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Re: Re: was weather, now fall color

> From: cathy carpenter <cathyc@rnet.com>
> Have yet to figure out the mysteries of fall leaf coloration. Don't

> really think it is a maturity driven phenomenon though.

Cathy, there was an extended discussion about this on Alpine-L a
while ago.  I saved several of the posts.  I have copied and pasted a
few of them that might help sort out the mysteries, although, it
seems to me, after reading many posts on the subject, that there is
no one definitive answer...one of the posters ended his post with "Or
maybe Jack Frost, like most artists, is moody with bursts of
creativity!"...maybe that's the solution to the mystery:-)

"Colour is 'mostly' genetic. Why does the same tree vary from year to
in colour intensity?. The most vivid colors should appear after a
warm dry
summer with high levels of sunshine. Long periods of wet weather in
fall produces a rather drab coloration. Droughts favour anthocyanin
formation principally due to the indirect effects of soil water
upon the metabolism of the plants. At the 'other end', drought
also favor red pigment formation due to the reduction of nitrate
absorption. Or:

 High sunshine levels produce sugars. Sugars form in the
leaves...lack of
moisture reduces sugar flow from the leaves into the rest of the
These sugars produce red pigments which are concentrated in the
Sugars (and pigment) build up in the leaves to create higher
intensity of
colour.  Lots of sunshine with low (but not drought!) precipitation
lead to intense colours."

"There is definitely a strong genetic component in determining the
development of autumn colour.....Climate also definitely plays an
important part, early frosts being necessary to stimulate the rapid
breakdown of chlorophyll which allows the other pigments which have
been there (more or less) throughout to show through. It is important
that the chlorophyll is metabolised before the leaves fall so that
the store of magnesium which it contains can be transported out into
the wood where it is stored for use the following growing season."

"Autumn in the UK is a slightly more gradual process than in
continental Europe because of the moderating influence of the
maritime climate and the Gulf stream but is none the less beautiful.
Sycamores (from North America) are often particularly bright reds,
oranges and yellows, but not all of them, indicating that genetics is
certainly a very strong factor. Day length, light intensities and
temperatures are simply triggers to which the genetics of the plant

The insistence that there must be a "why" or reason for autumn leaf
colours is not very logical. The cyananthins and carotenoids which
give the colours are very difficult compounds for the tree's
metabolism to break down and would use up more energy and resources
than the tree would gain.  They are just left in the leaves when the
other nutrients and metabolites have been withdrawn. When that
process is complete, abscission occurs.

The bright colours are simply the result of the carbon ring and
double bond structures of cyananthins and carotenoids which give them
their properties. It has been suggested that those properties make
them difficult for insects to digest in the same way that the plant
cannot easily break them down, offering some protection. But they are
there throughout the life of the leaf and only become apparent when
everything else has been removed.

So, unless you are a religious fundamentalist or zealot, there does
not have to be a reason or a "why" for everything in nature. A more
difficult question to answer is 'why' humans are programmed to find
plant colours and forms so pleasing."

"To the best of my belief (and here let me refer non-believers to the
excellent and succinct article on autumn color in Wyman9s Gardening
Encyclopedia, which still fails to deal with the "why" of my initial
question) the relevant factors are climatic: red colors are maximally
brought out in plants having the capacity to produce them (for plants
indeed differ genetically in this capacity) where the weeks before
frost bring the regular alternation of sunny days reaching
above 70F (21C) (maximizing sugar production) with nights falling
below 45F
(5C) (minimizing movement of sugars from the leaves into the rest of

Marge Talt, zone 7 Maryland
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