… develop white flowers while their species is supposed to have coloured flowers. This Gentiana germanica, found at Seiser Alm in the Dolomite Alps, is an example.

The plant at the right side has flowers without pigments (anthocyanins). It may be viewed as “Gentiana germanica albiflora”, as Ferdinand Schur has noted in his article “Beitraege zur Flora von Wien” (Oesterreichische Botanische Zeitschrift vol. 11/1860). The correct name should be Gentiana germanica f. albiflora.

Another example found this year in the Swiss region of Aargau is Ajuga reptans f. albiflora which has acquired some horticultural importance.

But neither the Gentianaceae nor the Labiatae (the family of the genus Ajuga) could be viewed as a family with a certain tendency towards developing white flowers – as it is the case with orchids. Maybe another family with an albiflora disposition are the Cactaceae. A charming web gallery of albiflora cacti has been set up by Gerd Weiss presenting more than 50 species.

In an e-mail exchange following his recent article in the Journal Europaeischer Orchideen (JEO), Richard Bateman, orchid specialist at Kew Gardens, wrote me that albiflora plants “are far more common among diploid Dactylorhiza species than tetraploid species”. A possible reason might be the “buffering of mutations by having four comparable genes in the tetraploid chromosomes”. Diploid species (with 40 chromosomes) are Dactylorhiza fuchsii, D. incarnata and D. sambucina. Tetraploid species (with 80 chromosomes) are D. majalis, D. praetermissa, D. maculata, D. elata, D. sphagnicola and D. traunsteineri.

Albiflora plants of Dactylorhiza fuchsii are quite often observed, and in Ireland there is also the intriguing D. fuchsii ssp. okellyi which is diploid as well. D. incarnata and D. sambucina are known for their colour dimorphism: red and yellow with D. sambucina, purple and yellowish-white with D. incarnata and its var. ochroleuca. In a recent article in the Annals of Botany (2009), Mikael Hedrén and Sofie Nordstroem presented the results of their reasearch about the colour dimorphism with D. incarnata. They observed that there was “no clear pattern of habitat differentiation … among the colour morphs”. With D. incarnata var. ochroleuca “the lack of anthocyanins is probably due to a particular recessive allele in homozygous form” – the diploid chromosome set has both alleles determining the lack of purple in the flowers.

Besides genetics, colour also affects the pollination function of orchid flowers. Bateman wrote me that “in at least a few cases, instantaneous loss of anthocyanins (or even just radical decrease in anthocyanin production) must affect pollinator preference, and lead to lineage divergence”. A potential example of such an evolutionary process could be Gymnadenia frivaldii as a relative of Gymnadenia conopsea.

But in general the question of a certain functionality of colour change is still unanswered. Following his mentioning of white flowers in the above mentioned JEO article, Bateman wrote me it would be “more correct to use the term ‘parallelism’ rather than ‘convergence’, since in most cases no-one has demonstrated a change of function or ‘behaviour’ in the abnormal white flowers”. He further noted “the probability that many different mutations and epimutations generate white flowers”. Recognising that there are quite many open questions, Bateman also asked “whether white is actually a colour at all”, pointing to the “very simple shifts between ‘white’ flowers and ‘green’ flowers in Platanthera”.

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.

Limodorum abortivum: Albiflora form (left; Photo: N.Griebl) and common form (right)

Limodorum abortivum is one of the rarest albiflora forms of orchids. Sometimes, literature is mentioning the existence of white flowering plants, e.g. Horst Kretzschmar states in his new “Die Orchideen Deutschlands und angrenzender Laender” (Wiebelsheim 2008), p.163: “In Southern Europe, there is a broad variation of flower colours, from white to purple to red, these colours have not been observed in Germany up to now, though.”  Norbert Griebl in Austria has sent me a photo of an albiflora form he found in northern Greece. He observed that the plant has a green stem and green sheathing leaves, “which prooves that Limodorum is not totally living saprophytic” (=myco-heterotrophic).

Compared with the violet stem colour of the regular form the green colour of the stem is indead striking. The existent chlorophyll is obviously covered by dominant anthocyanins. When these purple pigments are absent – as it is the case with the albiflora form – the green chlorophyll colour becomes clearly visible. A study published in 2006 (M. Girlanda, M. A. Selosse, D. Cafasso, F. Brilli, S. Delfine, R. Fabbian, S. Ghignone, P. Pinelli, R. Segreto, F. Loreto, S. Cozzolino and S. Perotto: Inefficient photosynthesis in the Mediterranean orchid Limodorum abortivum is mirrored by specific association to ectomycorrhizal Russulaceae. In: Molecular Ecology 15, 2006, S. 491-504) recognizes the existence of chlorophyll but stated that Limodorum abortivum’s photosynthesis “was found to be insufficient to compensate for respiration in adult plants”. It would be interesting to know how the albiflora orchid is behaving in this regard and if it is also dependent on nutrition by fungi.

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.”

Weiß blühende Orchideen mit einer anderen Standardfarbe werden oft als “Albino”-Form bezeichnet. Tatsächlich aber können Albino-Pflanzen, denen das für die Photosynthese unerlässliche grüne Pigment Chlorophyll fehlt, durchaus farbige Blüten haben, wie diese Aufnahme einer Cephalanthera rubra aus Thüringen von Holger Disse zeigt.

Umgekehrt haben die meisten Albiflora-Orchideen zwar weiße Blüten, aber durchaus kräftig grüne Blätter und auch einen solchen Stängel. Ihnen fehlen die für die Blütenfarbe relevanten Pigmente aus den Gruppen der Carotenoide und Anthocyanine. Sie verfügen aber über reichlich Chlorophyll, erhalten also auch Nährstoffe aus der Photosynthese.

Bei manchen Orchideen ist die Ausbildung der Blütenfarbstoffe reduziert, Anthocyanine werden aber noch in geringem Ausmaß gebildet, wie es bei der hier gezeigten Cephalanthera rubra der Fall ist. Diese weiß blühenden Pflanzen mit einem Hauch von Standardfarbe könnten als Teil-Albiflora-Form bezeichnet werden.