Comparative analysis of flower volatiles from nine citrus at three blooming stages

Variation in the Floral Scent Chemistry of Nymphaea 'Eldorado', a Valuable Water Lily, with Different Flowering Stages and Flower Parts

Nymphaea 'Eldorado', a valuable water lily, is a well-known fragrant plant in China. Studying the temporal and spatial characteristics of the floral components of this plant can provide a reference for the further development and utilization of water lily germplasm resources. In this study, headspace solid-phase microextraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS) was used to explore the types and relative contents of floral components at different flowering stages (S1: bud stage; S2: initial-flowering stage; S3: full-flowering stage; S4: end-flowering stage) and in different floral organs of N. 'Elidorado', combined with the observation of the microscopic structure of petals. A total of 60 volatile organic compounds (VOCs) were detected at different flowering stages, and there were significant differences in floral VOCs at different flowering stages and in different flower organs. The volatile compounds of N. 'Eldorado' can be divided into seven chemical classes,, namely, alkenes, alcohols, esters, aldehydes, ketones, alkanes, and others; the most common were alkenes and alkanes. A total of 39, 44, 47, and 42 volatile compounds were detected at S1, S2, S3, and S4. The VOCs present in high concentrations include benzaldehyde, benzyl alcohol, benzyl acetate, trans-α-bergamotene, α-curcumene, cis-α-farnesene, and so on. The types and total contents of volatiles at the full-flowering stage were higher than at other flowering stages. Comparing the VOCs in different parts of flower organs, it was found that the contents of alcohols, esters, and aldehydes were greatest in the petals, the alkenes in stamens were abundant with a relative content of up to 54.93%, and alkanes in the pistil were higher than in other parts. The types and total contents of volatiles in the stamens of N. 'Eldorado' were higher than those in other flower organs; they were the main part releasing fragrance. The observation of petal microstructure revealed that the size and quantity of the papillae on the epidermises of petals, the number of intracellular plastids, and the aggregates of floral components (osmophilic matrix granules) were significantly higher at the full-flowering stage than at the other flowering stages. This study suggested the main flowering stage and location at which the floral VOCs are released by N. 'Eldorado' and provided a reference for guiding the breeding of this water lily, exploring genetic patterns and developing related products.

Analysis of Volatile Components in Rosa roxburghii Tratt. and Rosa sterilis Using Headspace-Solid-Phase Microextraction-Gas Chromatography-Mass Spectrometry

Volatile organic compounds (VOCs) and flavor characteristics of Rosa roxburghii Tratt. (RR) and Rosa sterilis (RS) were analyzed using headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry (HS-SPME-GC-MS). The flavor network was constructed by combining relative odor activity values (ROAVs), and the signature differential flavor components were screened using orthogonal partial least squares discriminant analysis (OPLS-DA) and random forest (RF). The results showed that 61 VOCs were detected in both RR and RS: 48 in RR, and 26 in RS. There were six key flavor components (ROAVs ≥ 1) in RR, namely nonanal, ethyl butanoate, ethyl hexanoate, (3Z)-3-hexen-1-yl acetate, ethyl caprylate, and styrene, among which ethyl butanoate had the highest contribution, whereas there were eight key flavor components (ROAVs ≥ 1) in RS, namely 2-nonanol, (E)-2-hexenal, nonanal, methyl salicylate, β-ocimene, caryophyllene, α-ionone, and styrene, among which nonanal contributed the most to RS. The flavor of RR is primarily fruity, sweet, green banana, and waxy, while the flavor of RS is primarily sweet and floral. In addition, OPLS-DA and RF suggested that (E)-2-hexenal, ethyl caprylate, β-ocimene, and ethyl butanoate could be the signature differential flavor components for distinguishing between RR and RS. In this study, the differences in VOCs between RR and RS were analyzed to provide a basis for further development and utilization.

Artificial flowers as a tool for investigating multimodal flower choice in wild insects

Flowers come in a variety of colours, shapes, sizes and odours. Flowers also differ in the quality and quantity of nutritional reward they provide to entice potential pollinators to visit. Given this diversity, generalist flower-visiting insects face the considerable challenge of deciding which flowers to feed on and which to ignore. Working with real flowers poses logistical challenges due to correlations between flower traits, maintenance costs and uncontrolled variables. Here, we overcome this challenge by designing multimodal artificial flowers that varied in visual, olfactory and reward attributes. We used artificial flowers to investigate the impact of seven floral attributes (three visual cues, two olfactory cues and two rewarding attributes) on flower visitation and species richness. We investigated how flower attributes influenced two phases of the decision-making process: the decision to land on a flower, and the decision to feed on a flower. Artificial flowers attracted 890 individual insects representing 15 morphospecies spanning seven arthropod orders. Honeybees were the most common visitors accounting for 46% of visitors. Higher visitation rates were driven by the presence of nectar, the presence of linalool, flower shape and flower colour and was negatively impacted by the presence of citral. Species richness was driven by the presence of nectar, the presence of linalool and flower colour. For hymenopterans, the probability of landing on the artificial flowers was influenced by the presence of nectar or pollen, shape and the presence of citral and/or linalool. The probability of feeding increased when flowers contained nectar. For dipterans, the probability of landing on artificial flowers increased when the flower was yellow and contained linalool. The probability of feeding increased when flowers contained pollen, nectar and linalool. Our results demonstrate the multi-attribute nature of flower preferences and highlight the usefulness of artificial flowers as tools for studying flower visitation in wild insects.

Ecological convergence in phytochemistry and flower-insect visitor interactions along an Andean elevation gradient

The diversity of specialized molecules produced by plants radiating along ecological gradients is thought to arise from plants' adaptations to local conditions. Therefore, closely related species growing in similar habitats should phylogenetically converge, or diverge, in response to similar climates, or similar interacting animal communities. We here asked whether closely related species in the genus Haplopappus (Asteraceae) growing within the same elevation bands in the Andes, converged to produce similar floral odors. To do so, we combine untargeted analysis of floral volatile organic compounds with insect olfactory bioassay in congeneric Haplopappus (Asteraceae) species growing within the same elevation bands along the Andean elevational gradient. We then asked whether the outcome of biotic interactions (i.e., pollination vs. seed predation) would also converge across species within the same elevation. We found that flower odors grouped according to their elevational band and that the main floral visitor preferred floral heads from low-elevation band species. Furthermore, the cost-benefit ratio of predated versus fertilized seeds was consistent within elevation bands, but increased with elevation, from 6:1 at low to 8:1 at high elevations. In the light of our findings, we propose that climate and insect community changes along elevation molded a common floral odor blend, best adapted for the local conditions. Moreover, we suggest that at low elevation where floral resources are abundant, the per capita cost of attracting seed predators is diluted, while at high elevation, sparse plants incur a higher herbivory cost per capita. Together, our results suggest that phytochemical convergence may be an important factor driving plant-insect interactions and their ecological outcomes along ecological gradients.

Investigation of phenological, primary and secondary metabolites changes during flower developmental of Rosa damascena

Oil-bearing rose is a very valuable member of the Rosa genus. Despite the importance of oil-bearing rose, metabolic changes during flower development are not well understood. Thus, the objective of this study was to investigate the changes in phenological, primary and secondary metabolites and their interactions at five developmental stages of oil-bearing rose. Flower width, flower and petal fresh weights, petal area and petal relative water content increased from bud stage to blooming stage, while flower length and sepal area increased only at early stages. Thirty-seven essential oil components were identified at different stages of petal development and nonadecane, β-citronellol and n-heneicosane were the prevalent essential oil components regardless of stage. Sixteen fatty acids were identified and the amount of saturated fatty acids was higher than the mono and polyunsaturated fatty acids in all developmental stages. Eight organic acids were detected in petals and four of them (tartaric, malic, citric and succinic acids) showed significant changes, and total organic acids content decreased during flower development. Catechin and epicatechin were the most abundant phenolic compounds in petals. While total phenolic, flavonoid and free amino acids contents decreased during flower development, total free fatty acids content increased, but was not significant between the developmental stages. Correlation analysis between phenological traits and some metablolites revealed 20 significant correlations and 11 of which were positive. Results showed that flower development stages had significant effects on metabolite content and quality of products obtained, and significant shifts in metabolite type and content occurred at flower development stages III and IV.

Evaluation and Comparison of Pear Flower Aroma Characteristics of Seven Cultivars

Due to its ornamental and medicinal value, pear flower has been historically loved and used in China. However, the current understanding of their odor-active compounds and aroma profiles is rather limited. This work aimed to evaluate and compare the overall aroma profile of pear flowers; the volatiles in flowers of seven pear cultivars (Anli, Bayuesu, Golden, Brown peel, KorlaXiangli, Lyubaoshi, Xizilü) were analyzed using solid-phase microextraction–gas chromatography-mass spectrometry (SPME-GC-MS). A total of 93 volatile compounds were identified and quantified within the amount of volatiles in the range of 62.7–691.8 μg kg−1 (FW) and showed high and significant variability in different cultivars. Anli and Brown peel flowers showed a relatively higher volatile abundance, while KorlaXiangli flowers were significantly lower than other cultivars. Although the composition of volatiles depended on the existence of different chemical classes, the odor activity values (OAVs) and odor descriptions showed some aldehydes were part of their main peculiarities and were considered as the basic active odorants that presented strong intensity of citrus and floral odor. Moreover, multivariate analysis showed the pear flower of different cultivars could be arranged in different clusters by the identified odorants. This study provides first-hand knowledge regarding pear flower aroma profiles, and that the cultivar differences were critical for the overall pattern.

Aroma Components of Absolute Oil from Natsudaidai (Citrus natsudaidai Hayata) Flowers

The aim of this study was to identify and characterize the aroma components of absolute oil from natsudaidai (Citrus natsudaidai Hayata) flowers. A total of 43 aroma components were detected in the absolute oil of natsudaidai flowers using a headspace solid phase microextraction (SPME)-gas chromatography-mass spectrometry (GC-MS). The most abundant components from the absolute oil was linalool (31.14%), followed by methyl anthranilate, γ-terpinene, p-cymene, (E)-β-ocimene, limonene, indole and α-terpineol. The configuration of linalool from the absolute oil was assigned as (S)-form and its optical purities were determined as 89.36±0.36% enantiomeric excess using a SPME-chiral GC. These results indicated that the composition of aroma components in the absolute oil would influence the overall aroma qualities of natsudaidai flowers and the physiological effects on human.

Safety Assessment of Citrus Flower- and Leaf-Derived Ingredients as Used in Cosmetics

The Cosmetic Ingredient Review (CIR) Expert Panel (Panel) assessed the safety of 33 Citrus flower- and leaf-derived ingredients, which are most frequently reported to function in cosmetics as fragrances and/or skin-conditioning agents. The Panel reviewed the available data to determine the safety of these ingredients. Because final product formulations may contain multiple botanicals, each containing similar constituents of concern, formulators are advised to be aware of these constituents and to avoid reaching levels that may be hazardous to consumers. With Citrus flower- and leaf-derived ingredients, the Panel was concerned about the presence of the hydroperoxides of limonene and linalool in cosmetics. Industry should use good manufacturing practices to limit impurities that could be present in botanical ingredients. The Panel concluded that these ingredients are safe in the present practices of use and concentration when formulated to be non-irritating and non-sensitizing.

The Sensory Quality Improvement of Citrus Wine through Co-Fermentations with Selected Non-Saccharomyces Yeast Strains and Saccharomyces cerevisiae

Co-fermentation of selected non-Saccharomyces yeast strain with Saccharomyces cerevisiae is regarded as a promising approach to improve the sensory quality of fruit wine. To evaluate the effects of co-fermentations between the selected non-Saccharomyces yeast strains (Hanseniaspora opuntiae, Hanseniaspora uvarum and Torulaspora delbrueckii) and S. cerevisiae on the sensory quality of citrus wine, the fermentation processes, the chemical compositions, and the sensory evaluations of citrus wines were analyzed. Compared with those of S. cerevisiae fermentation, co-fermentations produced high sensory qualities, and S. cerevisiae/H. opuntiae co-fermentation had the best sensory quality followed by Sc-Hu and Sc-Td co-fermentations. Additionally, all the co-fermentations had a lower amount of ethanol and total acidity, higher pH value, and higher content of volatile aroma compounds, especially the content of higher alcohol and ester compounds, than those of S. cerevisiae fermentation. Therefore, co-fermentations of the non-Saccharomyces yeast strains and S. cerevisiae could be employed to improve the sensory quality of citrus wines. These results would provide not only methods to improve the sensory quality of citrus wine, but also a valuable reference for the selection of non-Saccharomyces yeast strains for fruit wine fermentation.

Dynamic trajectories of volatile and non-volatile specialised metabolites in 'overnight' fragrant flowers of Murraya paniculata

Ephemeral flowers, especially nocturnal ones, usually emit characteristic scent profiles within their post-anthesis lifespans of a few hours. Whether these flowers exhibit temporal variability in the composition and profile of volatile and non-volatile specialised metabolites has received little attention. Flowers of Murraya paniculata bloom in the evenings during the summer and monsoon, and their sweet, intense fragrance enhances the plant's value as an ornamental. We aimed to investigate profiles of both volatile and non-volatile endogenous specialised metabolites (ESM) in nocturnal ephemeral flowers of M. paniculata to examine whether any biochemically diverse groups of ESM follow distinct patterns of accumulation while maintaining synchrony with defensive physiological functions. Targeted ESM contents of M. paniculata flowers were profiled at ten time points at 2-h intervals, starting from late bud stage (afternoon) up to the start of petal senescence (mid-morning). Emitted volatiles were monitored continuously within the whole 20-h period using headspace sampling. The ESM contents were mapped by time point to obtain a highly dynamic and biochemically diverse profile. Relative temporal patterns of ESM accumulation indicated that the active fragrance-emitting period might be divided into 'early bloom', 'mid-bloom' and 'late bloom' phases. Early and late bloom phases were characterised by high free radical generation, with immediate enhancement of antioxidant enzymes and phenolic compounds. The mid-bloom phase was relatively stable and dedicated to maximum fragrance emission, with provision for strong terpenoid-mediated defence against herbivores. The late bloom phase merged into senescence with the start of daylight; however, even the senescent petals continued to emit fragrance to attract diurnal pollinators. Our study suggests that dynamic relations between the different ESM groups regulate the short-term requirements of floral advertisement and phytochemical defence in this ephemeral flower. This study also provided fundamental information on the temporal occurrence of emitted volatiles and internal pools of specialised metabolites in M. paniculata flowers, which could serve as an important model for pollination biology of Rutaceae, which includes many important fruit crops.

Integration of Metabolite Profiling and Transcriptome Analysis Reveals Genes Related to Volatile Terpenoid Metabolism in Finger Citron (C. medica var. sarcodactylis)

Finger citron (Citrus medica var. sarcodactylis) is a popular ornamental tree and an important source of essential oils rich in terpenoids, but the mechanisms behind volatile formation are poorly understood. We investigated gene expression changes combined with volatile profiling of ten samples from three developing organs: flower, leaf, and fruit. A total of 62 volatiles were identified with limonene and γ-terpinene being the most abundant ones. Six volatiles were identified using partial least squares discriminant analysis (PLS-DA) that could be used as markers for distinguishing finger citron from other citrus species. RNA-Seq revealed 1,611,966,118 high quality clean reads that were assembled into 32,579 unigenes. From these a total of 58 terpene synthase (TPS) gene family members were identified and the spatial and temporal distribution of their transcripts was measured in developing organs. Transcript levels of transcription factor genes AP2/ERF (251), bHLH (169), bZIP (76), MYB (155), NAC (184), and WRKY (66) during finger citron development were also analyzed. From extracted subnetworks of three modules constructed by weighted gene co-expression network analysis (WGCNA), thirteen TPS genes and fifteen transcription factors were suggested to be related to volatile terpenoid formation. These results provide a framework for future investigations into the identification and regulatory network of terpenoids in finger citron.

Repellent Effects of Methyl Anthranilate on Western Corn Rootworm Larvae (Coleoptera: Chrysomelidae) in Soil Bioassays

Methyl anthranilate (MA), a compound in maize roots that is repellent to western corn rootworm larvae (Diabrotica virgifera virgifera LeConte) was tested in behavioral bioassays in a soil environment. MA prevented larvae from locating roots of a maize seedling, and the repellency strengthened with increasing rates of MA. In a simple push-pull strategy between an MA-treated seedling and an untreated seedling, granules containing 0.1 mg/g MA pushed larvae to the untreated seedling. This push effect increased with dose, with 90% repellency observed for the highest dose tested (100 mg/g). Chemical analysis showed that MA concentrations remained high for 4 wk in dry, sterilized or unsterilized soil, but declined rapidly in moist soil. After 7 d, 50% less MA was recovered in moist, sterilized soil than in dry soil, and only a trace of MA remained in unsterilized moist soil, suggesting that both moisture and microbial activity contributed to the loss of MA. Various (MA) carrier granules were tested in bioassays after aging in moist soil. After 1 d, all of the MA granules were repellent at the 10 mg/g rate and clay granules were also effective at 1 mg/g. After 1 wk, only molecular sieve granules elicited repellency, but that activity disappeared after 2 wk. These results demonstrate that MA is repellent to western corn rootworm larvae in the soil environment and may have potential as a rootworm treatment if formulations can be developed that protect the material from decomposition in the soil.

Convergent evolution of caffeine in plants by co-option of exapted ancestral enzymes

Convergent evolution is a process that has occurred throughout the tree of life, but the historical genetic and biochemical context promoting the repeated independent origins of a trait is rarely understood. The well-known stimulant caffeine, and its xanthine alkaloid precursors, has evolved multiple times in flowering plant history for various roles in plant defense and pollination. We have shown that convergent caffeine production, surprisingly, has evolved by two previously unknown biochemical pathways in chocolate, citrus, and guaraná plants using either caffeine synthase- or xanthine methyltransferase-like enzymes. However, the pathway and enzyme lineage used by any given plant species is not predictable from phylogenetic relatedness alone. Ancestral sequence resurrection reveals that this convergence was facilitated by co-option of genes maintained over 100 million y for alternative biochemical roles. The ancient enzymes of the Citrus lineage were exapted for reactions currently used for various steps of caffeine biosynthesis and required very few mutations to acquire modern-day enzymatic characteristics, allowing for the evolution of a complete pathway. Future studies aimed at manipulating caffeine content of plants will require the use of different approaches given the metabolic and genetic diversity revealed by this study.

Floral Scent Chemistry of Luculia yunnanensis (Rubiaceae), a Species Endemic to China with Sweetly Fragrant Flowers

Luculia plants are famed ornamentals with sweetly fragrant flowers. Luculia yunnanensis Hu is an endemic plant from Yunnan Province, China. Headspace-solid phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) was used to identify the volatile organic compounds (VOCs) of the different flower development stages of L. yunnanensis for the evaluation of floral volatile polymorphism. The results showed that a total of 40 compounds were identified at four different stages. The main aroma-active compounds were 3-carene, α-cubebene, α-copaene, δ-cadinene, and isoledene. Floral scent emission had the tendency to ascend first and descend in succession, reaching its peak level at the initial-flowering stage. The richest diversity of floral volatiles was detected at the full-flowering stage. Principal component analysis (PCA) indicated that the composition and its relative content of floral scent differed at the whole flower development stage. In comparison with the other two species of Luculia (L. pinceana and L. gratissima), the composition and its relative content of floral scent were also different among the tree species.

Genomic Analysis of Terpene Synthase Family and Functional Characterization of Seven Sesquiterpene Synthases from Citrus sinensis

Citrus aroma and flavor, chief traits of fruit quality, are derived from their high content in essential oils of most plant tissues, including leaves, stems, flowers, and fruits. Accumulated in secretory cavities, most components of these oils are volatile terpenes. They contribute to defense against herbivores and pathogens, and perhaps also protect tissues against abiotic stress. In spite of their importance, our understanding of the physiological, biochemical, and genetic regulation of citrus terpene volatiles is still limited. The availability of the sweet orange (Citrus sinensis L. Osbeck) genome sequence allowed us to characterize for the first time the terpene synthase (TPS) family in a citrus type. CsTPS is one of the largest angiosperm TPS families characterized so far, formed by 95 loci from which just 55 encode for putative functional TPSs. All TPS angiosperm families, TPS-a, TPS-b, TPS-c, TPS-e/f, and TPS-g were represented in the sweet orange genome, with 28, 18, 2, 2, and 5 putative full length genes each. Additionally, sweet orange β-farnesene synthase, (Z)-β-cubebene/α-copaene synthase, two β-caryophyllene synthases, and three multiproduct enzymes yielding β-cadinene/α-copaene, β-elemene, and β-cadinene/ledene/allo-aromandendrene as major products were identified, and functionally characterized via in vivo recombinant Escherichia coli assays.

Methyl Anthranilate as a Repellent for Western Corn Rootworm Larvae (Coleoptera: Chrysomelidae)

Methyl anthranilate was identified as the active compound in extracts of maize (Zea mays L.) roots that were shown to be repellent to neonate western corn rootworm (Diabrotica virgifera virgifera LeConte) larvae. A bioassay-driven approach was used to isolate the active material from diethyl ether extracts of roots from germinating maize seeds. Separation of the extract on a Florisil column yielded an active fraction of 90:10 hexane:diethyl ether. Analysis with gas chromatography-mass spectrometry identified two compounds in the active fraction: indole (2,3-benzopyrrole) and methyl anthranilate (methyl 2-aminobenzoate). When tested in behavioral bioassays, methyl anthranilate elicited a significant (P < 0.05) repellent response at doses of 1, 10, and 100 µg. In subsequent single-choice bioassays, 1, 10, and 100 µg of methyl anthranilate prevented larvae from approaching 10 mmol/mol concentrations of carbon dioxide, which is normally highly attractive to the larvae. Indole, the other compound identified from the active fraction, did not elicit a behavioral response by the larvae. Methyl anthranilate has potential for development as a management tool for western corn rootworm larvae and may be best suited for use in a push-pull control strategy.

Identification of floral scent in chrysanthemum cultivars and wild relatives by gas chromatography-mass spectrometry

The objective of this study was to identify the major volatile compounds and their relative concentrations in flowers of different chrysanthemum cultivars and their wild relatives. The volatile organic components of fresh flowers were analyzed using a headspace solid-phase microextraction coupled with gas chromatography-mass spectrometry. In total, 193 volatile organic components were detected; the major scent components were monoterpenoids and oxygenated monoterpenoids, which accounted for 68.59%–99.93% of the total volatiles in all tested materials except for Chrysanthemum indicum collected from Huangshan, in which they accounted for only 37.45% of total volatiles. The major volatile compounds were camphor, α-pinene, chrysanthenone, safranal, myrcene, eucalyptol, 2,4,5,6,7,7ab-hexahydro-1H-indene, verbenone, β-phellandrene and camphene. In a hierarchical cluster analysis, 39 accessions of Chrysanthemum and its relatives formed six clusters based on their floral volatile compounds. In a principal component analysis, only spider type flowers were located closely on the score plot. The results of this study provide a basis for breeding chrysanthemum cultivars which desirable floral scents.

Structural and biochemical characteristics of citrus flowers associated with defence against a fungal pathogen

The constitutive characters of plants can be structural or biochemical and play an important role in their defence against pathogens. Citrus postbloom fruit drop (PFD) caused by Colletotrichum spp. is one of the most important fungal diseases of citrus. The pathogen infects the flowers, leading to premature fruit drop and reducing citrus production. However, flower buds smaller than 8 mm long are usually not infected by Colletotrichum spp. Thus, this study investigated whether there are constitutive mechanisms in flower buds related to Colletotrichum spp. infection. We studied flower buds that were 2, 3, 4, 8, 12 and 15 mm long and petals, after anthesis, of sweet orange 'Valência' using light and scanning electron microscopy and histochemistry. We evaluated the effect of volatile organic compounds (VOCs) in flowers (R-limonene and linalool) on the in vitro growth of Colletotrichum acutatum. We found that the arrangement of the epidermal papillae in the petal primordia, the occurrence of prismatic crystals and the distribution of oil glands are the main differences between buds smaller than 8 mm and buds 8-15 mm long. Osmophores at the tips of petals produced and accumulated phenols, terpenes and lipophilic compounds. Flower buds smaller than 8 mm long have constitutive structural and biochemical barriers to Colletotrichum spp. infection. In addition, this is the first time that osmophores have been reported in citrus. Our study shows that natural terpenes of Citrus flowers inhibit the fungal growth in vitro, highlighting the potential use of terpenes for the chemical control of PFD in citrus.