Effect Of Selenium Nutrient Fortification On Fruit Quality And Metabolites Of Gannan Navel Orange

Selenium is a kind of trace element that is beneficial to plants and essential to human body.However,most areas in China are in the state of selenium deficiency,and the daily dietary intake of selenium is generally insufficient.Selenium deficiency can cause a variety of diseases,thereby affecting people’s health.At present,selenium intake is mainly supplemented by medicines or by eating selenium-containing foods to maintain healthy selenium levels.The conversion of inorganic selenium into the safer organic form by plants or animals is a more food safe way.Gannan of Jiangxi Province has a high selenium content in China,and is also the largest navel orange producing area in China,and the navel orange in Gannan is of high quality.However,selenium content in Gannan navel orange fruits is low under natural conditions,and there are few related studies on how to improve selenium content in Gannan navel orange fruits.In this study,under field conditions,the experiment was carried out in different orchards for two years by foliar spraying with sodium selenite as an exogenous inorganic selenium fertilizer,and the spraying concentration and fruit growth period were used as variables.The inherent relationship between selenium and fruit quality was analyzed by measuring selenium components and content in fruit,fruit quality indicators,primary and secondary metabolites.In 2021,six treatments were set according to selenium spraying concentration and fruit development stage: the selenium fertilizer concentration was set at 100 mg/L,one spraying at the young fruit stage(T1)in June,one spraying at the fruit expanding stage(T2)in July,and one spraying at the young fruit stage and one spraying at the fruit expanding stage(T3).The selenium fertilizer concentration was set at 200 mg/L,spraying once at the young fruit stage(T4),once at the fruit expanding stage(T5),and once at the young fruit stage and the fruit expanding stage(T6).The control group(CK)was not treated.In 2022,five treatments were set based on the experimental results in 2021: spraying at the early young fruit stage(May),the young fruit stage(June),the fruit expansion stage(July)and the late fruit expansion stage(August).The spraying concentration in each period was 100 mg/L in May,200 mg/L in June,200 mg/L in July,and 100 mg/L in August.R1 treatment was the same as T6 treatment,spraying in May,June and July(R2),spraying in June,July and August(R3),spraying in May,June,July and August(R4),and R-CK did not do any treatment.This study aimed to investigate the effects of foliate selenium(Se)fortification on fruit quality and metabolites of navel orange,and to provide a scientific basis for the production of selenium-rich and high-quality navel orange.Key findings were as follows:1.Compared with the control,Se treatment increased the total Se content in navel orange fruits.The proportion of organic selenium was 97.4%-100%,and the organic selenium selenocystine was the main selenium form.With the increases of selenium concentration and spraying times,the total selenium content in fruits increased,and the total selenium absorption efficiency in pulp increased with the later of the spraying period.However,when the selenium content in the fruit reaches a certain concentration,the effect of plants on selenium absorption will be weakened even if the amount of selenium fertilizer is increased.The appropriate selenium enrichment treatments were spraying 200 mg/L sodium selenite(T6 and R1 treatments)at the young fruit stage and the fruit expansion stage,which could significantly increase the total selenium content in navel orange fruits.2.Compared with the control,T6 and R1 improved the fruit quality of navel orange.The contents of soluble solids and soluble total sugar in navel orange fruits were significantly increased by 5.2%-20.3% and 12.4%-16.2%,respectively,at the ripening stage.The content of vitamin C was also increased by 8.3%-8.8%,and the contents of sucrose and glucose in soluble sugar fractions were increased by 2.3%-26.5% and 13.7%-27.2%,respectively.However,there was no significant effect on total flavonoids,total phenols,solid-acid ratio and sugar-acid ratio in navel orange fruit.3.Based on the UPLC-MS/MS and GC-MS detection platforms,the number of differential metabolites in R4 treatment was the largest compared with R1,R2 and R3,which had the greatest effect on the metabolism of navel orange pulp.Selenium treatment might affect terpenoid biosynthesis and flavonoid biosynthesis in navel orange fruits.These pathways may affect the synthesis of volatile compounds and antioxidants in navel orange fruits.At the physiological level,such as the changes of vitamin C,total flavonoids and total phenols,selenium treatment may affect the synthesis of antioxidant and other related substances.4.The transcriptome of navel orange pulp was sequenced and analyzed.There were more upregulated genes than down-regulated genes in the selenium treatment group than in the R-CK group.Gene Ontology(GO)analysis was performed on the differentially expressed genes,and the differentially expressed genes were distributed in three categories: biological process,cell composition and molecular function.Among them,the selenium treatment group was significantly enriched in the biological process category compared with R-CK.In the cell composition class,it mainly affects the two functions of the cell anatomical entity and the protein-containing complex.Among all the annotated functions,cellular anatomical entities were the annotations containing the most differentially expressed genes.Compared with R1 treatment,Se treatment also had a greater effect on the biological process of navel orange fruit.5.Selenium treatment group had different effects on gene expression in navel orange pulp compared with R-CK.Differentially expressed genes enriched different number of metabolic pathways.R4 treatment had the greatest effect on navel orange pulp than other selenium treatment groups.Selenium treatment group and R-CK were mapped to plant hormone signal transduction,plant-pathogen interaction,carotenoid biosynthesis and phenylpropyl biosynthesis,which may affect some hormones,disease resistance and resistance in navel orange pulp.The differentially expressed genes of R1 in Se treatment groups jointly affect the metabolism of enriched starch and sucrose,which may affect the accumulation of sugars in navel orange pulp.6.There were co-enriched metabolic pathways among different selenium treatments compared with R-CK and the combined metabolomic and transcriptomic analysis among selenium treatment groups.Compared with R-CK,the main differentially expressed genes in R1 treatment were coenriched in tryptophan metabolism,and the expression of caffeic acid 3-O-methyltransferase in the metabolic pathway was down-regulated,which may affect lignin biosynthesis in navel orange fruits.Compared with R-CK,the expression of(+)-nementhol dehydrogenase was up-regulated in R3 and R4 treatments,which was co-enriched in monoterpenoids,indicating that spraying low concentration of sodium selenite at the late growth stage might affect the synthesis of terpenoids in navel orange fruits.Compared with R-CK,R4 had the most differentially expressed metabolites and co-enriched metabolic pathways,indicating a greater effect of selenium on navel orange fruits.In the combined metabolomic and transcriptome analysis,the expression of β-glucosidase in the metabolic pathway was down-regulated in R2 and R3 treated with selenium compared with R1.metabonomics and transcriptome analysis showed that Se treatment R4 was enriched in arginine and proline metabolism compared with R1.