Floral scents and fruit aromas are crucial volatile organic compounds (VOCs) in plants. They are used in defense mechanisms, along with mechanisms to attract pollinators and seed dispersers. In addition, they are economically important for the quality of crops, as well as quality in the perfume, cosmetics, food, drink, and pharmaceutical industries. Floral scents and fruit aromas share many volatile organic compounds in flowers and fruits. Volatile compounds are classified as terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. Many genes and transcription factors regulating the synthesis of volatiles have been discovered. In this review, we summarize recent progress in volatile function, composition, biosynthetic pathway, and metabolism regulation. We also discuss unresolved issues and research perspectives, providing insight into improvements and applications of plant VOCs.
Floral scents and fruit aromas are volatile organic compounds (VOCs) released by plants. These VOCs are lipophilic and characterized by low molecular weights and high melting points. Biosynthesis of VOCs occurs in all plant organs, including seeds, roots, stems, leaves, fruits, and flowers; floral scents and fruit aromas are relevant to our daily lives and have substantial economic value.
Floral VOCs are important traits of floral scents. The differences and abundances of these VOCs in scents vary widely among flowering plants (Farré-Armengol et al., 2017). According to their origin, biosynthesis, and function, floral scents are classified as terpenoids, phenylpropanoids, fatty acid derivatives and amino acids.
Fruit aroma is a key contributor to fruit quality and acceptance by animal and human. Fruit aroma consists of various chemical compounds (e.g., aldehydes, alcohols, ketones, esters, lactones, and terpenes) (Riu-Aumatell et al., 2004); the presence or absence of certain compounds determines differences among fruit aromas. In a wide range of fruits, the production of volatile compounds beginning from fruit-set to late-ripening is modulated by the accumulation of fruity esters, terpenes, and other compounds.
In the past decade, numerous studies of floral scents and fruit aromas have improved our understanding of their functions, components, biosynthesis, and regulation. In addition, previous reviews focused on floral scents or fruit aromas separately, whereas both have important volatile components contributing to the economic value of horticultural plants. Therefore, we review recent progress regarding floral scents and fruit aromas, focusing on the functions and compositions of volatiles, the factors and biosynthetic pathways that affect volatiles, and the regulation of their biosynthesis and metabolism.
Floral Scent Volatiles 花香挥发物
Floral volatiles attract effective pollinators for sexual reproduction (Schiestl, 2010; Gross et al., 2016). Although some floral VOCs attract a variety of pollinators (broad pollinators) (Schiestl et al., 2003), flowers release volatiles to signal to specific pollinators (Schiestl, 2015). Their selection behavior depends on differences in composition, amount, and emission of floral volatile compounds (Muhlemann et al., 2014). For instance, emitted terpenoids and benzenoids can attract pollinators and repel elective visitors (Farré-Armengol et al., 2013), while the same terpenoid compound may attract one animal but repel another (Eilers et al., 2021). Many flowers offer floral rewards as nectar, pollen or oil products, on which visitors depend (Steiner et al., 2011) but some plants attract pollinators without offering nectar by mimicking the scents and colors of neighboring plants (Kunze, 2001; Schiestl, 2005). High production of VOCs is necessary to guide pollinators to flowers, particularly to overcome the decreased conspicuousness of flowers at night (Muhlemann et al., 2014). β-ocimene is implicated in the attraction of pollinators. For example, in Mirabilis jalapa flowers, the emitted floral scent is dominated by trans-β-ocimene and its emission peaks in the evening, which match flower opening and hawkmoth pollinator activity (Effmert et al., 2005).
花挥发物吸引有效传粉者有性生殖(Schiestl, 2010;Gross et al.， 2016)。虽然一些花的挥发性有机化合物吸引了各种传粉媒介(广义传粉媒介)(Schiestl等，2003)，但花释放挥发性物质向特定的传粉媒介发出信号(Schiestl, 2015)。它们的选择行为取决于花挥发性化合物的组成、数量和释放量的差异(Muhlemann et al.， 2014)。例如，排放的萜类化合物和苯类化合物可以吸引传粉者并排斥选择性访客(farr<s:1> - armengol等人，2013)，而相同的萜类化合物可能吸引一种动物，但排斥另一种动物(Eilers等人，2021)。许多花提供花蜜，花粉或精油产品作为它的奖励，使传粉者具有对花的依赖性(Steiner et al.， 2011)，但有些植物通过模仿邻近植物的气味和颜色来吸引传粉者而不提供花蜜(Kunze, 2001;Schiestl, 2005)。高挥发性有机化合物的产生是引导传粉者到花上的必要条件，特别是为了克服夜间花的色差显著性下降的缺陷(Muhlemann et al.， 2014)。β-罗勒烯与吸引传粉者有关。例如，在紫茉莉(Mirabilis jalapa)花中，散发的花香以反式β-罗勒烯为主，其散发的峰值出现在晚上，这与花开放和飞蛾传粉者的活动相匹配(Effmert et al.， 2005)。
Other adaptive roles for floral scents include repellents (Piechulla and Pott, 2003; Kessler et al., 2008; Raguso, 2008) and the provision of physiological protection against biotic stresses in plants (Dudareva et al., 2006; Knudsen and Gershenzon, 2006). In plant allelopathy, different VOCs can function as cues to other plants (Caruso and Parachnowitsch, 2016). For instance, in plant-plant communication, plants use volatiles emitted by their neighbors to evaluate their environment, including the presence of herbivores (Heil and Karban, 2010) and competitors (Kegge and Pierik, 2010). Generally, exposure of higher plants to biotic stresses (e.g., herbivore damage) can result in the emission of VOCs to repulse the attack (Arimura and Pearse, 2017) and enhance the response to future attacks (Morrell and Kessler, 2014). Similarly, some flower VOCs can be used in plant allelopathy. For example, the olfactory cues of some non-host plants attract the pollinators of other plants (e.g., Malva moschata and Geranium sanguineum attract Chelostoma rapuncul bees, the pollinator of Campanula spp.) (Burger et al., 2021). However, the compositions of floral scents that attract pollinators or repel herbivores are unclear.
Some volatiles have antibacterial properties and function in defense against microbial pathogens (Junker et al., 2011; Huang et al., 2012). Certain defensive functions rely on a single compound, which is more effective than a mixture of compounds (Gershenzon and Dudareva, 2007). A single floral volatile can have multiple roles in flowers. For example, (E)-b-caryophyllene from Arabidopsis thaliana promotes defense against pathogens and enhances pollinator attraction (Huang et al., 2012). In rose geranium (Pelargonium graveolens), β-citronellol is an abundant compound with extensive antibacterial activity (Boukhris et al., 2015). The reported functions of floral scents are listed in Table 1.
TABLE 1. Functions of floral scents. 花香的功能列表
Fruit Aroma Volatiles 果香挥发物
Fruit aroma is a major contributor to fruit quality (color, texture, flavor, and aroma). In the wild, the aroma VOCs released from fruits influence herbivore behavior and attract animal dispersers (Dudareva et al., 2013). For example, fruit bats recognize ripe and non-ripe fruits based on the emitted volatiles (Hodgkison et al., 2007). VOCs have biological activities against bacteria, fungi, and insects. Volatiles extracted from citrus peels (Citrus reticulata Blanco) exhibit significant antifungal and antibacterial activities against pathogenic strains (Sultana et al., 2012). In another citrus species (Citrus hystrix), the essential oils extracted from fruit peels possess antibacterial activity against respiratory bacteria; the most effective components are α-terpineol, terpinene-4-ol, and limonene (Srisukh et al., 2012). The antibacterial activities of fruit VOCs in extracts of lemon (Citrus limonium), sweet lime (Citrus limetta), pomegranate (Punica granatum), apple (Malus domestica), and tomato (Solanum lycopersicum) against pathogenic bacteria isolated from a wound have been reported (Unnisa et al., 2012). Essential oils extracted from Alchornea cordifolia fruit have shown antibacterial activity against Staphylococcus aureus and antifungal activity against Aspergillus niger, while essential oils extracted from Canthium subcordatum fruit have shown antibacterial activity against Bacillus cereus, S. aureus, and A. niger (Essien et al., 2015). In strawberry fruit, the antifungal activity of fruit VOCs against Colletotrichum acutatum is contributed by (E)-hex-2-enal (Arroyo et al., 2007). Essential oil components from pepper fruit significantly inhibited the germination of Colletotrichum gloeosporioides; the active compounds were carvacrol, cinnamon oil, citral, trans-cinnamaldehyde, p-cymene, and linalool (Hong et al., 2015). Volatiles isolated from leaves, flowers, and fruits of three opuntia species had antifungal activity against fungal species such as Alternaria solani (Bergaoui et al., 2007). Although fruit VOCs have antibacterial and antifungal activities, the most important function of fruit aroma in horticulture is to attract humans. Indeed, fruit aromas have been selected by horticulture breeding to improve their edible quality and economic value.
水果香气是决定水果品质(颜色、质地、风味和香气)的主要因素。在野外，水果释放的香气挥发性有机化合物会影响食草动物的行为，并吸引动物散布者(Dudareva et al.， 2013)。例如，果蝠根据释放的挥发物来识别成熟和未成熟的水果(Hodgkison et al.， 2007)。挥发性有机化合物对细菌、真菌和昆虫具有生物活性。从柑橘皮(citrus reticulata Blanco)中提取的挥发物对致病菌株具有显著的抗真菌和抗菌活性(Sultana et al.， 2012)。在另一种泰国青柠(citrus hystrix)中，从果皮中提取的精油对呼吸道细菌具有抗菌活性；最有效的成分是α-松油醇、松油烯-4-醇和柠檬烯(Srisukh et al.， 2012)。据报道，中国柠檬(Citrus limonium)、白柠檬(Citrus limetta)、石榴(Punica granatum)、海棠(Malus domestica)和番茄(Solanum lycopersicum)提取物中的水果挥发性有机化合物对伤口分离的致病菌具有抗菌活性(Unnisa et al.， 2012)。从灯盏花(Alchornea cordifolia)果实中提取的精油显示出对金黄色葡萄球菌和黑曲霉的抗菌活性，而从心形鱼骨木(Canthium subcordatum)果实中提取的精油显示出对蜡样芽孢杆菌、金黄色葡萄球菌和黑曲霉的抗菌活性(Essien et al.， 2015)。在草莓果实中，水果挥发性有机化合物对炭疽病菌的抗真菌活性是由(E)-己-2-烯醛贡献的(Arroyo et al.， 2007)。辣椒果实精油成分对炭疽病菌萌发有显著抑制作用；活性化合物为香芹酚、肉桂油、柠檬醛、反式肉桂醛、对花香烃和芳樟醇(Hong et al.， 2015)。从三种仙人掌属植物的叶、花和果实中分离出的挥发物对真菌有抗真菌活性，如索拉尼链格孢菌(Alternaria solani） (Bergaoui et al.， 2007)。虽然水果的挥发性有机化合物具有抗菌和抗真菌活性，但水果香气在园艺中最重要的功能是吸引人。事实上，为了提高水果的食用品质和经济价值，园艺育种已经选择了水果的香味作为育种目标。
Volatile Composition 挥发性成分
Floral Scents 花香
Floral scents consist of a mixture of compounds; they are categorized as terpenoids, phenylpropanoids/benzenoids, fatty acids, and amino acids. Terpenoids are the largest class of volatiles and comprise more than 40,000 structures derived from five-carbon isoprene units as monoterpenoids, sesquiterpenoids, apocarotenoids, and others. Phenylpropanoids comprise more than 8,000 metabolites (Dong and Lin, 2021); benzenoids are the second class of VOCs derived from the amino acid phenylalanine. Fatty acids and amino acids are important VOCs present in floral scents and fruit aromas (Negre-Zakharov et al., 2009).
Rose (Rosa spp.) 玫瑰
Rose is one of the most important ornamental plants for the production of cut flowers and perfumes. Therefore, floral scent is the main economic trait of rose, and some cultivars are classified according to their fragrant components (Du et al., 2019). Rose scents are composed of mixtures of organic compounds. In Bulgarian rose (Rosa damascene) essential oil, the most abundant components are phenethyl alcohol, citronellol, heneicosane, pentadecane, eugenol, methyleugenol, and geraniol (Won et al., 2009; Kiralan, 2015; Koksal et al., 2015). The first rose water (R. damascene) comprised benzoic acid 2-hydroxy-3-methyl butyl ester, geranyl acetate, carbamic acid methyl ester, linalool, eucalyptol, citronellol, geraniol, and methyleugenol (Koksal et al., 2015). In other rose (Rosa L.) species (R. dumalis, R. canina, R. dumalis subsp. boissieri, R. gallica, and R. hirtissima), the major compounds are aldehydes, alcohols, monoterpenes, and sesquiterpenes (Demir et al., 2014). Moreover, the fragrant rose cultivar “Fragrant Cloud” contains monoterpene alcohols, terpene hydrocarbons, and acetates (Shalit et al., 2004). The predominant scent compounds in the petals of six Hybrid Rugosa roses are phenyl ethyl alcohol, β-citronellol, nerol, and geraniol (Sparinska and Rostoks, 2015). Rose-breeding programs are underway to produce new varieties of floral scents.
玫瑰是制作切花和香水最重要的观赏植物之一。因此，花香是玫瑰的主要经济性状，一些品种根据其香气成分进行分类(Du et al.， 2019)。玫瑰的香味是由多种有机化合物混合而成。在保加利亚玫瑰(Rosa damascene)精油中，最丰富的成分是苯乙醇、香茅醇、十六烷、十五烷、丁香酚、甲基丁香酚和香叶醇(Won等人，2009;Kiralan, 2015;Koksal et al.， 2015)。第一款玫瑰水(R.Damascene)由苯甲酸2-羟基-3-甲基丁基酯、香叶乙酸酯、氨基甲酸甲酯、芳樟醇、桉油醇、香茅醇、香叶醇和甲基丁香醇组成(Koksal等人，2015年)。在其他玫瑰(Rosa L.)种中(杜马里蔷薇、犬蔷薇、杜马里亚种蔷薇、 高卢蔷薇和R. hirtissima)，主要化合物是醛类，醇类，单萜烯和倍半萜烯(Demir et al.， 2014)。此外，香玫瑰品种“香云”含有单萜烯醇、萜烯烃和乙酸酯(Shalit et al.， 2004)。六种杂交Rugosa玫瑰花瓣中的主要气味化合物是苯乙醇、β-香茅醇、橙花醇和香叶醇(Sparinska和Rostoks, 2015)。玫瑰育种计划正在进行，以产生新的花香品种。
Orchid 兰花
Orchid is the largest family (Orchidaceae) of flowering plants (Julsrigival et al., 2013), comprising 20,000–30,000 species (Khuraijam et al., 2017). Approximately 75% of orchids are fragrant (De et al., 2019). In an economic context, they can be used to produce cut flowers and as potted plants (Khuraijam et al., 2017). The family Orchidaceae is divided into five subfamilies. Epidendroideae is the largest subfamily, comprising approximately 76% of the family (Freudenstein and Chase, 2015). In Epidendroideae, the major aromatic compounds of Thai fragrant orchid species (Rhynchostylis gigantea Ridl., R. gigantea var. harrisonianum Holtt., Vanda coerulea, and Dendrobium parishii Rchb. f.) are nerol, 2,3-dihydrofarnesol, nonanal, and 2-pentadecanone (Julsrigival et al., 2013). The emitted volatiles of Zygopetalum maculatum (an orchid species) are enriched in benzenoids, including O-diethylbenzene, p-diethylbenzene, benzyl acetate, and methyl salicylate; 2-phenylethylacetate is the major phenylpropanoid component (Bera et al., 2018). Cymbidium spp. flowers are rich in cineole, isoeugenol, and (-) selinene (Ramya et al., 2020). Phalaenopsis orchids contain monoterpenes, including linalool and geraniol (Chuang et al., 2017). The scent compounds in Vanda Mimi Palmer (an orchid hybrid) are ocimene, linalool, linalool oxide, and nerolidol as the major terpenoids; the major benzenoids and phenylpropanoids are benzyl acetate, methylbenzoate, phenylethyl acetate, and phenylethanol (Mohd-Hairul et al., 2010). The major components of the floral scents of Ophrys sphegodes, Ophrys bertolonii, and Neotinea tridentate are hydrocarbons, alcohols, aldehydes, and terpenes (Manzo et al., 2014).
Tulip (Tulipa spp.) 郁金香
The few fragrant tulip cultivars produce a range of floral scents (Oyama-Okubo and Tsuji, 2019). The major scent compounds of tulips (Tulipa L.) cultivars are monoterpenoids (eucalyptol, d-limonene, linalool, trans-β-ocimene, and α-pinene), sesquiterpenoids (α-farnesene, caryophyllene, geranyl acetone, and β-ionone), benzenoids (benzaldehyde, acetophenone, 3,5-dimethoxytoluene, benzyl alcohol, methyl salicylate, and 2-phenyl ethanol), and fatty acid derivatives (decanal, cis-3-hexenol, cis-3-hexenyl acetate, 2-hexenal, and octanal) (Oyama-Okubo and Tsuji, 2013).
Peony (Paeonia spp.) 牡丹
Peony (tree and herbaceous peony) includes many Paeoniaceae species and cultivars, which are important ornamental plants with rich fragrances. The major flavoring substances in peony are citronellol, geraniol, and linalool (Song et al., 2018; Wang et al., 2021). In 30 herbaceous peony cultivars, the main compounds are phenyl ethyl alcohol, β-caryophyllene, linalool, nerol, and (R)-citronellol (Song et al., 2018).
Lily (Lilium spp.) 百合
Lilies are commercially important ornamental plants because of their attractive colors and scents (Du et al., 2019). Volatile-emitting lily cultivars (Lilium spp.) release various scent compounds, predominantly three monoterpenoids (1.8-cineole, (E)-b-ocimene and linalool) and one benzenoid (methyl benzoate) (Kong et al., 2017). β-cis-ocimene represents the major component in Lilium spp. “Sweetness” (Aros et al., 2020).
Water Lily (Family: Nymphaeaceae) 睡莲
Water lily flowers (Nymphaea colorata) release 11 volatiles, comprising terpenoids (sesquiterpenes), fatty acid derivatives (methyl decanoate), and benzenoids (Zhang et al., 2020).
蓝星睡莲(Nymphaea colorata)释放11种挥发物，包括萜类(倍半萜类)、脂肪酸衍生物(癸酸甲酯)和苯类(Zhang et al.， 2020)。
Carnation (Dianthus spp.) 康乃馨
Benzenoids are the principal components of carnation flowers in addition to terpenoids and fatty acid derivatives (Kishimoto, 2020). Some scents are described as spicy because they contain eugenols (Kishimoto, 2020). In Dianthus rupicola Biv. (cliffs carnation), phenolic monoterpenes are the predominant components of the essential oil, followed by monoterpene hydrocarbons and oxygen-containing sesquiterpenes (Casiglia et al., 2014). In a carnation species (D. elymaiticus), the essential oil of flowers contains high levels of fatty acid derivatives and terpenoids; the major compounds are (Z)-3-hexenyl acetate, methyl benzoate, β-caryophyllene, and decanal (Azadi and Entezari, 2016).
Lavender (Lavandula spp.) 薰衣草
Lavender plants have excellent medicinal and aromatic properties (Guo and Wang, 2020). The major volatiles extracted from lavender essential oil (Lavandula angustifolia Mill) are linalool, linalyl acetate (Venskutonis et al., 1997; Won et al., 2009; Guo and Wang, 2020), 1,8-cineole, and α-terpineol (Tschiggerl and Bucar, 2010; Śmigielski et al., 2013), along with oxygenated derivatives of monoterpenes and monoterpene alcohols (Śmigielski et al., 2013). In L. officinalis, linalyl acetate was the predominant volatile in the essential oils (Xiao et al., 2017). Lavender flower (L. angustifolia) vapor consists of terpene hydrocarbons, oxygenated terpenes, and sesquiterpenes; linalool, terpin-4-ol, and linalyl acetate are predominant (An et al., 2001).
薰衣草植物具有优良的药用和芳香特性(Guo and Wang, 2020)。从薰衣草精油(Lavandula angustifolia Mill)中提取的主要挥发物是芳樟醇、乙酸芳樟酯(Venskutonis et al.， 1997;Won et al.， 2009;Guo and Wang, 2020)， 1,8-桉叶脑和α-松油醇(Tschiggerl and Bucar, 2010;Śmigielski等人，2013)，以及单萜烯和单萜烯醇的氧合衍生物(Śmigielski等人，2013)。在officinalis中，乙酸芳樟酯是挥发油中的主要挥发物(Xiao et al.， 2017)。薰衣草(L. angustifolia)蒸气由萜烯烃、氧化萜烯和倍半萜烯组成;芳樟醇、松油-4-醇和乙酸芳樟酯占主导地位(An et al.， 2001)。
Jasmine (Jasminum spp.) 茉莉
Jasmine flowers are well-known for their pleasant fragrances. In Jasminum species (J. sambac, J. auriculatum, J. grandiflorum, and J. multiflorum), linalool and (3E, 6E)-a-farnesene are the major monoterpene and sesquiterpene, respectively (Bera et al., 2015). J. sambac is a fragrant flower species distributed worldwide. The major compounds of the floral scent are methyl anthranilate, linalool, 4-nonanolide, 4-hexanolide, (E)-2-hexenyl hexanoate, 4-hydroxy-2,5-dimethyl-3(2 H)-furanone (Ito et al., 2002), eugenol, benzyl alcohol, benzyl acetate, benzyl benzoate, methyl salicylate, methyl anthranilate, (Z) 3-hexenyl benzoate, indole, and α-farnesene (Lin et al., 2013; Chen et al., 2017, 2020).
Daffodil (Narcissus spp.) 水仙花
Narcissus floral volatiles and their essential oils are used in the perfume industry (Terry et al., 2021). Chinese daffodil flowers (Narcissus tazetta) contain acetic acid phenethyl ester, E-ocimene, acetic acid benzyl ester, neo-allo-ocimene, allo-ocimene, α-linalool, 1,8 cineole, benzenepropyl acetate, and 3-methyl-2-buten-1-ol acetate as the major VOCs (Melliou et al., 2007; Song et al., 2007; Chen et al., 2013). Monoterpenes are the major VOCs in daffodil (N. pseudonarcissus); β-ocimene and β-myrcene are predominant (Li et al., 2018). In simple and double flower cultivars of Narcissus, the major scent compounds are monoterpenes and benzenoids, particularly cis-β-ocimene and benzyl acetate (Ruíz-Ramón et al., 2014). In the essential oil of N. serotinus, the major component is benzyl acetate; linalool oxides are the main characteristics of this species (Melliou et al., 2007). In two varieties of European Daffodil (N. pseudonarcissus L.), “Isha” and “Acropolis,” the major categories of VOCs are terpenes and ethers; the characteristic aromatic component is β-ocimene (Wang et al., 2013).
Hyacinth (Hyacinths spp.) 风信子
In Hyacinth (Hyacinths orientals) varieties, the major components of floral scents are terpenes, esters, and alcohols; the characteristic aromatic components are acetoxytoluene, β-ocimene, β-myrcene, and β-phenethyl alcohol (Wang et al., 2013).
Most studies of floral scents have focused on VOC profiles. However, the components of specific species (cultivars) should be evaluated by mass spectrometry and a sensory evaluation of floral fragrances. The chemical compositions of floral scents of major flower species are listed in Supplementary Table 1.
Fruit Aroma 果香
According to the modes of fruit development, there are three main groups (botanical classification): simple fruits, aggregate fruits, and multiple (or composite) fruits (Singh, 2004). However, based on the yield and commercial values, the fruits are normally classified into berries, melons, citrus fruits, drupes (stone fruits), pomes (apples and pears), and tropical fruits. Most fruits release a wide range of VOCs, which determine their aroma profiles. The VOCs released from fruits are esters, ketones, aldehydes, lactones, alcohols, and terpenoids (Ahmed et al., 2013). C10 monoterpenes and C15 sesquiterpenes are abundant and key determinants of the characteristic aroma of fruits (Ahmed et al., 2013). Each fruit species has a distinctive aroma based on the mixture of fruit VOCs (Tucker, 1993; Baietto and Wilson, 2015).
Banana (Musa spp.) 香蕉
Banana is the main fruit traded globally (Alam, 2014). Almost 200 volatile components are found in banana fruit (Alam, 2014). Hexanal is the major volatile compound in most banana cultivars. The typical volatile compounds are (E)-2-hexenal and acetoin (Cavendish), (E)-2-hexenal and hexanal (Plantain), and 2,3-butanediol (Frayssinette) (Aurore et al., 2011). In addition, VOC composition may change during fruit maturation. During the ripening of two banana cultivars (“Brazilian” and “Fenjiao”), the predominant volatile components are isoamyl acetate, butanoic acid, 3-methyl-3-methylbutyl ester, hexanal, trans-2-hexenal, and 1-hexanol. However, octanoic acid and propanoic acid 2-methylbutyl ester are only detected in Fenjiao (Zhu et al., 2018a).
香蕉是全球贸易的主要水果(Alam, 2014)。在香蕉果实中发现了近200种挥发性成分(Alam, 2014)。己醛是大多数香蕉品种的主要挥发性化合物。典型的挥发性化合物有(E)-2-己烯醛和乙偶姻(香芽蕉)、(E)-2-己烯醛和己醛(芭蕉)和2,3-丁二醇(弗赖西内特) (Aurore et al.， 2011)。此外，挥发性有机化合物的组成可能在果实成熟过程中发生变化。在两个香蕉品种(“巴西”和“粉椒”)的成熟过程中，主要挥发性成分是乙酸异戊酯、丁酸、3-甲基-3-甲基丁基酯、己醛、反式-2-己烯醛和1-己醇。而辛酸和丙酸2-甲基丁酯仅在粉椒中检测到(Zhu et al.， 2018a)。
Apple (Malus domestica) 苹果
More than 300 aromatic compounds have been identified in apples, including esters, alcohols, aldehydes, acids, ketones, and terpenoids (Yang S. et al., 2021). Esters are the most important determinant of the aroma of ripe apples, followed by alcohols (Espino-Díaz et al., 2016). Although the aroma is cultivar-specific, eight common VOCs are detected in 40 apple cultivars: esters (hexyl butyrate, hexyl 2-methylbutyrate and hexyl hexanoate), hexanal, (E)-2-hexenal, 1-hexanol, estragole, and α-farnesene (Yang S. et al., 2021). In addition, esters such as butyl acetate, hexyl acetate, and 2-methyl butyl acetate influence the aroma profiles of the apple cultivars “Discovery” and “Prima” (Dunemann et al., 2009). There are more than 7,500 cultivars of culinary and eating apples (Elzebroek and Wind, 2008); thus, more aroma components must be identified.
Grape (Vitis vinifera) 葡萄
Grape is of great commercial significance worldwide; it is divided into table (fresh consumption), juice, wine, and dried (raisins) types. The components of three varieties (Merlot, Cabernet Sauvignon, and Feteasca Neagra) of wine grape are: butanoic acid, tropilidene, methyl ester, 2-ethyl heptanoic acid, 2,4-dimethyl heptane, 2,4-dimethyl-1-heptene, n-nonane, 4-methyl octane, 2-propyl-1-pentanol, 6-methyl tridecane, 3,5-dimethyl octane, n-decane, O-cymene, terpinen-4-ol, undecane, linalool, and estragole (Palade and Popa, 2016). During development of muscadine grape (Vitis rotundifolia), myrcenol, β-ocimene, and L-limonene are common components at the first stage (green). Nonanal, decanal, and β-citronellol are detected at the second stage (soft and translucent and skin pink/red); at the third stage (purple to black), butyl-2-butenoate, propyl acetate, hexyl acetate, hexyl-2-butenoate, ethyl trans-2-butenoate, ethyl acetate, 1-octanol, butyl acetate, ethyl hexanoate, and β-citral are detected (Lee et al., 2016). The most common VOCs in white “Albariño” grapes are (E)-2-hexenal, (Z)-2-hexanol, 1-hexanol, benzaldehyde, phenylethanal, 2-phenylethanol, and cis pyran linalool oxide (Ripoll et al., 2017). In table grape cultivars (Centennial Seedless, Italia, Italia Rubi, Chasselas, Alphonse Lavallée, and Muscat de Hambourg), (E)-2-hexenal and hexanal are the two major volatiles, whereas monoterpenols are specific to Muscat varieties (Aubert and Chalot, 2017).
Strawberry (Fragaria spp.) 草莓
Several compounds are found at high concentrations (such as ketones and long-chain acids) in VOCs of strawberry, whereas several characteristic VOCs such as furanones (particularly 4-methoxy-2,5-dimethyl-3(2H)-furanone), esters (ethyl butanoate, ethyl hexanoate, methyl butanoate, and methyl hexanoate), terpenes (linalool and nerolidol) and sulfur compounds (methanethiol) are present at low levels (Yan et al., 2018). In white strawberry (Fragaria chiloensis), the major aroma compounds are ethyl butanoate, 2-hexenal, ethyl hexanoate, hexyl acetate, 2-hexen-1-ol acetate, furfuryl acetate, linalool, mesifuran, ethyl decanoate, benzyl alcohol, 2-phenylethyl acetate, 2,5-dimethyl-4-hydroxy-3(2H)-furanone, hydrocinnamyl alcohol, γ-decalactone, cinnamyl acetate, (E)-2,6-dimethylocta-2,7-dien-1,6-diol, (Z)-2,6-dimethylocta-2,7-dien-1,6-diol, and hexadecanoic acid (Prat et al., 2013).
草莓的挥发性有机化合物中存在高浓度的几种化合物(如酮类和长链酸)，而呋喃酮(特别是4-甲氧基-2,5-二甲基-3(2H)-呋喃酮)、酯类(丁酸乙酯、己酸乙酯、丁酸甲酯和己酸甲酯)、萜烯类(芳樟醇和橙花叔醇)和硫化合物(甲硫醇)等几种特征挥发性有机化合物的含量较低(Yan et al.， 2018)。在白草莓(Fragaria chiloensis)中，主要的香气化合物是丁酸乙酯、2-己烯醛、己酸乙酯、乙酸己酯、乙酸2-己烯-1-酯、乙酸糠酯、芳樟醇、4-甲氧基-2,5-二甲基-3(2h)-呋喃酮、癸酸乙酯、苯甲醇、乙酸2-苯乙酯、2,5-二甲基-4-羟基-3(2H)-呋喃酮、氢化肉桂醇、γ-癸内酯、乙酸肉桂酯、(E)-2,6-二甲基-2,7-二烯-1,6-辛二醇、(Z)-2,6-二甲基-2,7-二烯-1,6-辛二醇和十六酸(Prat等，2013)。
Citrus (Citrus spp.) 橙
Citrus fruits (e.g., orange and lemon) have high nutritional and economical value because of their contents of vitamin C, flavonoids, pectin, carotenoids, and calcium (Abobatta, 2019). Global production reached 98 million tons in 2021, according to the United States Department of Agriculture. The aromatic compounds in citrus fruits are present in peels and juices; their essential oils are used in the food, cosmetics, and pharmaceutical industries. Terpenes and terpenols are the major volatiles in orange juice. In orange beverage, limonene is the main volatile compound, followed by myrcene, ethyl butyrate, γ-terpinene, linalool, 3-carene, decanal, ethyl acetate and low levels of 1-octanol, geranial, β-pinene, octanal, α-pinene, and neral (Mirhosseini et al., 2007). Similarly, the main aromatic compounds in orange essence oil (from juice) are limonene followed by linalool, octanal, decanal, myrcene, and ethyl butyrate (Högnadóttir and Rouseff, 2003). In sweet orange (Citrus sinensis) juice, ethyl butanoate, nootkatone, linalool, and limonene are predominant (Kelebek and Selli, 2011; Herrera et al., 2016); the aroma profile of Jinchen sweet orange juice and peel oil comprises the same VOCs (Qiao et al., 2008).
The volatile oil of Citrus limon peels consists mainly of monoterpenes; limonene is the most abundant component (Ayedoun et al., 1996; Mahalwal and Ali, 2003), followed by camphenes, α-terpineol, α-phellandrene, and 4-terpineol, along with α-selinene (a predominant sesquiterpene), caryophyllene oxide, t-nerolidol, and valencene (Mahalwal and Ali, 2003). In eureka lemon, terpenoids are the main aromatic components; d-limonene is the major component in lemon juice and peel, followed by aldehydes and esters (Zhong et al., 2014).
柠檬皮挥发油主要成分为单萜；柠檬烯是最丰富的成分(Ayedoun et al.， 1996;Mahalwal和Ali, 2003年)，其次是莰烯、α-松油醇、α-水芹烯和4-松油醇，以及α-芹子烯(主要的倍半萜)、氧化石竹烯、反式-橙花叔醇和瓦伦烯(Mahalwal和Ali, 2003年)。在尤里卡柠檬中，萜类化合物是主要的芳香成分;D-柠檬烯是柠檬汁和柠檬皮的主要成分，其次是醛和酯(Zhong et al.， 2014)。
Mango (Mangifera indica) 芒果
Mango is a tropical fruit and a good source of vitamins, minerals, and fiber. According to the latest report of global fruit production in 20191, mango ranked sixth among the major fruits. The predominant aroma compounds in mango are (E)-β-damascenone, (E,Z)-nonadienal, (E)-2-nonenal, (E)-β-ionone, terpinolene, ethyl 2-methylpropanoate, ethyl butanoate, ethyl 2-methylbutanoate, limonene, myrcene, linalool, δ-3-carene, β-caryophyllene, γ-octalactone, nonanal, methyl benzoate, 2,5-dimethyl-4-methoxy-3(2H)-furanone, and hexanal (Pino, 2012). In addition to ethyl-2-methylpropanoate, ethyl butanoate, and methyl benzoate, the most abundant aromatic compounds in 20 mango cultivars are (E)-2-nonenal, (E,Z)-2,6-nonadienal, decanal, (E)-β-ionone, and 2,5-dimethyl-4-methoxy-3(2H)-furanone (Pino and Mesa, 2006). Some cultivars have typical aromas; examples include Colombian mangoes (α-pinene, α-phellandrene and terpinolene) (Quijano et al., 2007) and Harumanis mango (β-ocimene, transβ-ocimene, and allo-ocimene) (Zakaria et al., 2018). Mango skin produces glycosidically bound aromatic volatile compounds, the levels of which are strongly influenced by fruit part and maturity (Lalel et al., 2003a).
Peach (Prunus persica) 桃
Peach is an important stone fruit; it contains vitamins, minerals, and sugars. The volatile levels in white-fleshed peach skin are significantly higher than in other parts of the fruit. Unsaturated lactones and C6-compounds are detected mainly in the top and bottom mesocarp; benzaldehyde content is highest close to the stone (Aubert and Milhet, 2007). The essential oils of six peaches comprise aldehydes, lactones, alcohols, terpenes, esters, acids, norisoprenoids, phenylalanine derivates, and ketones (Eduardo et al., 2010). The characteristic volatiles and their contents in peach cultivars depend on the genotypic background and germplasm origin. For example, the highest contents of terpenoids and esters are present in Chinese wild peaches and “Wutao,” lactones are present in “Ruipan 14” and “Babygold 7,” and linalool is present in seven cultivars of American or European origin (Wang et al., 2009).
Apricot (Prunus armeniuca) 杏
The major volatiles in apricot are aldehydes, alcohols, esters, acetates, terpenes, and acids. The most abundant compounds are ethanol, hexanal, hexyl acetate, (Z)-3-hexenyl acetate, (E)-2-hexenyl acetate, (Z)-3-hexenol, 1-hexanol, and (E)-2-hexen-1-ol (Gokbulut and Karabulut, 2012). The major compounds in other apricot species are heptyl isobutyrate, citronellyl propionate, geranyl acetate, γ-hexalactone, δ-undecalactone, 5-hydroxy-7-decenoic acid lactone, and 5-hydroxy-2,4-decadienoic acid lactone (Zhang et al., 2008). In some apricot cultivars, ethyl acetate, hexyl acetate, limonene, β-cyclocitral, γ-decalactone, 6-methyl-5-hepten-2-one, linalool, β-ionone, menthone, and (E)-hexen-2-al are the most important aromatics (Guillot et al., 2006). In the apricot cultivar “Xinshiji,” the predominant compounds are hexyl acetate, β-ionone, butyl acetate, linalool, limonene, γ-decalactone, (E)-2-hexenal, and hexanal (Meixia et al., 2004; Chen et al., 2006). Aldehydes and terpenes decrease significantly, whereas lactones and apocarotenoids increase significantly, with apricot ripening. β-ionone, γ-decalactone, sucrose, and citrate are key flavor characteristics influencing consumer acceptance (Xi et al., 2016). In ripe “Xinshiji” and “Hongfeng” apricots, shared constituents include ionone, hexanal, hexenal, hexanol, hexenol, lactones, and terpenic alcohols (Meixia et al., 2004). In Japanese apricot (Prunus mume Sieb. et Zucc.), benzaldehyde, isolongifololyl acetate, linalool, butyl acetate, and palmitic acid are the dominant compounds (Miyazawa et al., 2009).
杏的主要挥发物是醛类、醇类、酯类、乙酸类、萜烯类和酸类。最丰富的化合物是乙醇、己醛、乙酸己酯、(Z)-3-乙酸己烯酯、(E)-2-乙酸己烯酯、(Z)-3-己醇、1-己醇和(E)-2-己烯-1-醇(Gokbulut和Karabulut, 2012)。其他杏种的主要化合物有异丁酸庚酯、丙酸香茅酯、乙酸香叶酯、γ-己内酯、δ-十一内酯、5-羟基-7-癸烯酸内酯和5-羟基-2,4-癸二烯酸内酯(Zhang et al.， 2008)。在一些杏品种中，乙酸乙酯、乙酸己酯、柠檬烯、β-环柠檬醛、γ-癸内酯、6-甲基-5-庚烯-2-酮、芳樟醇、β-紫罗兰酮、薄荷酮和(E)-己烯-2-醛是最重要的芳香化合物(Guillot et al, 2006)。在杏品种“新世纪”中，主要化合物是乙酸己酯、β-紫罗兰酮、乙酸丁酯、芳樟醇、柠檬烯、γ-癸内酯、(E)-2-己烯醛和己醛(Meixia et al.， 2004;Chen et al.， 2006)。随着杏的成熟，醛类和萜类物质显著减少，而内酯类和类胡萝卜素显著增加。成熟的“新世纪”杏和“红枫”杏共有的成分包括紫罗兰酮、己醛、己烯醛、己醇、己烯醇、内酯和萜烯醇(梅霞等，2004)。在日本杏(Prunus mume Sieb. et Zucc.)中，苯甲醛、乙酸异长叶叶酯、芳樟醇、乙酸丁酯和棕榈酸是主要化合物(Miyazawa等，2009)。
Pineapple (Ananas comosus) 菠萝
Pineapple is an important tropical fruit in which the most common VOCs are ethyl 2-methylbutyrate, methyl-2-methylbutyrate, ethyl 2-methylbutanoate, methyl 2-methylbutanoate, methyl hexanoate, ethyl hexanoate, decanal, and 2,5-dimethyl-4-hydroxy-3(2H)-furanone (Montero-Calderón et al., 2010; Wei et al., 2011; Zheng et al., 2012; Žemlička et al., 2013). Other VOCs include furaneol, 3-(methylthio) propanoic acid ethyl ester, 3-(methylthio) propanoic acid methyl ester, δ-octalactone (Zheng et al., 2012), and methyl butanoate (Montero-Calderón et al., 2010). The dominant compounds in pineapple at all growth stages are esters, terpenes, alcohols, 2-ketones, aldehydes, free fatty acids, and γ- and δ-lactones (Steingass et al., 2015).
Pineapple juice comprises mainly esters, decanal, acetic acid, ethyl octanoate, 1-hexanol, γ-octalactone, δ-octalactone, γ-hexalactone, γ-decalactone, and γ-dodecalactone (de Barretto et al., 2013). The major esters are methyl 2-methylbutanoate, methyl 3-(methylthio)-propanoate, methyl butanoate, methyl hexanoate, ethyl hexanoate, ethyl 3-(methylthio)-propanoate, 2,5-dimethyl-4-methoxy-3(2H)-furanone (mesifurane), and 2,5-dimethyl-4-hydroxy-3(2H)-furanone (furaneol) (Elss et al., 2005). In pineapple wine, the main components are ethyl octanoate, ethyl acetate, ethyl decanoate, and 3-methyl-1-butanol (Pino and Queris, 2010).
菠萝汁主要含有酯类、癸醛、乙酸、辛酸乙酯、1-己醇、γ-辛内酯、δ-辛内酯、γ-己内酯、γ-癸内酯和γ-十二内酯(de Barretto et al.， 2013)。主要酯类是2-甲基丁酸甲酯、3-(甲硫)-丙酸甲酯、丁酸甲酯、己酸甲酯、己酸乙酯、3-(甲硫)-丙酸乙酯、2,5-二甲基-4-甲氧基-3(2H)-呋喃酮和2,5-二甲基-4-羟基-3(2H)-呋喃酮(呋喃酮)(Elss等，2005年)。菠萝酒的主要成分是辛酸乙酯、乙酸乙酯、癸酸乙酯和3-甲基-1-丁醇(Pino和Queris, 2010)。
Durian (Durio zibethinus) 榴莲
Durian is a tropical fruit popular in southeast Asia (Chin et al., 2008); it is well-known for its strong aroma and unique taste. Durian produces various VOCs, including esters (ethyl ester and propanoic acid), aldehydes (acetaldehyde), and sulfur compounds (di-ethyl disulfide, di-ethyl trisulfide, and ethyl-propyl disulfide) (Ali et al., 2020).
The VOCs of most major fruits have been identified. The chemical compositions of fruit aromas are listed in Supplementary Table 2.
