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HYB: ?bicolor/amoena yellows/pinks


Linda, the one to whom you might address this question with the greatest gain
is Chuck Chapman.  He has made an effort to get a firm handle on the genetics
of yellows.

In the meantime, I'll offer to share what experience and opinion (not the same
thing, of course, nor of equal significance) I have about yellow genetics.

First of all, TWOI, I think, in its presentation on yellows describes three
different yellow genetic pathways.  Chuck Chapman also points out (as do some
abstracts of journal articles that can be found on the Internet) that there
are several yellow carotinoid pigments that differ, and other yellows as well,
such as xanthophylls.

For the moment, I'm ignoring the differences among the carotinoids and going
by "yellow" as seen, remembering that anything that applies to these yellows
also applies to pinks, apricots, oranges, etc.

There are (1) selfs, (2) yellows with spots or zones of white beginning at the
beard tip, with the non-yellow zones of various size in different cv's.  This
is well illustrated in the three "yellow" seedlings from the R-60 cross I
posted on Iris-photos last night.  (3) White irises with varying widths of
yellow banding on the edge of the falls and standards, some of which have
complicating features such as solid yellow on the outside of the standard,
solid yellow on the underside of the fall, but bands of yellow on the other
side of the petal, and the ones at the center of your question, (4) white
standards, carotenoid-colored falls (yellow, cream, ivory, pink, apricot or
orange).  Some of these latter also have a flush upward into the standard of
the fall color, most intensely at the base, progressively paler toward the
petal edge.

To further complicate the matter, Y genetic pathways are dosage dependant
dominants.  For instance, with yellow self, one dose produces a pale ivory,
more or less, two a light yellow, three a medium yellow, and four a rich
yellow.  All of these should be noted as "more or less," not just the first.

To still further complicate the issues, the four differing distributions
described above are not mutually exclusive.  The same flower can have more
than one of the four at the same time.  A great example is the historic 'Happy
Days.'  It is a yellow bitone.  Jean Stevens used this, along with other
sources, to produce 'Pinnacle,' the great historic first of the yellow
"amoenas," isolating the factor(s) that express yellow in the fall only.
'Happy Days' clearly has both this and a light yellow self present in the same
flower.

The reason I put "amoenas" in quotes there is that some folks reacted with an
uproar at the redefinition of what applied only to the 'Wabash' type to
something entirely different.  With time the uproar died down, then exploded
again, less huffily this time, when "amoena" got applied to the 'Whole Cloth'
type.  Now, the term seems to mean anything with a white top and a set of
falls with any color other than white and no one seems to mind the
generalization of what at one time was a very precise, narrowly defined term.

Now, back to the question of differing kinds of yellow.  There are, among the
carotenoids, at least alpha-carotene, beta-carotene, gamma-carotene (the
alpha, beta and gamma are the first three letters in the Greek alphabet, by
the way), lycopene and undoubtedly others.  Are these all controlled by the
same gene?  No.  The genetic process of producing a pigment--any pigment--is
complex, with *each* step governed by an enzyme or protein put together by the
pattern of DNA components responsible for it.  The pigments are assembled step
by step, and I do not think it unlikely that the various carotenoids share
many of the steps in their synthesis within the flower.  In other words, a
whole group of DNA segments is responsible for yellow color, not just "a" gene
for yellow.

Where are the yellows located in the petal?  Unlike anthocyanin (violet, blue,
or in other genera even red) pigments which are ink-like soluble pigments
located in the main part of the cell solution (the vacuole), the carotenoids
are elsewhere.  They are not water-soluble inks.  They are oil-soluble solids
or semi-solids and occur in small, globular structures scattered around in the
cell, more often near its outer wall, called "plastids."

The cell layers of the iris petal differ.  Because of the banded type of
yellows with different patterns on the inside and outside of the standards (or
top side, underside of the falls), the non-yellow areas quite white, the
yellows must be at or near the surface of the petal.

One interesting thing I noted about yellows, even rich selfs, maybe as long
ago as fifty years or more, is that when the yellow petals are abraded, or
scraped, the damaged area looks white.  This also suggests the color is very
near the surface.  The same is true of the "Umbrata" pattern color, but not
true of blue or violet selfs.

Now, I've told you far more than you ever wanted to know, and every bit of it
is more opinion than scientifically established fact.  Are you even more
befuddled from what I've said?  Or perhaps this may have helped clarify the
question, even if it didn't answer it?

(a smile or two borrowed from WMB)
Neil Mogensen             zone 7 western NC mountains

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