Yellow Pigments of Australian Acacias

[DaveO]

Please read the following as the ramblings of a retired chemist.

 

I chanced across a paper entitled Yellow Pigments of Australian Acacias by James Matthew Petrie http://www.biochemj....957/0180957.pdf . There was along tradition of “natural products chemistry” at Australian Universities, this paper came from the University of Sydney in 1924, in the “heroic” days of chemical analysis long before instrumental methods such as IR spectroscopy were introduced. Identification of components in extracts of plant material depended on careful crystallisation and measurement of melting points.

The paper examined aqueous extracts of the yellow flowers of Acacia discolor, linifolia, decurrens and longifolia. I have posted UV images of Acacia longifolia http://www.ultraviol...-sallow-wattle/ which resemble those of all the other Acacia I have photographed so far in having very dark UV images of the flowers which, when examined at 100% look like nothing as much as black knotted string!

 

Acacia longifolia 100%

Ultraviolet Light: Pentax K-5 Full Spectrum Modification, Nikon Rayfact PF10545 MF-UV 105 mm f/4.5 lens, Nikon SB-14 flash, 1/180s @ f/11 ISO 200, Baader UV-Pass Filter.

Image Reference: DO53517

 

The yellow pigment was identified as a rhamnose glucoside of kaempferol which is a tricyclic flavonol found in a large range of plants.

Kaempferol (or one of its derivatives) has been found in many plant species used in traditional medicine and in many food items, such as tea, broccoli, witch-hazel, endive, leek, tomatoes. Some studies have found a positive association between consumption of such foods and reduced risk of cardiovascular disorders or cancer.

 

A couple of mouse clicks lead to the chemical properties of kaempferol where we find it has UV absorption maxima at 265 and 356 nm. This says nothing about any medical effects but it may explain the observed UV images. A similar phenomenon exists with all the Eucalyptus blossoms that I have photographed, where regardless of the visual coloration, the UV signature is black. I have yet to track down references to the identity of the pigments in those flowers.

[Andrea B.]

Very interesting, Dave. Thank you for finding this connection between the Acacia's UV-dark appearance and the underlying pigment. I've read before of the flavonols, but nothing so specific as this observation.