Content changes of bitter compounds in 'Guoqing No.1' Satsuma mandarin (Citrus unshiu Marc.) during fruit development of consecutive 3 seasons

Roles of citrus fruits on energy expenditure, body weight management, and metabolic biomarkers: a comprehensive review

Citrus fruits are widely consumed for their nutritional and health benefits. They belong to the Rutaceae and have many varieties, such as sweet orange (Citrus sinensis), which is the most popular. Citrus fruits are rich in water (>80%), dietary fiber, and vitamins. They also contain bioactive components, which may modulate energy metabolism and lipid oxidation through various mechanisms. These mechanisms include stimulating β3-adrenergic receptors, increasing mitochondrial biogenesis and thermogenesis, activating AMP kinase and peroxisome proliferator-activated receptor-gamma coactivator-1α pathways, inhibiting lipogenesis and lipid accumulation, and inducing browning of white adipose tissue. This review summarizes the mechanisms and outcomes of citrus fruits and their metabolites on energy metabolism and body weight in different experimental models. The literature was searched for in vitro and in vivo animal and human studies that investigated the effects of citrus consumption on energy expenditure, thermogenesis, adipogenesis, and lipid accumulation. Citrus fruits and their metabolites have shown promising effects on energy metabolism and lipid oxidation in in vitro and in vivo animal studies. However, the evidence from human studies is limited and inconsistent. Possible reasons for the discrepancy are briefly discussed, and knowledge gaps and research needs are identified for future studies. Citrus fruits may have beneficial effects on energy metabolism and body weight, but more rigorous and well-designed human trials are needed to confirm their efficacy and safety.

Comparison of different drying technologies for brocade orange (Citrus sinensis) peels: Changes in color, phytochemical profile, volatile, and biological availability and activity of bioactive compounds

This study evaluated the effects of freeze drying (FD), heat pump drying (HPD), microwave drying (MD), and far-infrared drying (FID) on the quality of brocade orange peels (BOPs). Although the most attractive appearance, maximum levels of ascorbic acid (0.46 mg/g dry weight (DW)), carotenoids (total 16.34 μg/g DW), synephrine (15.58 mg/g DW), limonoids (total 4.60 mg/g DW), phenols (total 9142.80 μg/g DW), and antioxidant activity were observed in FD-BOPs, many aroma components in FD-BOPs were in the minimum levels. HPD-, and MD-BOPs depicted similar trends to FD-BOPs, but they contained the highest concentrations of limonene and β-myrcene. Phenols and ascorbic acid in MD-BOPs generally featured the highest levels of bioavailability, being to 15.99% and 63.94%, respectively. In comparison, FID was not beneficial for the preservation of bioactive compounds and volatile. Therefore, considering time and energy costs, HPD and particularly MD are more appropriate for the commercial production of dried BOPs.

Quantitative metabolomic analysis reveals the fractionation of active compounds during lemon fruit juicing

In this study, metabolomic analysis was employed to investigate the separation (fractionation) of active compounds into lemon juice (LJ) and lemon pomace (LP) during lemon juicing. A total of 968 metabolites were identified, and 438 differentially abundant metabolites (DAMs) were screened out between LJ and LP, suggesting significant metabolite fractionation during juicing. The "flavonoids", "phenolic acids", and "saccharides and alcohols" were mainly retained in the LP, while the fractionation of major "organic acids" was differentiated. Seven of the 12 potential bitter metabolites were more abundant in the LP and two were more abundant in the LJ, suggesting that LP would be more bitter. L-Ascorbic acid, thiamine, and acitretin were significantly lost during juicing, while riboflavin was newly dissolved during juicing. The antioxidant capacity of LP was significantly higher than that of LJ, which was closely related to the higher abundance of phenolic acid metabolites in LP. These findings suggtested that promoting the release of flavonoids and phenolic acids from LP is a potential strategy to improve the quality of LJ. Results also provides important information and reference for developing high-value products by using LP.

Combined Transcriptome and Metabolome Analyses Reveal Candidate Genes Involved in Tangor (Citrus reticulata × Citrus sinensis) Fruit Development and Quality Formation

Tangor, an important citrus type, is a hybrid of orange and mandarin and possesses their advantageous characteristics. Fruit quality is an important factor limiting the development of the citrus industry and highly depends on fruit development and ripening programs. However, fruit development and quality formation have not been completely explored in mandarin-orange hybrids. We sequenced the metabolome and transcriptome of three mandarin-orange hybrid cultivars at the early fruiting [90 days after full bloom (DAFB)], color change (180 DAFB), and ripening (270 DAFB) stages. Metabolome sequencing was performed to preliminarily identify the accumulation patterns of primary and secondary metabolites related to fruit quality and hormones regulating fruit development. Transcriptome analysis showed that many genes related to primary metabolism, secondary metabolism, cell wall metabolism, phytohormones, and transcriptional regulation were up-regulated in all three cultivars during fruit development and ripening. Additionally, multiple key genes were identified that may play a role in sucrose, citric acid and flavonoid accumulation, cell wall modification, and abscisic acid signaling, which may provide a valuable resource for future research on enhancement of fruit quality of hybrid citrus. Overall, this study provides new insights into the molecular basis of pulp growth and development regulation and fruit quality formation in mandarin-orange hybrids.

Grapefruit Debittering by Simultaneous Naringin Hydrolysis and Limonin Adsorption Using Naringinase Immobilized in Agarose Supports

Naringin and limonin are the two main bitter compounds of citrus products such as grapefruit juice. The aim of this investigation was to evaluate the reduction in both bitter components simultaneously using a combined biochemical and physical approach. The proposed strategy was based on the use of heterofunctional supports with glyoxyl groups that allow for the covalent immobilization of naringinase, which hydrolyses naringin and alkyl groups that allow for the adsorption of limonin. The supports were butyl-glyoxyl agarose (BGA) and octyl-glyoxyl agarose (OGA), which were characterized in terms of aldehyde group quantification and FTIR analysis. The optimal pH and temperature of free and immobilized enzymes were assessed. The maximum enzyme loading capacity of supports was analyzed. Debittering of grapefruit juice was evaluated using soluble enzyme, enzyme-free supports, and immobilized catalysts. Enzyme immobilized in BGA reduced naringin and limonin concentrations by 54 and 100%, respectively, while the use of catalyst immobilized in OGA allowed a reduction of 74 and 76%, respectively, obtaining a final concentration of both bitter components under their detection threshold. The use of OGA biocatalyst presented better results than when soluble enzyme or enzyme-free support was utilized. Biocatalyst was successfully applied in juice debittering in five repeated batches.

Variation in Phytochemical, Antioxidant and Volatile Composition of Pomelo Fruit (Citrus grandis (L.) Osbeck) during Seasonal Growth and Development

Citrus fruits exhibit a high level of different phytoconstituents, of which the changes in the different parts of the fruit during ripening have not been thoroughly studied yet. Thus, in this study, we have investigated how different parts of pomelo fruit (Citrus grandis L.) are modified throughout the development of two consecutive growing seasons. In detail, the main phytochemical compounds, such as total phenolic content, total flavonoid content, antioxidant capacity, DPPH free radical scavenging activity, Ferric reducing antioxidant power (FRAP), and naringin and tannin content, were analyzed. A systematic metabolism of these compounds was found during the development of the fruit, but some pomelo tissues showed a fluctuating trend, suggesting a dependence on the different growing season. Focusing on the tissue distribution of these compounds, the fruit membrane contained the highest level of total phenolic and flavonoid content; fruit flavedo displayed the highest antioxidant capacities and FRAP activities, whereas maximum accumulation of naringin was noticed in fruit albedo. Instead, the highest DPPH free radical scavenging activity and tannin contents were found in the pomelo juice. Regarding the distribution of compounds, a possible bias pattern for the accumulation of those compounds has been noticed throughout the fruit development. From the GC-MS analysis, a total of 111 compounds were identified, where 91 compounds were common in both seasons. Overall, these results could be useful for the food processing industry as guidelines for excellent quality foods and for introducing health-beneficial products and components into our daily diets.

Primary Bitter Taste of Citrus is Linked to a Functional Allele of the 1,2-Rhamnosyltransferase Gene Originating from Citrus grandis

1,2-Rhamnosyltransferase (1,2RhaT) catalyzes the final step of production of flavanone neohesperidoside (FNH) that is responsible for the primary bitter taste of citrus fruits. In this study, species-specific flavonoid profiles were determined in 87 Citrus accessions by identifying eight main flavanone glycosides (FGs). Accumulation of FNHs was completely correlated to the presence of the 1,2RhaT gene in 87 citrus accessions analyzed using a novel 1,2RhaT-specific DNA marker. Pummelo (Citrus grandis) was identified as the genetic origin for a function allele of 1,2RhaT that underpinned FNH-bitterness in modern citrus cultivars. In addition, genes encoding six MYB and five bHLH transcription factors were shown to coexpress with 1,2RhaT and other flavonoid pathway genes related to FNH accumulation, indicating that these transcription factors may affect the fruit taste of citrus. This study provides a better understanding of bitterness formation in Citrus varieties and a genetic marker for the early selection of nonbitterness lines in citrus breeding programs.

Emerging approaches to determine maturity of citrus fruit

Owing to their health-boosting properties and other appreciable properties, citrus fruit is widely consumed and commercialized worldwide. Destination markets around the world vary in their fruit quality requirements and are also highly influenced by climatic conditions, agronomical and postharvest practices. Hence, harvesting decisions are arduous. Maturity indices in citrus fruit are highly variable and dependent on the species and varieties, growing regions, and destination markets. For decades, determination of the maturity of citrus fruit and predicting the near time of harvesting was a challenge for producers, researchers, and food safety agencies. Thus, the current review provides a correlation between maturity and internal components and an overview of techniques of maturity determination for citrus fruits. Also, stress has been given to the destructive and nondestructive methods to determine the maturity level of different citrus species. The techniques presented in this review portray continuous productiveness as an excellent quality assessment, particularly as ripening and maturity analysis tools for citrus fruits. Traditional techniques are time-consuming, laborious, costly, destructive, and tedious. Thus, these nondestructive techniques hold great potential to replace conventional procedures.

The MYB transcription factor CiMYB42 regulates limonoids biosynthesis in citrus

BACKGROUND

Limonoids are major bioactive compounds that are produced by the triterpenoid metabolic pathway. The detailed biochemical process of limonoid biosynthesis and the mechanism of its molecular regulation remain elusive. The identification of transcription factors that regulate limonoid biosynthetic pathways is very important for understanding the underlying regulatory mechanisms. This information could also provide tools for manipulating biosynthesis genes to modulate limonoid production.

RESULTS

In this study, the CiMYB42 transcription factor was isolated to identify its role in limonoid biosynthesis. Multiple alignment analysis and phylogenetic analysis demonstrated that CiMYB42 is a typical R2R3MYB transcription factor that shares high similarity of its amino acid sequence with AtMYB42. Limonoids contents were higher in Citrus sinensis and Citrus grandis than in other species. Limonoid accumulation during leaf development also showed diverse trends in different genotypes. The expression of CiMYB42 was significantly related to the limonoid content and the expression of CiOSC in some citrus accessions. The overexpression of CiMYB42 in sweet orange resulted in significant accumulation of limonin, whereas the downregulation of CiMYB42 by RNAi resulted in a dwarf phenotype and less nomilin accumulation. Furthermore, the results of a yeast one-hybrid assay and EMSA indicated that CiMYB42 binds exclusively to the TTGTTG sequence (type II MYB core) in the promoter of CiOSC. Together, these results suggest that CiMYB42 positively regulates limonoid biosynthesis by regulating the expression of CiOSC by binding to the TTGTTG sequence (type II MYB core) of its promoter.

CONCLUSIONS

CiMYB42 is an important transcription activator involved in limonoid biosynthesis that regulates the expression of CiOSC by binding to the TTGTTG sequence (type II MYB core).

Metabolite profile comparison of a graft chimera 'Hongrou Huyou' (Citrus changshan-huyou + Citrus unshiu) and its two donor plants

BACKGROUND

Chimeras synthesized artificially by grafting are crucial to the breeding of perennial woody plants. 'Hongrou Huyou' (Citrus changshan-huyou + Citrus unshiu) is a new graft chimera originating from the junction where a Citrus changshan-huyou ("C") scion was top-grafted onto a stock Satsuma mandarin 'Owari' (C. unshiu, "O"). The chimera was named OCC because the cell layer constitutions were O for Layer 1(L1) and C for L2 and L3. In this study, profiles of primary metabolites, volatiles and carotenoids derived from different tissues in OCC and the two donors were investigated, with the aim of determining the relationship between the layer donors and metabolites.

RESULTS

The comparison of the metabolite profiles showed that the amount and composition of metabolites were different between the peels and the juice sacs, as well as between OCC and each of the two donors. The absence or presence of specific metabolites (such as the carotenoids violaxanthin and β-cryptoxanthin, the volatile hydrocarbon germacrene D, and the primary metabolites citric acid and sorbose) in each tissue was identified in the three phenotypes. According to principal component analysis (PCA), overall, the metabolites in the peel of the chimera were derived from donor C, whereas those in the juice sac of the chimera came from donor O.

CONCLUSION

The profiles of primary metabolites, volatiles and carotenoids derived from the peels and juice sacs of OCC and the two donors were systematically compared. The content and composition of metabolites were different between the tissues and between OCC and the each of the two donors. A clear donor dominant pattern of metabolite inheritance was observed in the different tissues of OCC and was basically consistent with the layer origin; the peel of the chimera was derived from C, and the juice sacs of the chimera came from O. These profiles provide potential chemical markers for genotype differentiation, citrus breeding assessment, and donor selection during artificial chimera synthesis.

Integrative Analysis of Terpenoid Profiles and Hormones from Fruits of Red-Flesh Citrus Mutants and Their Wild Types

In citrus color mutants, the levels of carotenoid constituents and other secondary metabolites are different in their corresponding wild types. Terpenoids are closely related to coloration, bitterness, and flavor. In this study, terpenoid profiles and hormones in citrus fruits of two red-flesh mutants—Red Anliu orange and Red-flesh Guanxi pummelo—and their corresponding wild types were investigated using GC/MS, HPLC, and LC-MS/MS. Results showed that Red Anliu orange (high in carotenoids) and Anliu orange (low in carotenoids) accumulated low levels of limonoid aglycones but high levels of monoterpenoids; conversely, Red-flesh Guanxi pummelo (high in carotenoids) and Guanxi pummelo (deficient in carotenoids) accumulated high levels of limonoid aglycones but low levels of monoterpenoids. However, isopentenyl diphosphate was present at similar levels. A correlation analysis indicated that jasmonic and salicylic acids might play important roles in regulating terpenoid biosynthesis. Additionally, the similarities of carotenoid and volatile profiles between each mutant and its corresponding wild type were greater than those between the two mutants or the two wild types. The flux balance of terpenoid metabolism in citrus fruit tends toward stability among various citrus genera that have different terpenoid profiles. Bud mutations could influence metabolite profiles of citrus fruit to a limited extent.

Variation in limonin and nomilin content in citrus fruits of eight varieties determined by modified HPLC

The nomilin and limonin content in citrus fruits of different varieties was determined at fruit growth and maturation stages by HPLC. The results showed that the two limonoids can be separated, identified, and quantified in citrus fruits within 10 min by the developed method. The method exhibited good precision, repeatability, stability, and recovery rate. The content of limonin and nomilin in most citrus fruits presented an increasing trend initially, and then decreased during fruit growth and maturation; a peak was observed at the young fruit or fruit expansion stage. The dropped fruits also contained some amount of limonoids, suggesting their industrial application. The variation and cluster analyses results revealed that the orange varieties contained the highest amount of limonoids at the mature stage. The results of this study will enable better use of citrus limonoids.

Largely different carotenogenesis in two pummelo fruits with different flesh colors

Carotenoids in citrus fruits have health benefits and make the fruits visually attractive. Red-fleshed ‘Chuhong’ (‘CH’) and pale green-fleshed ‘Feicui’ (‘FC’) pummelo (Citrus maxima (Burm) Merr.) fruits are interesting materials for studying the mechanisms of carotenoid accumulation. In this study, particularly high contents of linear carotenes were observed in the albedo tissue, segment membranes and juice sacs of ‘CH’. However, carotenoids, especially β-carotene and xanthophylls, accumulated more in the flavedo tissue of ‘FC’ than in that of ‘CH’. Additionally, the contents of other terpenoids such as limonin, nomilin and abscisic acid significantly differed in the juice sacs at 150 days postanthesis. A dramatic increase in carotenoid production was observed at 45 to 75 days postanthesis in the segment membranes and juice sacs of ‘CH’. Different expression levels of carotenogenesis genes, especially the ζ-carotene desaturase (CmZDS), β-carotenoid hydroxylase (CmBCH) and zeaxanthin epoxidase (CmZEP) genes, in combination are directly responsible for the largely different carotenoid profiles between these two pummelo fruits. The sequences of eleven genes involved in carotenoid synthesis were investigated; different alleles of seven of eleven genes might also explain the largely different carotenogenesis observed between ‘CH’ and ‘FC’. These results enhance our understanding of carotenogenesis in pummelo fruits.

Effect of drying temperature on the sugars, organic acids, limonoids, phenolics, and antioxidant capacities of lemon slices

Changes in contents of sugars, organic acids, limonoids, phenolics contents, and antioxidant capacities of lemon slices dried at different temperatures were evaluated. Air drying (AD) promoted losses of sugars, citric acid, ascorbic acid, extractable phenolics (EPs), and non-extractable phenolics (NEPs), while it introduced an increase in limonoids. Phenolics of lemon were mainly presented in their extractable form. Hesperidin and eriocitrin were the main EPs; protocatechuic acid and poncirin were the predominant NEPs. The decrease in extractable phenolic acid, EP, and NEP content in lemon is lower at low drying temperatures, while the increase in non-extractable phenolic acid content is higher at high drying temperatures. The antioxidant capacity of EP was higher than that of NEP. Phenolics contributed to antioxidant capacities of lemon slices, and flavonoids were the main contributors among phenolics. Considering limonoids contents and the high levels of EP, NEP, and antioxidant capacities, AD at 60 °C could be an appreciate treatment for dehydrating lemon slices.

An Integrated Approach to Mandarin Processing: Food Safety and Nutritional Quality, Consumer Preference, and Nutrient Bioaccessibility

Mandarins are a member of the Citrus genus and are the focus of growing commercial interest, with satsuma mandarins (Citrus unshiu) and the common mandarin (Citrus reticulata Blanco) being the most important mandarin varieties. The possible health benefits and functional properties of those fruits are often associated with the antioxidative function of vitamin C, carotenoids, and phenolic compounds. While most mandarins are consumed fresh, many are processed into juices (mostly cloudy), usually via thermal processing which can lead to the creation of off-flavors and may diminish nutritional quality. The aim of this review is to summarize the most significant and recent information on the safety, sensorial properties, and nutritional benefits of mandarins and their processing into juice. The article also discusses recent information regarding the bioaccessibility of valuable, mandarin specific, compounds.

Characterization and Metabolic Diversity of Flavonoids in Citrus Species

Flavonoids are widely distributed in plants and play important roles in many biological processes. Citrus fruits are rich dietary sources of flavonoids. However, there have been very few reports about the comprehensive metabolic profile and natural diversity of flavonoids in different tissues of various Citrus cultivars. In this study, based on the 7416 metabolic signals detected with non-targeted metabolomics approach, Principal Component Analysis revealed the flavedo has the largest differences from other tissues in metabolite levels; as many as 198 flavonoid signals were then detected in 62 Citrus germplasms from 5 species mainly cultivated worldwide, while 117 flavonoids were identified, including 39 polymethoxylated flavonoids (PMFs), 7 flavones, 10 C-O-glycosylflavonoids, 44 O-glycosylflavonoids, 10 C-glycosylflavonoids and 7 newly annotated O-glycosylpolymethoxylated flavonoids. Tissue-specific accumulations were observed: O-glycosylated flavonoids were abundant in all fruit tissues, while PMFs were accumulated preferentially in the flavedo. Among different species, mandarins had the highest levels of PMFs and O-glycosylpolymethoxylated flavonoids, followed by sweet oranges. Based on the flavonoid profiles, 62 germplasms could be clearly grouped into five distinct clusters via hierarchical clustering analysis, which were perfectly matched with their species, with sweet oranges and mandarins clustering closely and being further away from other three species.

Identification of Putative Genes Involved in Limonoids Biosynthesis in Citrus by Comparative Transcriptomic Analysis

Limonoids produced by citrus are a group of highly bioactive secondary metabolites which provide health benefits for humans. Currently there is a lack of information derived from research on the genetic mechanisms controlling the biosynthesis of limonoids, which has limited the improvement of citrus for high production of limonoids. In this study, the transcriptome sequences of leaves, phloems and seeds of pummelo (Citrus grandis (L.) Osbeck) at different development stages with variances in limonoids contents were used for digital gene expression profiling analysis in order to identify the genes corresponding to the biosynthesis of limonoids. Pair-wise comparison of transcriptional profiles between different tissues identified 924 differentially expressed genes commonly shared between them. Expression pattern analysis suggested that 382 genes from three conjunctive groups of K-means clustering could be possibly related to the biosynthesis of limonoids. Correlation analysis with the samples from different genotypes, and different developing tissues of the citrus revealed that the expression of 15 candidate genes were highly correlated with the contents of limonoids. Among them, the cytochrome P450s (CYP450s) and transcriptional factor MYB demonstrated significantly high correlation coefficients, which indicated the importance of those genes on the biosynthesis of limonoids. CiOSC gene encoding the critical enzyme oxidosqualene cyclase (OSC) for biosynthesis of the precursor of triterpene scaffolds was found positively corresponding to the accumulation of limonoids during the development of seeds. Suppressing the expression of CiOSC with VIGS (Virus-induced gene silencing) demonstrated that the level of gene silencing was significantly correlated to the reduction of limonoids contents. The results indicated that the CiOSC gene plays a pivotal role in biosynthesis of limonoids.

Deficit irrigation and maturation stage influence quality and flavonoid composition of 'Valencia' orange fruit

BACKGROUND

Effects of continuous deficit irrigation (DI) and partial rootzone drying (PRD) treatments (50% ETc) in comparison with full irrigation (CI, 100% ETc) were investigated during 'Valencia' orange fruit maturation. Ultra-high-performance liquid chromatography/high-resolution mass spectrometry was used to quantify hesperidin, narirutin, tangeritin, nobiletin, didymin and neoeriocitrin in the fruit juice and peel.

RESULTS

No significant effect of irrigation was found on yield, juice soluble solids or acidity. Juice color was not influenced by irrigation or harvest date, whereas peel color increased during maturation and was more pronounced in CI and PRD fruits. Juice acidity reached a peak in May, while soluble solids increased linearly throughout maturation. Hesperidin was the major flavanone detected during maturation, with concentrations 200-fold higher in the fruit peel than in the juice. In the peel, narirutin, didymin and neoeriocitrin decreased while hesperidin, nobiletin and tangeritin increased with maturation. Narirutin synthesis in the orange fruit was insensitive to irrigation strategy. In fruit peels, PRD and DI induced the decline of hesperidin, nobiletin and tangeritin only in June, whereas in the juice, deficit irrigation treatments induced an increase in hesperidin and didymin.

CONCLUSION

These results suggest that deficit irrigation, in particular the conditions imposed with PRD, may cause a significant accumulation shift of total flavonoids from the fruit peel into the juice, with a positive impact on juice quality and nutritional value. Fruit compositional changes during maturation also suggest that late harvest can improve fruit palatability and nutritional quality under the cultural and environmental conditions of this study. © 2016 Society of Chemical Industry.

Comprehensive comparative analysis of volatile compounds in citrus fruits of different species

The volatile profiles of fruit peels and juice sacs from 108 citrus accessions representing seven species were analyzed. Using GC-MS 162 and 107 compounds were determined in the peels and juice sacs, respectively. In the peels, monoterpene alcohols were accumulated in loose-skin mandarins; clementine tangerines and papedas were rich in sesquiterpene alcohols, sesquiterpenes, monoterpene alcohols and monoterpene aldehydes. β-pinene and sabinene were specifically accumulated in 4 of 5 lemon germplasms. Furthermore, concentrations of 34 distinctive compounds were selected to best represent the volatile profiles of seven species for HCA analysis, and the clustering results were in agreement with classic citrus taxonomy. Comparison of profiles from different growing seasons and production areas indicated that environmental factors play important roles in volatile metabolism. In addition, a few citrus germplasms that accumulated certain compounds were determined as promising breeding materials. Notably, volatile biosynthesis via MVA pathway in C. ichangensis 'Huaihua' was enhanced.

Largely different contents of terpenoids in beef red-flesh tangerine and its wild type

BACKGROUND

Niurouhong (Citrus reticulata Blanco. Niurouhong) (NRH) is a spontaneous beef-red flesh mutant with distinctive flavor compared with its wild type orange-red flesh Zhuhongju (ZHJ). To illustrate the biochemical mechanism of its special flesh color and flavor, fruits at commercial mature stage were used to profile the volatiles in the flavedo and determine the levels of carotenoids, limonoid aglycones and phytohormones in the juice sacs in two seasons.

RESULTS

Our results showed the content of total volatile terpenoids in NRH was 1.27-fold that in ZHJ. The components of volatiles were found to be common between the two tangerines. This result indicates that the distinctive flavor of NRH might not be derived from the presence/absence of specific volatiles; instead, it was derived from the altered concentrations or balance of α-citral, β-citral, 2-cyclohexen-1-one, (S)-3-methyl-6-(1-methylethenyl) and n-hexadecanoic acid. Analyses of the contents of total and specific carotenoids indicated that the beef-red color of NRH flesh might be largely attributed to the over accumulation of β-cryptoxanthin and β-carotene. However, lower ABA level was found in NRH than in ZHJ, reflecting a possible feedback regulation of ABA biosynthesis on carotenogenesis and the balance in the metabolism among terpenoids.

CONCLUSIONS

Collectively, our study suggested that the MEP pathway was enhanced in NRH tangerine. However, a certain unknown co-regulatory mechanism might be present in the metabolism pathway of secondary metabolites (especially terpenoids) in beef-red flesh mutant. Our study provides new insights into the regulatory network of terpenoid metabolism and mutation mechanism of red-fleshed citrus.

The Chemistry and Pharmacology of Citrus Limonoids

Citrus limonoids (CLs) are a group of highly oxygenated terpenoid secondary metabolites found mostly in the seeds, fruits and peel tissues of citrus fruits such as lemons, limes, oranges, pumellos, grapefruits, bergamots, and mandarins. Represented by limonin, the aglycones and glycosides of CLs have shown to display numerous pharmacological activities including anticancer, antimicrobial, antioxidant, antidiabetic and insecticidal among others. In this review, the chemistry and pharmacology of CLs are systematically scrutinised through the use of medicinal chemistry tools and structure-activity relationship approach. Synthetic derivatives and other structurally-related limonoids from other sources are include in the analysis. With the focus on literature in the past decade, the chemical classification of CLs, their physico-chemical properties as drugs, their biosynthesis and enzymatic modifications, possible ways of enhancing their biological activities through structural modifications, their ligand efficiency metrics and systematic graphical radar plot analysis to assess their developability as drugs are among those discussed in detail.

Influence of storage temperature and low-temperature conditioning on the levels of health-promoting compounds in Rio Red grapefruit

Commercial operations use low‐temperature conditioning of citrus fruits to reduce the incidence of chilling injury (CI) during cold storage and quarantine treatments. Rio Red grapefruits (Citrus paradisi Macf) were stored for 12 weeks at 11°C or 5°C; an additional set was temperature conditioned at 16°C for 7 days before storing at 5°C (CD). Every 3 weeks, samples were assessed for chilling injury (CI) and health‐promoting compounds such as ascorbic acid, carotenoids, limonoids, flavonoids, and furocoumarins. Low‐temperature conditioning significantly reduced CI but did not affect the total soluble solids, acidity, and ripening ratio. After 12 weeks of storage, grapefruits showed no significant differences in lycopene, narirutin, poncirin, furocoumarins, and radical scavenging activity in all the three treatments. Limonin was significantly higher (p < .05) in CD fruits, nomilin was significantly higher in fruits stored at 11°C, whereas fruits stored at 5°C had lower levels of naringin, neohesperidin, and didymin after 12 weeks of storage. Low‐temperature conditioning treatment helped fruits to retain similar or higher levels of most of the health‐promoting compounds by the end of storage period while maintaining better quality than the nonconditioned fruits.

Temporal and spatial variations on accumulation of nomilin and limonin in the pummelos

Limonoids are the important secondary metabolites in the citrus. In this study, the accumulation of limonoids at different fruit developmental stages and distribution among different genotypes, tissues and developmental stages were investigated in 12 pummelo varieties. The large variations on limonoids concentration were found among different varieties, which ranged from 233.78 mg/kg FW to 4090.41 mg/kg FW in the seeds at full color stage of the fruit. Classification of pummelos based on the limonoids content divided 12 varieties into three groups. It was matched well with the geographic origination of the pummelo varieties, suggesting that the accumulation of limonoids was mainly determined by the genotype of the pummelo. Accumulation of the limonoids in different tissues was highly variable, and in a tissue specific fashion. The trend of the change on the levels of nomilin and limonin in the seeds and segment membrane were corresponded to the physiological development of the fruit. The rapid accumulation of nomilin and limonoids was observed from the physiological ripening of the seeds. It suggested that physiological maturation of the seeds is a key point that the seeds accelerate the accumulation of nomilin and limonin. In most of pummelo varieties, 10% color break of the fruit was a phenotypic landmark associated with the maximum level of nomilin accumulated in the seeds.

Spatio-temporal distribution and natural variation of metabolites in citrus fruits

To study the natural variation and spatio-temporal accumulation of citrus metabolites, liquid chromatography tandem mass spectrometry (LC-MS) based metabolome analysis was performed on four fruit tissues (flavedo, albedo, segment membrane and juice sacs) and different Citrus species (lemon, pummelo and grapefruit, sweet orange and mandarin). Using a non-targeted metabolomics approach, more than 2000 metabolite signals were detected, from which more than 54 metabolites, including amino acids, flavonoids and limonoids, were identified/annotated. Differential accumulation patterns of both primary metabolites and secondary metabolites in various tissues and species were revealed by our study. Further investigation indicated that flavedo accumulates more flavonoids while juice sacs contain more amino acids. Besides this, cluster analysis based on the levels of metabolites detected in 47 individual Citrus accessions clearly grouped them into four distinct clusters: pummelos and grapefruits, lemons, sweet oranges and mandarins, while the cluster of pummelos and grapefruits lay distinctly apart from the other three species.

Features of citrus terpenoid production as revealed by carotenoid, limonoid and aroma profiles of two pummelos (Citrus maxima) with different flesh color

BACKGROUND

Terpenoids are major components of carotenoids, limonoids and aromas in citrus fruits, resulting in fruit coloration, bitterness and aroma. In this study the carotenoid, limonoid and volatile profiles of red-flesh Chuhong pummelo (CH) and pale green-flesh Feicui pummelo (FC) were investigated by HPLC and GC/MS.

RESULTS

Large differences were found in constituents of carotenoids and limonoids in juice sacs and flavedo and of aromas in flavedo of the two pummelos. For carotenoids in juice sacs, CH contained 57 times the amount in FC, mainly all-trans-lycopene and phytoene, whereas in flavedo it contained only 25% of that in FC, the latter showing a high proportion of β-carotene and other chloroplastic carotenoids. In comparison with FC, limonin and nomilin aglycone production was boosted in juice sacs of CH while being almost absent in flavedo. For volatiles in flavedo, the total amount was significantly higher in CH. PCA suggested that germacrene-type sesquiterpenoids, etc. were principal in distinguishing volatile profiles of the two pummelos.

CONCLUSION

The data showed a different tissue-biased pattern of carotenoid and limonoid aglycone synthesis in pummelos with different flesh color, and the possible independently regulated synthesis of those metabolites in different fruit tissues. Furthermore, decreased carotenoid and limonoid aglycone production accompanied by increased accumulation of volatile terpenoids in flavedo of red-flesh CH was identified, indicating that a total capacity or a balance of production of various terpenoids might exist in pummelo fruit tissues. It was also suggested that substrate concentration is not the key factor affecting product concentrations during the synthesis of monoterpene derivatives.