Polyphenols 94: 17th International Conference, Palma de Mallorca, Spain, 23-27 May 1994: 201-202 (1995)
Spectacular colour modification in a Lathyrus hybrid — a chemical rationalization
K. R. Markham, K. A. Mitchell
New Zealand lnstitute for lndustrial Research and Development
PO Box 31310 Lower Hutt

Flower colour in the sweet pea (Lathyrus odoratus) commonly varies from white through pink to magenta and blue, including combinations thereof. No true yellow flower has yet been produced. Yellow is not uncommon however, in flowers of other Lathyrus species. The chemical nature of this yellow and its inheritance through hybridization has formed the subject of recently completed work (Markham et al., 1992); Markham and Hammett, 1994). With the objective of introducing yellow colouration into the sweet pea flower, a white L. odoratus cultivar "Mrs. Collier" was crossed with the newly discovered (Maxted and Goyder, 1988), carotenoid-rich L. belinensis. Embryo rescue techniques were used to obtain the resultant sterile hybrid. Colour chemistry is the only aspect of this multidisciplinary research which will be discussed in this presentation.

2D-PCs of extracts from freeze-dried petals provided much of the information and the material upon which this paper is based, although HPLC (RP-18 column) was used for quantification and anthocyanidin identification. Known compounds were identified by direct comparison. Ether and MeOH:H2O:HOAc (70:23:7) extracts were used for the quantification of carotenoids and flavonols/anthocyanins respectively by absorption spectroscopy. The results are presented in Table 1 as OD at λmax for 50mg d.w. in 20 ml volume, along with the petal spectra.

Table 1 General flower characteristics

  L. odoratus "Mrs Collier" F1 Hybrid L. belinensis
Petals (std) λmax 360nm λmax 521nm λmax 452, 486, 510nm
  White Magenta Orange
(wing) λmax 360nm λmax 360, 544sh, 588, 650nm λmax 364, 448, 481nm
Pigments      
OD (std / wing) White Blue Yellow
Flavonols 3.15 / 3.25 < 0.7 / 4.4 <1 / 3.0
Carotenoids 0.15 / 0.08 0.19 / 0.18 0.84 / 0.67
Anthocyanins Nil / Nil 0.97 / 0.26 0.6 / Nil

White "Mrs Collier" is shown by these means to be a "wild type" ("original") L. odoratus with elevated flavonol levels and a biosynthetic block at the dihydroflavonol-4-reductase (DFR) or later steps. In L. belinensis, carotenoids produce the yellow in the wing petals, and when superimposed on magenta veins (containing the 3-glucosides of malvidin, petunidin and delphinidin) give an orange appearance to the standard petals (Table 1). The markedly different colours in the petals of the hybrid (Table 1) approximate to those of the "wild type" and are produced by the same anthocyanins, the 3-rhamnoside-5-glucosides of malvidin (wing) and petunidin + delpinindin (standard). Flavonoid structures were determined by standard procedures (Markham, 1982) and the results are summarized and interpreted in terms of their biosynthetic relevance in Table 2.

Table 2 Effects of hybridization on flavonoid/anthocyanin biosynsthesis

Petal "Mrs Collier" L. belinensis Hybrid Enzyme gain implicated cf. "Mrs Collier"
Wing K, Q
3rha
[7rha, 7glu]
[K], Q, My
3glu
7glu
[K], Q, My
3rha, 3glu
[7rha, 7glu,
K only]
Flavonol 3'5' dihydroxylase
Flavonol 3 glucosyl-transferase
Increased specificity of 7 glycosyl-transferases
No anthocy. No anthocy. [D, P], M Complementary post-DHF enzymes
Std. K, Q Q, My Q Active flavonol 3' hydroxylase (but not 3'5'dihydroxylase)
No anthocy. D, P, M D, P, [M] Anthocyanidin 3 rhamnosyl- and 5(3glu (3rha 5glu) glucosyl-transferases
plus complementary post DHF enzymes.

Key. D = delphinidin, M = malvidin, P = petunidin, M = myricetin etc; 7glu = 7 glucoside etc. [ ] = low level; DHF = dihydroflavonol

A rationale for the observed colour changes, based on the pigment chemistry detailed in Tables 1 and 2, is summarized below.

  1. Since the high L. belinensis carotenoid levels were not inherited, no yellow colour is evident in the hybrid.
  2. Anthocyanins have been restored in "Mrs Collier", via hybridization, through the inheritance of a complete set of post-DHF biosynthetic enzymes. "Mrs Collier" has thus reverted to the "original" or wild-type anthocyanin pattern.
  3. The colour difference between wing and standard petals can be accounted for by:
    (i) The different anthocyanidin mix, M (wing) and D + P (standard).
    (ii) Copigmentation, cf. pigment: copigment ratio 1:2 (standard) and 1:120 (wing).