Re: Chimeric variegation and its pitfalls


I have a fascination with variegation (as well as contorted plant forms)
and have been waiting for the weekend and some time to get in on this
discussion. 

For those who would like to read more about chimeric variegation, I
recommend:

http://aggie-horticulture.tamu.edu/syllabi/clasnotes/201notes/chimeralec/
chimeras.html

(Please note that's all one long line.) This is a lecture by Dan
Lineberger at Texas A&M, and I found it a concise summary. Unless you are
already very familiar with this subject, though, don't expect to speed
read it - it takes some thought.

It's easy to see how variegation can result from a chimera in which one
group of cells for some reason lacks (some or all) chlorophyll or other
pigment. It's also easy to understand how such a chimera might "revert" -
any time the plant is grown under conditions which sufficiently favor
"normal" cells, the variegated line of cells might well die out, leaving
only the normal. 

Often I have seen, in the growing hints for variegated plants, something
to the effect of "needs more light than the normal form to retain good
variegation". I've always assumed that this was because in high light the
normal cells could produce plenty of food to go around, while in lower
light those cells which must live on charity find survival much harder. I
haven't seen anyone actually state this, though it seems consistent with
observed results: leaves of variegated plants grown in low light lose
some or all of their variegation, compared with older leaves which formed
in higher light, and when moved back to high light, new leaves usually
regain their abnormal coloring in all its glory. Once any leaf has opened,
however, its coloring is fixed and won't be affected by changes in light
level.

My further assumption is that even if a variegated plant is grown in low
light for some time, so long as the variegated cell line is still present
in the meristem, the plant can regain its coloration when moved to higher
light. A question: does that mean that such a plant, if left in low light
conditions long enough, would completely revert to normal because all
variegated cells, even in the meristem, would die? I'm not sure I can
accept it - it doesn't seem like the cells in the meristem would be
selected against for presence or absence of chlorophyll. 

The URL given above also discusses briefly why some kinds of variegation
are more stable than others - some are very consistent, while others
routinely produce both non-variegated shoots which much be pruned off and
completely achlorophyllous shoots, as well as shoots with varying amounts
of "normal" variegation.

Apparently there are several other causes of variegation, including the
viruses that Sean mentioned, something called "chloroplast mosaics" about
which I know nothing, and of course genetics. I believe Jim Waddick gave
the statistic that 95% of all variegation was NOT caused by virus, which I
am pleased to hear.

I'd appreciate any pointers from the group to good resources on
variegation and contortion, whether on the Web or not.

Steve

-- Steve Marak
-- samarak@arachne.uark.edu



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