Donald, I may be able to throw some light on the
question(s) you are asking--or the following may just stir up dust and
confusion. I have hopes it is of some help.
Concerning the synthesis pathways for the more
common carotenes, on p. 578 of Buchanan, Gruissem & Jones, eds,
*Biochemistry & Molecular Biology of Plants* there is a diagram of the
sequence of chemical changes which result in alpha- and beta-Carotene. I
am giving the source so that you will know that I am not pulling this out of my
own straw hat or out of "whole cloth" so to speak.
The sequence moves forward to LYCOPENE first, then
by the work of an enzyme named for its action, not its
structure, "Lycopene beta-cyclase," both ends of the Lycopene molecule are
twisted around into rings, identical on both ends, but the right end upside
down in relation to the left. A number of other carotenoid compounds and
derivitives follow in the sequence from beta-Carotene by the action of other
Then, in an alternate pathway, the same enzyme
active above, Lycopene beta-cyclase, acts together with another, Lycopene
epsilon-cyclase, one twisting one end of the Lycopene parent molecule into
the beta ring, and the other enzyme twisting the other end of the Lycopene into
a slightly different "epsilon" ring to form the pigment
Further development of alpha-Carotene gives rise to
Lutein through the action of two more enzymes.
The research that uncovered this was done on leaf
plastids, but there is no reason to believe that what happens in the plastids of
an iris petal would be different from those of the leaves, as the one is derived
directly from the other through successive cell divisions. It also doesn't
matter what genus of plants was used in the research. Basic biochemistry
of this nature is quite consistent from phyla to phyla, genus to
The curious thing I've noted in the back of TWOI,
pp. 426-427 is that MELODY LANE is the only one of the three cv's
noted that contain Lycopene, the other two lacking alpha-Carotene entirely, but
Melody Lane showing both Lycopene and alpha-Carotene. Beta-Carotene,
however, is present in all of them.
It is unfortunate that relative quantities were not
noted in the chemical tests published in TWOI. Only presence or absence is
noted. It would be very helpful if the relative quantity of alpha-Carotene
in Melody Lane were known.
Conspicuous by its absence, alpha-Carotene is not
found in the other two Lycopene-bearing varieties listed in TWOI--PINK
SENSATION and PINK TAFFETA.
This suggests something to me. First, there
appear to be separate genetic controls for the synthesis of alpha-Carotene and
beta-Carotene. These would be the genes responsible for the synthesis of
the two significantly different enzymes noted below.
Secondly, the synthesis of beta-Carotene does not
seem to be affected when Lycopene is present in the flower. That of
alpha-Carotene apparently is.
In those irises where we have tangerine beards,
yellow falls and standards with both yellow and a flush of pink there may be a
strong presence of both Lycopene and beta-Carotene in the beard, beta-Carotene
alone in the blade of the falls. The standards may
have beta-Carotene in the blade distributed evenly, but with a flush of
Lycopene pink upward from the center base of the standards.
Considering MELODY LANE, some alpha-Carotene may be present also.
This is a pattern found in a number of irises, and
also occurs in those cv's in which the falls are white bordered
apricot (a mixture apparently of beta-Carotene and Lycopene) or yellow, the
standards as described above. CHAMPAGNE WALTZ is an example.
There is some reason to believe that some genetic clusters of genes
affect standards and falls differently, or separately, the activity of those
genes perhaps switched on or off depending on position in the flower.
Others appear to affect the flower over all. This may account for the
difference in distribution of the pink color. Or, it is possible that the
Lycopene concentrated in the beard of the fall may appear as a flush in the
standard, no beard being present.
From the information in Buchanan, et al., together with what is noted from
TWOI, I am going to hazard a suggestion that "t" is actually a defective or
altered form of the enzyme "Lycopene epsilon-cyclase" so that when the plant is
"tttt" what occurs (normally) is that some Lycopene is converted to
beta-Carotene, but little or none is converted to alpha-Carotene. The
resulting colors range from yellow to pink, depending on the relative dosage
levels of the genes responsible for the enzymes Lycopene beta-cyclase and
Lycopene epsilon-cyclase, along with the precursor enzymes responsible for
synthesis and apparent dosage levels of the ancestral Lycopene from which
the subsequent pigments are formed. Colors ranging from white through
ivory, cream to various grades of yellow result when the two Lycopene cyclases
are in normal form.
A bit of supporting data comes from Chuck Chapman in Ontario, Canada, who
has noted that alpha-Carotene appears to be absent when Lycopene is
This also suggests that it is an exercise in frustration to try to get a
yellow iris with a really rich RED beard, as beta-Carotene is always going to be
present in the beard along with the red Lycopene.
I find this direction in research exciting, as it puts real experimental
and biochemical feet under the phenomena we observe when breeding with the
various t-bearded sorts.
Neil Mogensen z 7 Reg 4 western NC mountains
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