Floral Scent - Part 5

Lathyrus odoratus (sweet pea) photographed at dusk (D. Rhodes)

Another plant species with extremely colorful and fragrant flowers is the sweet pea (Lathyrus odoratus) (see: Sweet pea - Wikipedia).

Porter et al. (1999) analyzed the floral volatiles of three varieties of the sweet pea ('Royal Wedding', 'Diana' and 'Old Time') and found a total of 49 compounds at levels above the threshold for detection. 41 of these compounds were common to all 3 varieties. The major components of sweet pea floral scent were: (E)-B-ocimene (range 35.3 to 46.5% of total scent volatiles), linalool (range 20.7 to 26.2%), (Z)-B-ocimene (range 2.3 - 7.3%), phenylacetaldehyde (range 6.5 to 2.9%), nerol (range 3.3 to 5.1%), geraniol (range 4.5 to 5.9%), (E)-alpha-bergamotene (range 1.3 to 6.8%), geranyl acetate (range 0.3 to 1.4%), neryl acetate (range 0.3 to 1.4%), and B-caryophyllene (range 0.1 to 2.3%). With the exception of phenylacetaldehyde, these compounds are all terpenoids/isoprenoids (see Part 3 of this series for a discussion of the pathway(s) of synthesis of terpenoids/isoprenoids in snapdragon).

The benzenoids (methyl benzoate, benzaldehyde, benzyl alcohol, methyl salicylate) were detectable in the scent, but each comprised less than 0.6% of total scent volatiles in all cultivars. Notably, 2-phenylethanol was absent from L. odoratus scent (Porter et al. (1999)).

Sexton et al. (2005) confirmed the above findings: "The major components of the scent of cut sweet pea flowers (Lathyrus odoratus L. cv. 'Royal Wedding') are (E) and (Z)-ocimene, linalool, nerol, geraniol and phenylacetaldehyde. The aroma is almost exclusively produced by the standard and wing petals, with very little emanating from the keel petals and other floral structures."

We have previously seen that there are diverse pathways of phenylacetaldehyde synthesis in plants. These are listed below:

• Petunia hybrida (petunia):

Phenylalanine ---> Phenylacetadehyde (via phenylacetaldehyde synthase (PAAS)) (Kaminaga et al. (2006)) (as discussed in Part 1)

• Rosa damascena and Rosa x hybrida (rose - winter):

Phenylalanine ---> Phenylacetadehyde (via L-aromatic amino acid decarboxylase (AADC)) (Hirata et al. (2016)) (as discussed in Part 4)

• Rosa damascena and Rosa x hybrida (rose - summer):

Phenylalanine ---> Phenylpyruvate ---> Phenylacetadehyde (via aromatic amino acid aminotransferase (AAAT) and phenylpyruvate decarboxylase (PPDC) (Hirata et al. (2016)) (as discussed in Part 4)

• Solanum lycopersicum (tomato):

Phenylalanine ---> Phenylethylamine ---> Phenylacetadehyde (via aromatic amino acid decarboxylase (AADC) and an amine oxidase) (Tieman et al. (2006)) (not previously discussed)

• Angraecum sesquipedale (Madagascan orchid):

Phenylalanine ---> N-hydroxyphenylalanine ---> Phenylacetaldioxime ---> Phenylacetaldehyde (via as yet uncharacterized enzymes in this species) (Nielsen and Møller (2015)) (not previously discussed)

Antirrhinum majus (snapdragon) lacks phenylacetaldehyde and can be presumed to have no capacity to produce this scent compound (Weiss et al. (2016)) (discussed in Part 3).

Lathyrus odoratus (sweet pea) produces phenylacetaldehyde by a pathway that has not yet been characterized (Sexton et al. (2005)). It remains to be seen which pathway of phenylacetaldehyde synthesis operates in sweet pea.

Both Lathyrus odoratus (Porter et al. (1999)) and Antirrhinum majus (Weiss et al. (2016)) lack production of 2-phenylethanol (2-PE). In contrast, Petunia hybrida, Rosa damascena, Rosa x hybrida, Solanum lycopersicum and Angraecum sesquipedale all produce 2-PE from phenylacetaldehyde via the catalytic action of phenylacetaldehyde reductase (PAR). It can be reasonably speculated that Lathyrus odoratus (sweet pea) may lack PAR activity.

This post is a continuation of my earlier posts: Floral Scent - Part 1, Floral Scent - Part 2, Floral Scent - Part 3 and Floral Scent - Part 4.

Please feel free to comment or ask questions about this post in the comments section below. I will try to answer questions as soon as possible.