Steve Topletz wrote:
I am looking for some violacea alba.
After expressing some skepticism about people misrepresenting plants they were selling as albas, he said:
I'm even dubious of yellow in the flower....
I had followed most of his arguments to that point, but this seems to be a misunderstanding because many of us use the term alba to mean white. This is sort of sloppy jargon or shorthand. The related term albino works fine in describing people and other mammals because there is only one type of pigment in our skin, melanin or its relative pheomelanin, and single point mutations in enzymes necessary for melanin production lead to complete loss of skin pigmentation. In contrast, plants have several classes of pigments, the major ones being anthocyanins, carotenoids, and chlorophyll. Since they are synthesized by independent pathways, a simple recessive mutation of the kind he mentioned will only remove one type of pigment. When we talk about alba mutations in flowering plants, we usually mean loss of anthocyanin because loss of either carotenoids or chlorophyll is lethal to the plant. The yellow color in the flowers is due to carotenoid pigments and thus is not relevant to the way we usually use the term alba.
That being said, there are, of course, completely white flowered orchids in Phaleanopsis and other genera. If there is no anthocyanin pigment in the flowers or elsewhere in the plant, it's likely that they carry a recessive "alba" mutation - the inability to synthesize anthocyanins. Lack of carotenoids (or chlorphyll) in the flowers is due to changes in the tissue-specific regulation of pigment production, not the inability to synthesize the pigment. Mutations (or natural polymorphisms) that prevent yellow or green pigmentation of the flowers can be either recessive or dominant. These effects are often referred to as suppression of pigmentation. Similar mutations can suppress anthocyanin in the flowers as well. This is seen clearly in the many orchids that have pure white flowers but anthocyanin in the leaves and in the white petals and sepals of semialba phaleanopsis or cattleyas that have a beautifully contrasting, pigmented lip.
Thus some of the "scamming" Topletz refers to is probably due to an incomplete understanding of the genetics rather than intentional misrepresentation.
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Steve Beckendorf
Berkeley, California
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