2nd International Electronic Conference on Synthetic Organic Chemistry (ECSOC-2), http://www.mdpi.org/ecsoc/, September 1-30, 1998 [dp285]

Investigation of flower color at the cellular level
MARKHAM, Kenneth R.1, SMITH, Gerald J.1, GOULD, Kevin S.2
1 School of Biological Sciences, University of Auckland, Auckland, New Zealand
2 Industrial Research Ltd, PO Box 31 310, Lower Hutt, New Zealand

Traditional studies of flower color chemistry generally fail to take into account the subtleties of cell to cell color variation which are not perceived by the eye. In some flowers these can be very large. It is thus of importance also to understand the different physicochemical influences on pigment(s) at a cellular level. Frequently the color of the isolated pigment is markedly different from the color it acquires in the vacuolar environment. Delphinidin glycosides for example can appear pink, magenta, purple or blue in vacuoles and "colorless" flavonol glycosides can appear bright yellow. Although inter- and intra-molecular copigmentation, pH, metal ions and concentration differences are known to bring about color changes in vitro, in vivo studies have rarely been reported. The yellow appearance of colorless flavonols appears not to have been explained. The current communication describes progress to date on the understanding of some of these phenomena.

Yellow coloration in Lathyrus chrysantha

Microscopic examination of the epidermal peel of L. chrysantha standard petals reveals vacuolar yellow in only about half of the cells. In addition, a petal cross-section revealed that the yellow coloration is confined to the epidermal cells. The compounds involved in this coloration are a series of 3- and 7-glycosylated flavonols, primarily quercetin. In an attempt to understand the yellowing phenomenon, the fluorescence excitation and emission spectra of kaempferol and quercetin-7-glucosides were determined both in aqueous solution and adsorbed on cellulose. The spectra indicate that at near saturation in H2O, or in microcrystalline form on a cellulose matrix, these flavonols tautomerize substantially to the ground state enolic tautomers (IIe, IIz, Figure). These absorb with a maximum at around 440nm, i.e. they appear yellow, compared with 350-370nm for flavonols in their keto-form (I). The tautomerism in these cases is thought to be facilitated by flavonol-flavonol association in a manner similar to that previously observed with 7-hydroxyquinoline (Bohra et al., 1994). From absorption measurements on yellow and colorless epidermal cells (l max ca 400nm and <300nm respectively), and a determination of their depth (= optical path length), it was found that the flavonol concentration in yellow cells was ca. 2 x 10-2M (or ca. 11g/l). However, the "near saturation" concentrations used in the fluorescence studies were only ca. 5 x 10-5M, which suggests that some solubilizing matrix (carbohydrate?) may be influencing solubility/color in the vacuole.

Purple lisianthus (Eustoma grandiflorum) cell color

The major pigments in this flower are delphinidin-3-O-b-D-[6-O-a-L-rhamnogalactoside]-5-O-b-D-[(E and Z)-p-coumaroylglucoside]. These pigments are common to both the almost black inner portion of the petal and the purple outer portion. The color difference is not accounted for by the copigment : pigment ratios which are comparable (7:1 vs 9:1), but could be due to the higher anthocyanin levels in the inner portion. However, levels in the outer petal (>6 x 10-3M) already exceed those that can be produced in vitro at moderate pH (e.g. 6 x 10-4M at pH4). A solution to this dilemma was provided by microscopic examination of the epidermal peel. Much of the pigment in the inner petal cells, and to a lesser extent in the outer, is not in solution. "Gelatinous" blobs of color are evident in most vacuoles, varying in size such that inner petal vacuoles contain much larger, more intensely colored structures than outer petal vacuoles. Large red-purple to almost black rounded structures filled most inner petal vacuoles whereas outer petal vacuoles contained much smaller, redder, star-like structures, surrounded by pigmented vacuolar liquid. The existence of such non-solubilized pigment in the inner cells would account for the very intense coloration of the inner petal. A study of these structures by electron microscopy failed to detect the presence of a surrounding membrane, thus eliminating the possibility that these structures are organelles. Our preliminary investigation suggests that this novel strategy for intensification/modification of flower color is not unique to lisianthus.

Reference

BOHRA, A., LAVIN, A., COLLINS, S., 1994. Ground state triplet tranfer in 7-hydroxyquinoline. J. Phys. Chem., 98, 11424-7.