Why can we find orchids of one and the same species with different flower colours? The answer depends on the genetic design of a given species.
1. Some orchids have a genetic blueprint which allows flowering in different colours. For example, Dactylorhiza sambucina mostly has yellow flowers, but there are also quite many plants with red flowers. Yellow-flowered plants produce yellow-flowered offsprings and vice versa. But cross-pollination has also produced intermediate colours (Svante Malmgren/Henric Nyström). This colour dimorphism seems to be a characteristic feature of Dactylorhiza sambucina.
2. Colour variation is one of several options how orchids of the same species vary. Species with a great morphological variety often show a broad divergence of flower colours as well - e.g. Anacamptis morio:
3. Beyond this broad spectrum of variations some orchids also show individual genetic characteristics which result in white flowers. These forms are often described as genetical disorders, as an absence of pigments which define the colour of a flower. Terms associated with this phenomenon are hypochromia (in contrast to hyperchromia which means excessive pigmentation, i.e. a very intense colouring) and albiflora. Alba or albiflora forms are devoid of any coloured pigmentation, and are pure white (Olaf Gruss: Albino Forms of the Slipper Orchids. In: Orchid Digest. Vol 69 (4), 2005. p.204). The formation of red flower pigments can be curbed or blocked with some individuals. Flowers with red pigments often have different colour hues - when these pigments are absents, these flowers are white (Wolfgang Wucherpfennig: Die Orchidee des Jahres 2007: das Schwarze Kohlröschen Nigritella nigra subsp. rhellicani, ein Kleinod der Berge. In: Berichte aus den Arbeitskreisen Heimische Orchideen Jg. 24 - Heft 1 - 2007. p.26). An example is the white variation of Anacamptis pyramidalis.
4. Some people describe this individual genetic specialty as albinism: Thus, a plant whose flowers are devoid of any red pigmentation is colloquially termed an albino - these flowers may be light green, yellowish, or white. But in a strict biological sense, an albino is a plant which lacks chlorophyll: Thus, albinism is the complete absence of green pigment that would normally be present. Since most of the orchids presented here have green foliage leaves, they are not albinos, but albiflora forms. Sometimes the white colour dominates just a part of the flower. In other cases the whole flower is pure white, even the pollinia have lost all colour.
Only a plant without any colour pigments, the foliage leaves included, should be named an albino as it has been described with Epipactis helleborine or with Malaxis bayardii in Massachusetts named as Malaxis bayardii forma kelloggiae (Paul Martin Brown: An Albino Adder's Mouth from Cape Cod, Massachusetts. In: North American Native Orchid Journal, 11/2005. p. 4-5).
There is one common characteristics of albiflora forms and albinism: Both phenomena are genetically recessive - an offspring might retain the white flowers but can also develop flowers in the standard colour. But with some species the white flowered forms are quite common and develop stable populations. With tropical orchids somw white forms have got their own horticultural value and are being cultivated accordingly - with sometimes interesting results. It was, for example, possible to produce a coloured orchid by crossing two albiflora orchids.
Terms like disorder, anomaly or deficiency refer to a certain norm or standard and define the white flowered orchid just by the negative fact of not complying with this norm. This concept with its deprecative connotations shall not to be used here. Instead white flowered orchids of a otherwise coloured species are viewed as special individuals or as way of nature exploring a new path of evolution. The result will be known only in a few million years.
Nature as a lab: Orchis simia var. albiflora and Orchis purpurea x simia
Alteration of the genetic design resulting in white flowering is not a sufficient condition to determine a new species. Even a classification as a subspecies seems not to be appropriate. As early as 1905, an orchid guide printed in the U.S. stated: Nearly every plant has albino freaks, and that alone would not be sufficient to split those two closely resembling orchids into two species (Leeming Jelliffe, Helena Dewey/Gibson, William Hamilton: Our native orchids. New York; Doubleday, 1905, p.37).
Regarding taxonomy, i.e. the scientific classification of certain species, the colour of flowers is irrelevant. There are even historical reasons for this fact: The scientifically correct description of a species requires a herbarium record. But with pressed and dried plants the colour is no longer discernible. So the white-flowered plants of a given species with a different flower colour are quaracterised as a variety or a form indicated with the appendix var. or f., e.g. var. albiflora. The process of describing a variety or a form is the same as with a new species. Only the taxa described with all these requirements are scientifically valid. Among them there has to be a description in Latin. Here follows, as an example, the description of Orchis quadripunctata var. albiflora, registered by Chryssoula and Antoine Alibertis in L‘Orchidophile 87/1989, p.112:
Since in many cases there are is no such descriptions, the white forms may be described with quotation marks: Anacamptis morio "albiflora".
Terms used for white-flowered or relative forms:
The first botanist who has crossed two species of plants was the British gardener Thomas Fairchild (1667-1729). He combined two species of carnations, putting the pollen of Dianthus barbatus on the pistil of Dianthus caryophyllus, to create a hybrid form of both. Fairchild was heavily criticised for manipulating God‘s creation, but the interest to get new kinds of flowers was greater.
A long time it was assumed that crossing of flowers with different colours is just like mixing paint on an artist‘s palette. Thus, a hybrid of a red and a white-flowered plant would have a pink colour. In 1866, the Augustinian monk Gregor Johann Mendel proved this theory false. Mendel (1822-1884) discovered the concept of recessive alleles, one of the insights which were later called the laws of Mendelian inheritance. A genetic allele (or DNA sequence) is recessive, when an individual of a certain species needs two copies of the relevant genes so that a certain genetic trait is expressed. If the individual has only one copy, by the male or the female side of inheritance, the trait is not expressed - in contrast to a dominant allele.
Mendel studied the flower colour of peas and found out that the dominant allele is purple and the recessive allele is white. Individuals with both alleles purple (BB) have a purple colour as well as individuals with one allele purple and one allele white (Bb). Only peas with both alleles white (bb) develop white flowers.
Following Mendel, Charles Chamberlain Hurst (1870-1947) was the first who studied albinism in orchids. He discovered that actually two genes are responsible for a certain flower colour: Factor C enabled the formation of colour, while the other factor, R, determined what particular colour would appear (Rohrl, Helmut: For Heaven‘s Sake, It‘s Xanthic! Albinism in Orchids. In: Orchid Digest. Vol 69 (4), 2005. p.241). Each of these genes also exist in an inactive form, c and r. Coloured plants have inherited one or two of the active alleles: CC and RR, Cc and RR, Cc and Rr or CC and Rr. White flowers have either cc or rr. C and R are understood as dominant alleles which determine certain enzymes required by the production of pigments - among them the anthocyanins, which are especially important for flower colour.
Since then, much more detailed reasearch into the genetic processes of determining flower colour has been done. At least 35 genes are known to affect flower colour in petunia (Timothy A. Holton/Edwina C. Cornish: Genetics and Biochemistry of Anthocyanin Biosynthesis. In: The Plant Cell, Vol. 7,1995, p. 1071). Among them are regulatory genes, which influence the timing, distribution, and amount of anthocyanin pigmentation.
The gene, a certain region of the chromosome, contains the coding for creating the enzymes which are necessary for the biosynthesis of pigments, for their "biochemical pathway". Thus, the genetic makeup, which is called the genotype, defines the phenotype of a flower, its visible character.
Scientists are producing transgenic plants in order to introduce pigment relevant genes which are not available naturally. Thus, there are transgenic petunia plants with foreign genes which allow the production of pelargonidin, an anthocyanin with a deeply red colouring. There are two different ways for such a genetic engineering in order to change flower colour:
(1) introduction of genes encoding novel enzyme activities, and
(2) inactivation of endogenous genes (Holton/Cornish 1995, p.1077). This can be done by the method of RNA interference (RNAi), when special genes are inactivated so they can no longer produce their specific amino messengers.
In the case of our albiflora orchids, nature is going the second way: the biosynthesis of flower colour pigments is blocked. Since there are so many different genes, regulatory genes and enzymes participating in this complex process, there are also different possibilities how this process is blocked or reduced.
There are four classes of pigments providing not only different colours but also fulfilling essential functions:
1) Chlorophyll: This green pigment is essential for the process of photosynthesis, which transforms light energy and carbon dioxide (CO2) to glucose and oxygen (O2). Chlorophyll makes good use of the blue and red portions of the wavelength of light, but is not good in using the green portions: Red und blue light is being absorbed, green light is reflected. Therefore, the parts of the plant which contain chlorophyll, have green colour. Chemically, the pigment molecule consists of a magnesium core with 4 surrounding parts of nitrogen as well as 5 or 6 parts of oxygen, 28 to 72 parts of hydrogen and 35 to 55 parts of carbon. They are not water, but lipid soluble and located in the chloroplasts of the cell. The name's roots are Greek: χλωρός (chloros is "green") and φύλλον (phyllon is "leaf").
2) Flavonoids: These water soluble pigments are based on the amino acid Phenylalanine, which is found in high levels in the breast milk of mammals. There are two different groups of flavonoids:
2 a) Anthocyanins: These pigments provide a broad range of colours from orange/red to violet/blue. The specific colour is determined by other pigments, metal ions and the pH value (they change from change from red in acids to blue in bases) (Yoshikazu Tanaka/Nobuhiro Sasaki/Akemi Ohmiya: Biosynthesis of plant pigments: anthocyanins, betalains and carotenoids. In: The Plant Journal, 54/2008 , p. 733-749). Anthocyanins are most prominent in the petals of flowers - all the albiflora varieties of orchids are missing them. The colour provided by anthocyanins has several biological functions. One of them is to reflect light waves to the chlorophyll regions of the plants to increase the production of glucose. Furthermore, anthocyanins protect sensible parts of the plant from possibly destructive light effects by absorbing blue-green and UV light.
Last but not least the colour of the flower is attracting pollinators. There are probably more than 550 different kinds of anthocyanins. Among them are the brick-red pelargonidin, the red cyanidin and the blue delphinidin pigments.
In a complex process of biosynthesis more than five enzymes are needed to produce the water soluble anthocyanins in the vacuoles of the cell. Any even minor disruption in any of the mechanism of these enzymes by either genetic or environmental factors would halt anthocyanin production (Wikipedia:Anthocyanins). The name is derived from the Greek ἀνθός (anthos is "flower") and κυανός (kyanos is "blue").
2 b) Flavones and Flavonols. These flavonoids are called co-pigments, because they are colourless for the human eye, but can influence the colour of anthocyanins. The difference between both is that Flavonols have an additional hydroxyl in their molecular structure. As they absorb UV, which insects recognize, they give colour and patterns to flowers to attract insects (Tanaka/Sasaki/Ohmiya 2008, p.737). Flavones and Flavonols can be found in most white petals. There are no white pigments with plants, but white flowers reflect all visible light and are therefore white. Noncoloured flavonoids provide 'depth' to many white or cream flowers (Erich Grotewold: The Genetics and Biochemistry of Floral Pigments. In: Annual Review of Plant Biology 2006, p.770).
Flavones: The molecular structure of the flavonoids is built by oxygen and hydroxyl. Depending on the R1 and r2 groups there are 3 major flavones: Apigenin (R1 and R2: H), Tricetin (R1 and R2: OH) and Luteolin (R1: OH, R2: H)
Flavonols: Equipped with an additional OH group. Depending on the R1 and r2 groups there are 3 major flavonols: Kaempferol (R1 and R2: H), Myricetin (R1 and R2: OH) and Quercetin (R1: OH, R2: H)
3) Carotenoids: Belonging to the group of terpenoids, these pigments cover colour wavelengths from yellow to red. Together with certain red or purple anthocyanins they enable brown or bronze hues (Grotewold 2006, p.766).
Carotenoids can be found in all parts of a plant - but they are often hidden by chlorophyll. As the chlorophylls they are lipid soluble, their containers are called chromoplasts. Similar to the anthocyanins, the carotenoids support the photosynthesis and they serve as a protection screen against destructive light. There are more than 600 carotenoids, their chemical structure being rather complex. The degradation of certain carotenoids have an importing role in producing the odours of flowers. The name comes from the Greek καρότον (karoton) and the Latin carota, both meaning carrot.
4) Betalains: There are two different groups, the betacyanins with pigment colours from red to violet and the betaxanthins which appear yellow to orange. These pigments are similar to the anthocyanins, as they are water soluble components of the vacuoles. But in contrast to them, they contain nitrogen. Betalains can only be found in a few plant families, orchids are not among them. Betalains and Anthocyanins are mutually excluded (Grotewold 2006, p.765). The name was derived from the beet (Beta vulgaris) with their deeply red coloured roots.
While the human eye has just one single lens, insects view their environment with an compound eye consisting of thousands of ommatidia. Each has its own set of photoreceptors to receive visual signals: Bees have six green receptor cells responsible for motion vision and small target detection as well as one or two UV and blue receptors (Lars Chittka/Nigel E. Raine: Recognition of Flowers by Pollinators. In: Current Opinion in Plant Biology 2006, p.429). Other insects like the swallowtail Papilio glaucus have also red receptors - as butterflies generally have a better ability to recognize colours than
bees (Adriana Briscoe: What colors do insects see? In: Orchid Digest 2005, p.265). The resolution of the compound eye is about 100 times worse than that of the human eye. Bees are near-sighted: A flower needs a giant size of 26 cm in diametre to be recognized in a distance of 1 meter; for viewing a 1 cm diameter flower a bee has to shorten its distance to 11.5 cm (Chittka/Raine 2006, p.428f.).
Much better than sight is the odor perception of insects with bees having at least 130 receptors in their antennae.
It would be a good evolutionary strategy to develop enormous flowers in order to attract pollinators. But larger flowers are more energetically costly to produce and flower size is
constrained by a complex series of interactions between the genes and processes governing organ development (Heather M. Whitney/Beverley J. Glover: Morphology and development of floral features recognised by pollinators. In: Anthropod-Plant Iteractions 2007, p.148). Flower shape is more variable, but not as variable as its colour which is regulated by genes, enzymes, metal ions and pH value. There have been experiments showing that both bumblebees and hummingbirds exhibit strong discrimination on the basis of petal colour. These results make it likely, although not certain, that colour is the only significant factor in the choices made by pollinators (Whitney/Glover 2007, p.153).
It seems that insects are learning the colours and shapes of flowers which have awarded them with nectar. Since many orchids don‘t produce nectar it may be vital for them to be mistaken for a plant rewarding its visitor with nectar.
But a successful pollination depends on multiple factors which are beyond the control of the individual plant or species (Chittka/Raine 2007, p.433).
One decisive factor is the vicinity of other flowers. Since white, UV-absorbing (typically bee bluegreen) flowers are the most common in practically all temperate European and Mediterranean habitats (Chittka/Raine 2007, p.433),
it might be a good strategy to develop white flowers: An orchid with a common color can be fairly sure to find itself in the vicinity of other flowers with similar color (Chittka/Raine 2007, p.434).
In addition to colour, the patterns of petals may as well influence the way how insects recognise flowers. The lines and blotches on petals and sepals generate a "flicker" effect due to their changing light intensity while an insect is flying over a flower with a marked pattern, and in experiments the bees preferred those (flowers) with the most flicker (Harold Koopowitz: More on Insect Vision in Flowers. In: Orchid Digest69/2005. p.267). In some albiflora varieties of orchids the colour of the petal pattern is preserved, while the surrounding area has lost all its pigments. But pure white flowers may have their own patterns as well, created by UV absorbing flavonols in the flower cells: Flowers that are visited by insects with eyes sensitive to ultraviolet light often have patterns or markings that the insects are sensitive to, but which humans cannot see (Koopowitz 2005, p.268).