It is assumed that VOCs were originally antimicrobial compounds that later also served to combat pests, thus providing plants with a kind of immune system (Turlings & Tumlinson, 1992). In vegetative tissues, VOC patterns have coevolved with phytophagous insects, and their chemical diversity has escalated, probably to gain improved defenses (Becerra & Venable, 1999). It has been proposed that the different VOCs could act synergistically, as in conifer resins, for simultaneous protection against pests and pathogens (Phillips & Croteau, 1999). Recent data have demonstrated that VOCs serve as signals for communication between plants and between distal parts within the same plant (Qualley & Dudareva, 2001). They are also involved in protecting the plant against abiotic stress, defending the plant against pests and pathogens, and attracting herbivore predators and pollinators (Gershenzon & Dudareva, 2007; Kessler et al., 2008). It is well documented through genetic engineering experiments that specific terpenoid compounds emitted by leaves can intoxicate, repel or deter herbivores (Aharoni et al., 2003), or they may attract natural predators and parasitoids of damaging herbivores, thus protecting plants from further damage (Kappers et al., 2005). It has also been demonstrated that specific volatile compounds emitted by flowers greatly contribute to the plant's reproductive success and survival in natural ecosystems (Kessler et al., 2008).
Our knowledge regarding VOC synthesis and accumulation in fruits is much less extensive than that related to flowers and leaves. There are few references that have considered specific VOCs or VOC blends in mature fleshy fruits for the attraction of legitimate disperser organisms (Lomáscolo et al., 2010) and no references considering VOCs in interactions with putative predators, probably as a result of the difficulties and complexities involved in the measurement and analysis of VOC contents and emission from fruits under different developmental and environmental conditions in ecological contexts. In contrast, the importance of the interaction of fruit VOCs with specific insects or microorganisms in agricultural contexts has been the subject of extensive research because of its economic impact (Bruce et al., 2005), although there are few works on the interactions of fruit VOCs with vertebrates in crops (Borges et al., 2011).
VOCs in fleshy fruits
In general, flowers and fruits release the widest variety of VOCs, with emission rates peaking before pollination and at ripening, respectively (Dudareva et al., 2004). In addition, flowers, leaves and fruits often show different VOC profiles, suggesting that their functions in different tissues or organs may also be different (Fig. 1; A. Rodríguez et al., unpublished). For example, mono- and sesquiterpenes are major compounds of mango leaves and fruits, although specific VOCs can be ascribed to each tissue, such as esters that are not detected in leaves (Lalel et al., 2003; Silva et al., 2012). Similarly, important scent VOCs in ripe peach (linalool and C10 lactones) are absent from leaves (Horvat & Chapman, 1990). Specific fruit and leaf VOCs have also been reported in citrus (Dugo & Di Giacomo, 2002) and in the wild Schinus molle (Maffei & Chialva, 1990). Based on principal component analysis, Oliveira et al. (2010) showed that the peel, pulp and leaves from different fig cultivars can be distinguished by their distinct abundance of monoterpenes, sesquiterpenes and aldehydes.
Figure 1. Terpene volatile profile of different citrus tissues and organs: flower, peel from immature fruit, peel from mature fruit and leaf. Relative amounts of sesquiterpene- and monoterpene-derived volatiles are presented as a percentage of each class with respect to the total.
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VOCs in fruits are diverse, consisting of different chemical products comprising only 10−7–10−4 of the fresh fruit weight (Jiang & Song, 2010). Hundreds of VOCs are identified in most fruits, and this diversity is partially responsible for the unique scent found in different fruit species and cultivars. The aroma properties of fruits depend on the combination of VOCs produced and on the concentration and odor threshold of each in the blend. Most can be divided into four major classes according to their metabolic origin (Negre-Zakharov et al., 2009): terpenoids (e.g. mono- and sesquiterpenes and apocarotenoids), phenylpropanoids/benzenoids (e.g. eugenol, benzaldehyde), fatty acid derivatives (e.g. hexenal, hexenol) and amino acid derivatives (e.g. thiazole, 2- and 3-methylbutanal). Among them, terpenoids and lipid derivatives are probably the most abundant and expensive to produce in terms of energy and nutrients (Gershenzon & Dudareva, 2007). From a chemical view, these VOCs can be classified as esters, alcohols, aldehydes, ketones, lactones and terpenoids.
To attract seed disseminators and thus to ensure reproductive and evolutionary success, many plants release diverse blends of VOCs from their fruits. With their huge number of compounds and high structural diversity, terpenoids are one of the largest classes of VOCs in fruits, especially monoterpenes, sesquiterpenes and irregular terpenes of low molecular weight. In lulo, myrtle, coriander, mango and citrus mature fruits, monoterpenes are the most representative type of volatiles, their content varying between 50% to almost 100% of the total VOCs (Table 1). Esters are also key contributors to the fruity aroma. For example, the ester fraction has been described as the determinant for the characteristic varietal aroma in apple cultivars and Lambrusco and hybrid grapes (Jiang & Song, 2010; Yang et al., 2011). In Ficus racemosa, esters dominate volatile profiles (86–94% of total), as also occurs in Ficus benghalensis fruits during the night (Borges et al., 2011). Some other groups of minor volatiles are also important for fruit scent in terms of concentration, for example, apocarotenoids, also called norisoprenoids, derived from carotenoids by oxidative cleavage. Studies in tomato, melon, peach and watermelon indicate that the carotenoid profile has a clear impact on aroma via the determination of the suite of synthesized apocarotenoids (Lewinsohn et al., 2005; Rodrigo et al., 2012). Other compounds, such as sulfur volatiles, mainly arising as degradation products of cysteine, cystine, methionine, glutathione and some vitamins, are also characterized by their extremely low aroma thresholds (Du et al., 2011). VOCs derived from amino acids are important flavor constituents of many ripe fruits, such as strawberries, tomato, melon and apples (Goff & Klee, 2006; Gonda et al., 2010).
Table 1. Changes in the total amount and selected groups of volatile organic compounds during the development and ripening of different fleshy fruits
| ||Increase during ripening||Aldehydes||Esters||Monoterpenes||Sesquiterpenes||References|
|Solanum vestissimum ||> ×30|| a ||1.4||> ×6.0||75.0||> ×0.5||1.2||nd||nd||Suárez & Duque (1992)|
|Schinus molle ||·/· 1.5||nd||nd||nd||nd||×1.0||85.9||×1.0||10.8||Hosni et al. (2011)|
|Myrtus communis ||×3.0||nd||nd||·/· 3.3||0.1||×0.9||71.6||·/· 4.5||3.06||Aidi Wannes et al. (2009)|
|Coriandrum sativum ||×30.7||·/· 2.1||1.2||·/· 14.7||2.4||×1.6||90.4||·/· 1.9||2.0||Msaada et al. (2009)|
|Psidium guajava ||×2.9||·/· 12.5||2.3||×8.8||54.0|| b ||10.0|| b ||33.5||Soares et al. (2007)|
|Psidium salutare ||·/· 2.2||nd||nd||·/· 1.2||2.5||·/· 2.2||58.5||·/· 2.6||20.0||Pino & Queris (2008)|
|Mangifera indica ||nd||nd||nd||×44.6-b ||7.7–38.4||·/· 1.2–1.6||53.6–78.3||·/· 1.9||7.9–10.8||Lalel et al. (2003)|
|Fragaria ananassa ||×4.0–19.0||·/· 60.3||0.5–4.2||×1.3||78.0–91.0||×3.3||nd||nd||nd||Menager et al. (2004); Azodanlou et al. (2004)|
|Actinidia deliciosa ||×5.6||·/· 1.7–2.0||9.3–15.8||×330–409||71.6–82.8||×1.0–1.1||0.0–0.1||nd||nd||Garcia et al. (2012); Wang et al. (2011)|
|Actinidia chinensis ||×60–117||·/· 1.1–×1.6||0.3–0.7||×1116–2381||76.0–83.0||×1.0–4.4||0.0–0.9||nd||nd||Wang et al. (2011)|
|Malus × domestica ||×1–30||×1.7–18.3||3.3–54.3||×9–515||11.5–97.0||nd||nd||nd||nd||Villatoro et al. (2008); Ortiz et al. (2011)|
|Prunus persica ||·/· 1.0–4.6||·/· 3.1–×1.2||0.3||nd||nd||·/· 1.7- b ||0.3–40||nd||nd||Engel et al. (1988); Aubert et al. (2003)|
|Carica papaya ||×3.3||nd||nd||×1.6||50.8–95.1|| b ||1.6||nd||nd||Almora et al. (2004); Fuggate et al. (2010)|
|Capsicum annuum ||·/· 1.2–1.3||×1.3||13.8||·/· 1.7||52.8||×1.9||18.1||·/· 1.4||10.4||Forero et al. (2009)|
|Solanum lycopersicum ||×1.9–1077.8||·/· 2.7-×5.5||13–82.0||nd||nd||×1.3–1.6||<0.1||nd||nd||Birtic et al. (2009); Ortiz-Serrano & Gil (2010)|
|Ficus scortechinii ||×18.0||nd||nd||×3.1||61.1||nd||nd||·/· 17.3||0.1||Hodgkison et al. (2007)|
|Ficus hispida ||×30||nd||nd||×2.6||46.4||nd||nd||·/· 6.9||0.1||Hodgkison et al. (2007)|
|Ficus benghalensis |
|Diurnal||nd||nd|| ||nd||13.34||nd||12.16||nd||35.86||Borges et al. (2011)|
| Ficus racemosa ||nd||nd||0.5–0.6||nd||85.6–93.2||nd||3.1–3.6||nd||nd||Borges et al. (2011)|
| Citrus sinensis ||×4.2||·/· 48||0.48|| b ||0.01||×1.2||96||·/· 8||0.34||Rodríguez et al. (2011)|
VOC changes during ripening
The following examples, without being an extensive review of the published literature, illustrate how VOC profiles change during fruit ripening. VOC production increases between 1 and > 1000 times during the maturation of most fruits (Table 1). Concomitantly, qualitative changes in the VOC profile take place during ripening (Table 1). For example, short-chained aldehydes, which provide the ‘fresh green’ odor, are abundant in numerous unripe green fruits, and their concentration decrease with ripening in fruits, such as nectarines, guavas, apples, coriander, strawberries and kiwis. In other fruits, such as neutral grapes, few volatiles other than C6 compounds accumulate (Yang et al., 2011). In this case, VOCs with a green flavor increase until the period of ripening and then decrease. The monoterpene profile also changes during the ripening of lulo, myrtle, coriander and citrus fruits (Table 1). For example, in oranges, there is a reduction in the linalool content as maturation progresses, whereas the limonene content increases from 30- to 100-fold between the green and color break stages (Dugo & Di Giacomo, 2002; Rodríguez et al., 2011). In white guava, mono- and sesquiterpenes, which are absent in unripe fruit tissues, accumulate and increase during ripening (Soares et al., 2007).
The concentration of esters and lactones, responsible for the spicy floral and fruity scent of many fruits, increases extraordinarily during ripening. In apple, mango, strawberry, kiwi, papaya, guava and lulo fruits, ester production increases by a factor ranging from 1.3 to > 2300 during maturation and, at the ripe stage, esters can account for up to 97% of the total VOCs (Table 1). It is interesting to remark that, in the case of strawberries, apples and lulos, a burst in ester production has been associated with the onset of ripening (Suárez & Duque, 1992; Menager et al., 2004; Villatoro et al., 2008; Table 1). In nectarines, lactones are characteristic of ripe fruits, their concentration increasing during maturation to reach up to 45.7% of the total VOCs (Engel et al., 1988).
The accumulation of low-strength ripeness-specific fruity aromas, such as apocarotenoids, sulfur volatiles and furan-related compounds, also increases during fruit ripening. In most carotenogenic fruits, the pigment profile changes during maturation, thus changing the apocarotenoid profile as well. In tomato, apocarotenoid VOCs are not emitted until relatively late in fruit ripening, and, during this process, the amount of apocarotenoids increases by a factor of 40 (Mathieu et al., 2009). In peaches, the apocarotenoid level also increases during fruit ripening, representing c. 40–60% of the total VOCs at the ripe stage (Aubert et al., 2003; Brandi et al., 2011). In strawberries, most sulfur volatiles increase by as much as 100% with maturity (Du et al., 2011). Methyl sulfanyl compounds increase considerably in kiwi with ripening, dimethyl sulfide being one of the key components that differentiates the aroma of yellow and green cultivars (Garcia et al., 2012). Finally, furanone-derived compounds also increase in concentration with the maturation of some fruits. In peaches, furan-related VOC accumulation starts just before color change, and its concentration reaches a maximum at the ripe stage (Brandi et al., 2011). In strawberries, furan-derived compounds are not detected until the fruit reaches a red color, and their content increases by c. 100-fold during maturation (Menager et al., 2004).
In summary, the influence of the ripening stage on fruit scent is clearly evident, and it is well documented that VOC composition changes both quantitatively and qualitatively during maturation. Indeed, the analysis of principal components has been successfully applied to discriminate between ripening stages depending on the presence/absence of some VOCs in many fruits, such as apples (Villatoro et al., 2008), grapes (Yang et al., 2011), mangos (Lebrun et al., 2008), strawberries (Azodanlou et al., 2004), figs (Hodgkison et al., 2007) peaches and nectarines (Lavilla et al., 2002). Some of these VOC modulations have been related to aroma chemical changes associated with ripening. For example, sugars, the concentration of which increases with ripening, are precursors of furanones, and, in tomatoes, a direct relationship has been established between sucrose and VOC production (Zanor et al., 2009). Fatty acids are quantitatively the major precursors responsible for the synthesis of esters, aldehydes, alcohols and acids found in fleshy fruits. Because lipid biosynthesis and membrane fluidity increase during ripening, a wider assortment of lipid-derived precursors of aroma-contributing VOCs is found in the tissues of fully ripe fruits (Sanz et al., 1997).
In addition, recent molecular findings support the idea that the de novo synthesis of VOCs is induced at ripening. Transcriptional regulation has been described for terpene-, carotenoid-, fatty acid- and phenylpropanoid-derived VOCs, and, in most cases, gene expression is induced on ripening concomitantly with the production of important flavor compounds (Rodrigo et al., 2012). Moreover, some genes have been shown to display a fruit-specific expression, such as those involved in different steps of alcohol and ester biosynthesis in melon (Yahyaoui et al., 2002; Manríquez et al., 2006). The expression of genes involved in the biosynthesis of amino acid-derived VOCs is also much higher in ripe fruits than in vegetative and unripe fruits (Gonda et al., 2010). Similarly, sesquiterpene synthase activity is evident in the rind from ripe melon, whereas it is not found in the flesh and unripe rind, where no sesquiterpenes accumulate (Portnoy et al., 2008). Moreover, sesquiterpene synthase genes are found to be transcriptionally regulated during fruit development and are likely to be associated with the VOC differences responsible for the unique aroma of different melon varieties (Portnoy et al., 2008). It is common in VOCs that a single enzyme catalyzes the synthesis of multiple products from different substrates (Pichersky & Gang, 2000). Therefore, it has been proposed that this broad substrate specificity is the result of convergent evolution, in which new enzymes with the same function have evolved independently in separate plant lineages from a shared pool of related enzymes with similar, but not identical, functions, providing an extraordinary versatility to VOC blend production patterns in specific plant tissues (Pichersky & Gang, 2000).