Model of phylogeny and classifying existing plants, by Robert Bateman

In an essay about the taxonomic mess with European orchids, Richard Bateman stresses the importance to develop clear criteria for classification instead of individualistic conclusions. The article titled “Evolutionary classification of maximising explicit evidence and minimising authoritarian speculation” (In: Journal Europäischer Orchideen, Vol. 41/2, July 2009) explains how a monophyletic classification (i.e. a classification with a group of organisms forming a clade with a common ancestor and comprising all the descendants of this ancestor) can be derived from the phylogeny (evolution) of species.

Bateman recognises the value of morphological characteristics, but stresses that molecular data should be the main source for a scientific taxonomy. He criticizes that other classifications are often only “the result of personal opinion” and says: “21st Century data are being constrained by voluntarily retaining an 18th Century approach to biological classification”. Regarding taxonomic debates about the genera Dactylorhiza/Coeloglossum, Neottia/Listera or Gymnadenia/Nigritella he complains: “Authors decide which names they will accept and which they will reject from the many classifications already available, as though they were selecting products from the supermarket shelves.”

Arguing for clear rules founded on molecular research, Bateman also mentions the “origination of white-flowered individuals in many orchid lineages” as a result of a convergent evolution – this term describes the development of similar features with species who have no genetic relationship, often initiated by functional or environmental adaptation. But is there really such an adaptation when orchids develop white flowers, “achieved by suppressing any one stage in the biosynthetic pathway that generates anthocyanin pigments”? And what would be the object of such an adaptation?

“The palette of colors in nature is almost infinite”, says the botanist Hilke Steinecke of the Palmengarten in Frankfurt. There, this palette is displayed in an exhibition which can be visited until November 1st. The exhibition also explains the role of pigments in the colors of flowers and how fertilizing insects see colors.

An interesting demonstration shows the acid sensitivity of anthocyanins. When a drip of vinegar is placed on the violet flower of  Ipomoea, its color changes to pink. “In an acid milieu many anthocyanins are rose-pink, in an alkaline milieu blue”, as it is stated in the catalogue. This phenomenon could also explain the color variations of Nigritella nigra ssp. rhellicani in the Dolomite Alps – these occur especially in a region with rather acid soil.

Yoshikazu Tanaka of the Institute of Plant Science in Osaka sent me an article about the biosynthesis of plant pigments und and pointed out in an e-mail exchange that white petals often contain pigments from the group of flavones and flavonols. “Flavonols and flavones are very pale-yellow and are mostly invisible to the human eye”, Tanaka and his co-authors Nobuhiro Sasaki and Akemi Ohmiya explain. “As they absorb UV, which insects recognize, they give color and patterns to flowers to attract insects.” 

Just as anthocyanins flavons and flavonols belong to the flavonoids. Under the impact of the enzyme dihydroflavonol 4-reductase (DFR) certain flavonols are transformed to a pigment of the anthocyanin group. With certain species, the authors explain with regard to orchids of the tropical genus Cymbidium, DFR does not unfold this effect due to a strict substrate specifity. “This is the reason that these species lack pelargonidin-based anthocyanins and thus lack flowers of an orange/brick red color.”