Module 2: Floral Pigments & Color

Additionally, a fourth category - betalains (red betacyanins, yellow betaxanthins) - is found only in the Caryophyllales order (beet, cactus, Portulaca). Betalains are derived from tyrosine, not phenylalanine, and never co-occur with anthocyanins in the same plant.

All flavonoids derive from the shikimate pathway via phenylalanine. The entry enzyme PAL (phenylalanine ammonia-lyase) deaminates phenylalanine to trans-cinnamic acid, releasing NH₃. This single step commits carbon to the phenylpropanoid pathway - diverting up to 20% of a plant's carbon skeleton budget.

PAL catalyses a nonoxidative deamination via a methylidene-imidazolone (MIO) prosthetic group that forms covalently from three active-site residues (Ala-Ser-Gly). The rate\(k_{\text{cat}} \approx 30\,\text{s}^{-1}\) and \(K_M \approx 50\,\mu\text{M}\) make PAL one of the most abundant plant enzymes (up to 2% of total soluble protein in stressed tissues).

2.1 Committing to Anthocyanins

Cinnamate is 4-hydroxylated (C4H, cytochrome P450) to p-coumarate, CoA-activated (4CL) and then condensed with 3 molecules of malonyl-CoA by chalcone synthase (CHS) - the committed step of flavonoid biosynthesis. Chalcone is isomerised (CHI) to the flavanone naringenin, hydroxylated at C3 (F3H) to dihydroflavonol, reduced (DFR) to leucoanthocyanidin, and finally oxidised by anthocyanidin synthase (ANS) to the coloured anthocyanidin (aglycone):

The three parent anthocyanidins differ only in the hydroxylation pattern of the B-ring (ring with the free phenol):

Each additional hydroxyl on the B-ring extends conjugation and shifts \(\lambda_{\max}\)bathochromically (redshift) by roughly 17 nm. Methylation at 3' or 5' gives peonidin, malvidin, petunidin - further fine-tuning hue.

3.1 pH and the Flavylium Cation

The chromophore is the positively charged flavylium cation (AH⁺), stable only below pH ~3. Above pH 4 it loses a proton from the 7-OH to form the quinoidal base - blue - and also hydrates at C2 to give a colourless carbinol. Further alkaline conditions open the pyran ring to a yellow chalcone:

This is why anthocyanins turn red in acidic media (strawberry juice) and blue in alkaline media (baking-soda solutions), and why hydrangea petal colour depends on soil pH through a more indirect mechanism - aluminium availability - rather than direct vacuolar pH.

3.2 Copigmentation & Metal Chelation

Vacuolar pH in petals is typically 5.0-5.5 - neither strongly red nor blue. Yet true blue flowers exist (delphinium, gentian, cornflower). Three tricks:

• Copigmentation: stacking of flavonols or aromatic acids on the flavylium cation shifts absorption to longer wavelengths by charge-transfer interaction (up to +50 nm, 10-fold intensity increase).
• Metal chelation: delphinidin 3,7,3',5'-O-acylated with caffeic acid chelates Fe³⁺, Mg²⁺ or Al³⁺ via the catechol hydroxyls on the B-ring, stabilising the blue quinoidal form. Commelina communis's blue pigment commelinin is a supramolecular complex of six anthocyanin molecules, six flavone copigments and two Mg²⁺.
• Vacuolar alkalisation: morning glory (Ipomoea tricolor) has NHX1, a tonoplast Na⁺/H⁺ antiporter that raises vacuolar pH from 6.6 to 7.5 during petal opening, converting pink to blue in hours.

Not all flower colour is due to pigments. Some flowers exhibit iridescence - a colour that changes with viewing angle - generated by periodic nanostructures. Hibiscus trionum's petal base displays a UV-reflecting diffraction grating (striated cuticle, ~1 um period) that functions as a bee-visible "blue halo."

For a one-dimensional diffraction grating with period \(d\), the m-th order is observed when \(d (\sin\theta_i + \sin\theta_r) = m\lambda\). For\(d = 1\,\mu\text{m}\) at normal incidence, the first-order reflection varies from 400 nm at 24 deg to 700 nm at 44 deg - spanning the full visible spectrum. Bees are particularly sensitive to this chromatic modulation because their UV + blue + green photoreceptors sample overlapping portions of the iridescent spectrum.

In Pollia condensata (peacock plant), the fruit wall uses a chiral cellulose helicoid (Bouligand structure similar to insect cuticle - see Bee M0) as a cholesteric liquid-crystal reflector, producing the most intensely blue colour in biology.

Honeybees (and most Hymenoptera) have three photoreceptor types with peaks around 344 nm (UV), 436 nm (blue) and 544 nm (green). They cannot see red but are exquisitely sensitive to UV - extending down to ~300 nm. Many apparently monochrome flowers have a striking UV "bullseye": Rudbeckia hirta, Potentilla, Oenothera, Helianthus. The central UV-absorbing patch contains flavonol glycosides (quercetin-3-rutinoside) that absorb 320-380 nm; the outer margin either reflects UV or absorbs weakly.

Honey guides provide three functions:

• Landing accuracy: focal target reduces the bee's approach error, shortens handling time.
• Heat cue: UV-absorbing centres can run 2-4 deg C warmer than the margin - a reward that small-bodied bees may exploit.
• Protection: flavonols double as UV screens for gametes in the centre of the flower, protecting pollen and stigma from UV damage.

5.1 UV-SVG: Rudbeckia Honey Guide