soluble sugar,total phenol,total flavonoids,antioxidant activity,etc physiological and biochemical indexes.There were three metabolite detection platforms used in this study,with high performance liquid chromatography(HPLC),electricity Spray extraction ionization mass spectrometry(EESI-MS)and ultra performance liquid chromatography-time-of-flight mass spectrometry(UPLC-QTOF MS);Combined with real-time fluorescence quantitative PCR technology(qRT-PCR)to analyze the expression of genes related to glucosinolates synthesis.The main findings were as follows:1.Different light quality treatments had a significant effect on the growth index and nutritional quality of broccoli microgreens.The whole plant fresh weight of the microgreens with R5B1 treatment was 43.73 mg,which was significantly higher than other treatments.Compared with the W(CK)treatment,the ratio of red and blue light contributed to increase the dry weight,water content and chlorophyll content of microgreens.The hypocotyl length(29.51 mm)and soluble sugar content(0.89 mg/g FW)of microgreens were significantly higher than those under other treatments;The contents of soluble protein and vitamin C were not significantly different among the treatments,but showed an upward trend with the increase of the proportion of blue light.Considering,R5B1 could be used as an optimized light quality for broccoli microgreens cultivation.2.Different light quality has obvious influence on the total phenol,total flavonoids and their composition of broccoli microgreens.There was no significant difference in the total phenol content between red combined blue light treatment and white light treatment.Among them,R5B3 had the highest total phenol content(3.63 mg/g DW),while red light treatment was significantly lower than the remaining 4 groups;R5B1 had the highest total flavonoid content(2.73 mg/g DW),blue light played an important role in the synthesis of total flavonoids in phenolic compounds.Seven kinds of phenolic substances such as protocatechuic acid,chlorogenic acid,rutin,p-hydroxybenzoic acid,ferulic acid,gallic acid,and caffeic acid were detected from broccoli microgreens by HPLC.The total amount of 7 compounds were highest in R5B5(328.03 mg/100 g DW).Antioxidant activity test results showed that the antioxidant activity of the three groups of red and blue treatments was higher than that of R and W treatments.Therefore,compared with the R and W treatments,the ratio of red and blue light can increase the content of phenols in broccoli microgreens and improve the antioxidant capacity of microgreens.3.19 compounds were identified from alcohol extracts of broccoli microgreens using EESI-MS,including 6 amino acids:serine,proline,valine,phenylalanine,tyrosine and tryptophan;7 kinds of glucosinolates and 1 kind of glucosinolates degradants:sulforaphane,myrosin,glucosinolate,3-butenylthioside,2-hydroxy-3-butenylthioside,3-methyl Indolylthioglycoside,4-methoxy-indole-3-methylthioglycoside and indole-3-methanol;5 kinds of organic acids:malic acid,citric acid,chlorogenic acid,p-hydroxybenzoic acid,crude Catechin.The ratios of red and blue light were conducived to the accumulation of the above biologically active substances.The glucosides,amino acids and other components with R5B5 treatment were significantly higher than W treatment group,while the R treatment was significantly lower than W treatment.The determination results of phenols were similar to HPLC,and all presented protocatechuic acid>chlorogenic acid>p-hydroxybenzoic acid.Principal component analysis(PCA)and partial least squares discriminant analysis(OPLS-DA)showed that broccoli microgreens under 5 light quality treatments could be clearly distinguished,and 11 compounds had VIP values>1.EESI-MS could quickly detect metabolites in microgreens and provide technical support for the quality identification of microgreens.4.Six kinds of glucosides in broccoli microgreens were detected by UPLC-QTOF MS and combined with external standard method for semi-quantitative analysis.Among them,the content of aliphatic glucosides such as glucoraphanin and progoitrin was relatively rich.The total glucosinolates content under R5B5 treatment was up to 1.59 mg/g,while the minimum R treatment was 1.33 mg/g.There was no significant difference in total glucosinolates content between red and blue combined light and white light treatment.Experiments on the expression levels of glucosinolate synthesis genes showed that the expression levels of aliphatic glucosinolate-related genes such as MYB28,CYP79F1,CYP83A1,and AOP3 showed an upward trend with the increase in the proportion of blue light,and were significantly higher in R5B5 treatment than other treatments.The relative content of glucosinolates determined by EESI-MS and UPLC-QTOF MS was consistent,indicating that R5B5 treatment significantly affected the relative expression content of glucosinolates synthesis-related genes in broccoli microgreens,thereby promoting the accumulation of glucosinolates.Comprehensive consideration,compared with the red and white light treatment,the ratios of red to blue light were more conducived to promoting the growth and nutritional quality of broccoli microgreens.Particularly R5B5 treatment could significantly affect the relative expression level of glucosinolatess synthesis-related genes in broccoli microgreens.And also significantly promote the content of phenols,glucosides and other biologically active substances.Therefore,R5B5 light ratio was beneficial to improve the nutritional quality of broccoli microgreens.