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HOW DIVERSE ARE FLORAL SCENTS?

The volatile compounds emitted from flowers belong to several different classes, but are united by their low molecular weight (30-300 amu) and vapor pressure sufficient to be released and dispersed into the air under normal temperature regimes. The individual compound classes are widely distributed among the flowers of different species, probably reflecting the fact that the major biosynthetic pathways leading to them are present in all plants.

So far, more than 1700 compounds have been identified in the floral headspace of 990 taxa (most at the species level) belonging to 90 families and 38 orders (Knudsen et ah, 2006). The majority of these taxa (78%) belong to the following 19 plant families, in each of which floral scent composition has been characterized from at least ten or more taxa listed in descending order of taxa (taxa number given in parentheses): Orchidaceae (417), Araceae (55), Arecaceae (40), Magnoliaceae (26), Rosaceae (24), Cactaceae (21), Rutaceae (21), Solanaceae (21), Caryophyllaceae (20), Nyctaginaceae (20), Fabaceae (18), Amaryllidaceae (17), Moraceae (15), Ranunculaceae (14), Asteraceae (13), Lecythidaceae (13), Oleaceae (13), Apiaceae (11), Rubiaceae (10). For details on species identity and references, see Knudsen et ah (2006).

Chemical Compound Classes in Floral Scents

Most of the 1700 compounds reported in headspace samples of floral scent are lipophilic (Knudsen et ah, 2006). The two largest groups (see Table 4.1) are terpenoids, synthesized by the mevalonate or methylerythritol phosphate pathway (Gershenzon and Kreis, 1999; Rodriguez- Concepcion and Boronat, 2002), and aliphatics, synthesized predominantly from fatty acids.

A total of 556 terpenoids have been identified and these include monoterpenes, sesquiterpenes, diterpenes, and irregular terpenes. The monoterpenes are divided about equally between compounds with acyclic or cyclic skeletons; the latter can be mono-, bi- or tricyclic skeletons (Table 4.1, Figures 4.1 and 4.2). Sesquiterpenes are also characterized by both acyclic and cyclic skeletons, but cyclic skeletons are much more common (Table 4.1, Figure 4.3). Because the enzymes producing terpene skeletons, the terpene synthases, often form multiple cyclic and acyclic products from a single substrate, either geranyl diphosphate or farnesyl diphosphate (Gershenzon and Kreis, 1999; Chen et ah, 2003), dividing compounds into acyclic and cyclic categories does not necessarily follow strict biosynthetic criteria. A number of cyclic sesquiterpene skeletons are shown in Figure 4.3C-J. The irregular terpenes include compounds varying in number of carbon atoms from 8 to 18. Among these are apocarotenoids, which are biodegradation products of carotenoid compounds (C40) like [3-carotene (Kaiser, 2002; Eugster et ah, 1969; Eugster and Marki-Fischer, 1991) (Table 4.1, Figure 4.4A-C). Others like the Cll and C16 irregular terpenes 4,8-dimethyl-l,3,7-nonatriene and 4,8,12-trimethyl-1,3,7,11- tridecatetraene are acyclic homoterpenes derived from nerolidol and geranyllinalool, respectively (Donath and Boland, 1994, 1995) (Figure 4.4E and H).

The aliphatics include 528 compounds with chains having between one and 25 carbon atoms, the majority having between 2 and 17 carbon atoms (Table 4.1, Figure 4.5). The C6 aliphatic compounds include the well known green-leaf volatiles, like (Z)-3-hexenyl acetate (Figure 4.5D) found in vegetative as well as floral scents of numerous plants and probably playing a role in plant defense

TABLE 4.1

Distribution of Floral Scent Compounds According to Their Supposed Biosynthetic Origin

Compound Class

No. of Compounds

Aliphatics

Cl

5

C2

59

C3

33

C4

41

C5

59

C6

64

C7

33

C8

39

C9

19

CIO

49

C11-C15

76

C16-C20

44

C21-C25

7

Sum

528

Benzenoids and Phenylpropanoids

C6-C0

23

C6-C1

133

C6-C2

78

C6-C3

83

C6-C4, -C5. -C7

12

Sum

329

C5-branched Chain Compounds

Saturated

40

Unsaturated

53

Sum

93

Terpenoids

Monoterpenes

Acyclic

Regular

136

Irregular

11

Cyclic

Menthanes

91

Bicyclo[2.2.11

14

Bicyclo[3.1.0)

12

Bicyclo[3.1.1]

27

Bicyclo[4.1.0]

3

Tricyclic

1

Sum

295

Sesquiterpenes

Acyclic

44

Cyclic

114

Sum

158

(Continued)

TABLE 4.1 (Continued)

Distribution of Floral Scent Compounds According to Their Supposed Biosynthetic Origin

Compound Class

No. of Compounds

Diterpenes

Acyclic

4

Cyclic

2

Sum

6

Irregular Terpenes

Apocarotenoid

52

C8

7

C9

2

CIO

8

Cll

10

CI2

3

CI3

5

CI4, CI6. C18

10

Sum

97

Nitrogen Compounds

Ammonia

1

Acyclic

41

Cyclic

19

Sum

61

Sulfur Compounds

Acyclic

37

Cyclic

4

Sum

41

Miscellaneous Cyclic Compounds

Carbocyclic

60

Heterocyclic

51

Sum

111

Compounds in each class are divided according either to carbon chain length, saturatedness, or skeletal structure.

(Pare and Tumlinson, 1999). Another large group consists of benzenoids and phenylpropanoids, which are synthesized either from the phenyl propanoid pathway starting with deamination of phenylalanine or from an intermediate of the shikimate pathway prior to phenylalanine (Jarvis et al., 2000; Wildermuth et al., 2001). A total of 329 benzenoids and phenylpropanoid compounds have been identified in floral scents (Table 4.1, Figure 4.6). Another group is the C5-branched chain compounds (Table 4.1, Figure 4.7). These are probably derived directly from branched chain amino acids, but there is little direct evidence to support this assumption (Rowan et al., 1996). Nitrogen compounds, like indole, and sulfur-containing compounds are also most likely derived from amino acid metabolism (Frey et al., 2000). The nitrogen containing compounds (Table 4.1, Figure 4.8A-F) include cyclic and acyclic compounds, whereas the sulfur-containing compounds are mainly acyclic (Table 4.1, Figure 4.8G-K). The 111 miscellaneous compounds are either carbocyclic or heterocyclic, the latter including both carbon and other atoms in the cyclic structures (Table 4.1, Figure 4.9). The group contains compounds of uncertain biosynthetic origin, but many of these are derived from fatty acids or amino acids.

Acyclic monoterpenes in floral scents

FIGURE 4.1 Acyclic monoterpenes in floral scents: regular skeleton, A, (£)-f$-ocimene; B, neral; C, ipsdi- enone; D, linalool; E, methyl geranate; F, tran.v-linalool oxide (furanoid); irregular skeleton, G, lavandulol.

Cyclic monoterpenes in floral scents

FIGURE 4.2 Cyclic monoterpenes in floral scents: /;-menthane skeleton, A, limonene; B, (+)-carvone; C, terpinen-4-ol; D, 1,8-cineole; bicyclo[2.2.1] skeleton, E, camphene; F, camphor; bicyclo[3.1.1] skeleton, G, a-pinene; H, verbenone; bicyclo[3.1.0] skeleton, I, sabinene; J, cA-sabinene hydrate; bicyclo[4.1.0] skeleton, K. 3-carene; tricyclic skeleton, L, tricyclene.

Sesquiterpenes in floral scents

FIGURE 4.3 Sesquiterpenes in floral scents: acyclic, A, (E, £)-a-farnesene; В (Z)-nerolidol; cyclic, C, P-bisabolene; D, y-cadinene; E, a-cubebene; F, germacrene D-4-ol: G, (3-elemene; H, P-gurjunene; I, caryo- phyllene oxide; J, a-santalene.

 
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