tinguished by the presence of additional hydroxyl and
methyl (-CH3) groups. There are six common anthocyanins (pelargonidin, cyanidin, peonidin, delphinidin, petunidin and malvidin). To date, only cyanidin has been
found within Phalaenopsis flowers. Molecules of glucose,
sinapic acid and malonic acid are attached to the cyanidin in Phalaenopsis to create 3-malony-glucosyl-7, 3’-di(sinaply-glucosyl) cyanidin (figure 4 a; Griesbach, 1990; Tatsuzawa et. al., 1997).
There are two major classes of co-pigments (flavonols
and flavones). The flavones are characterized by the presence of an oxygen (=O) at the 4 position and hydroxyl
groups at the 5 and 7 positions. The flavonols are characterized by the presence of an oxygen at the 4 position and
the presence of hydroxyl groups at the 3, 5 and 7 positions. The major co-pigment in Phalaenopsis is a flavone
(6, 7-diglucosyl apigenin) commonly known as saponarin (figure 4 b; Griesbach, 1990; Tatsuzawa et. al., 1997).
Within the vacuole, the anthocyanins and co-pigments
form a chemical complex whose color depends upon
the vacuolar pH. As the pH becomes more alkaline, the
complex becomes more stable and the color bluer. For
example, pH is responsible for the color difference between the purple (pH 4.9) and blue (pH 5.7) forms of
Phalaenopsis pulcherrima (Griesbach, 1997). The higher
pH of Phalaenopsis pulcherrima ‘Indigo Bunting’ HCC/
AOS was the result of a single recessive gene. When ‘Indigo Bunting’ was crossed to the typically colored Phalaenopsis pulcherrima ‘Arlington’ AM/AOS, all of the
seedlings were purple. When the hybrid was backcrossed
to ‘Indigo Bunting’, about half of the progeny were blue
and the other half purple. This is the expected ratio for a
single recessive gene.
The flavonoid biosynthetic pathway has been extensively
studied. Each step in pathway is catalyzed by an enzyme
encoded by a structural gene. Before we discuss flower
color genetics, two terms need to be defined - genotype
and phenotype. The phenotype is the observable properties of an organism. In a white-flowered plant, the phenotype is the absence of pigmentation. The genotype is
the genetic constitution of the plant leading to that phenotype. A capital and italic letter (R) refers to a gene that
encodes a functional enzyme; while a small and italic letter (r) refers to a gene that encodes a non-functional enzyme. An enzyme is denoted by a capital and non-italic
letter (R). Because most plants are diploid, they will have
two copies of every gene, one from each parent. Only a
single copy of a functional gene is needed for enzyme activity. Therefore, both RR and Rr genotypes results in a
functional R enzyme. While the rr genotype results in a
non-functional R enzyme. If both copies of the two genes
Figure 4a. The major anthocyanin found within Phalaenopsis flowers
is 3-malony-glucosyl-7,3’-di-(sinaply-glucosyl) cyanidin.
Figure 4b. The major co-pigment found within Phalaenopsis flowers
is 6,7-diglucosyl apigenin.
are identical (RR or rr), then the genotype is called ‘homozygous’. If the two copies are different (Rr), then the
genotype is called ‘heterozygous.’
One of the very first inheritance studies carried out was
on Cattleya flower color (Hurst, 1909). Hurst proposed a
two gene model (R and C). In order to have pigmented
flowers, both the R and C enzyme were required. If either the R or C enzyme was absent, then the flowers were
white. An R- C- genotype resulted purple flowers; while,
an R- cc or rr C- genotype resulted in white flowers. It
was possible for two plants to have the same phenotype
(white flowers) but different genotypes (R- cc or rr CC).
Today we know that are not two enzymes in the anthocyanin biosynthetic pathway, but at least a dozen. Approximately half of the enzymes result in colorless products. The other half of the enzymes are responsible for
adding different molecules to the anthocyanin skeleton.
For example, 3-O-glucosyltransferase attaches a glucose
molecule to the hydroxyl group (OH) at position 3. Recent studies suggest that Hurst’s R encodes chalcone synPhalaenopsis | Third Quarter | Volume 23 | 2013
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