Leaf colour in autumn

Jon Monroe monroejd at jmu.edu
Thu Oct 17 13:45:09 EST 1996

Many thanks to those who replied to my query about the evolutionary
significance of anthocyanin production in leaves.  This message is long but
it is clear to me that many folks are interested in this topic so I'll try
to explain what I now know about it.  The jury is still out (if even formed
yet) but there is an interesting new development described below.  As for
the TV interview, it was postponed until Saturday.  I'm looking forward to
it simply because this is one of the few opportunities we have as plant
biology educators to talk to the general public!

=46or those of you that do not live in regions of the world in which fall
leaf coloration happens (you need to have deciduous woody plants and
sub-freezing temperatures part of the year) I will briefly summarize.  Each
fall the leaves of many deciduous trees turn bright colors before falling
off.  In the eastern part of North America this event brings LOADS of
people out to look at them.  Here in the western end of Virginia we are
close to the Shenandoah National Park which gets a huge increase in
visitors this time of year.  Many of those visitors wonder why leaves turn
colors and look to us for info...

Prior to leaf abscision, chlorophyll is broken down to colorless molecules
which are transported back into the stem.  This energy-requiring process
makes sense since each chlorophyll molecule contains 4 atoms of nitrogen
and nitrogen is a valuable resource for the plant.  Carotenoids (yellow
pigments involved in photosynthesis and always present in green leaves)
contain no N and are not recycled, perhaps because it is not energetically
worthwhile.  The result is that the leaves of many trees and shrubs turn
bright yellow prior to falling off due to the unmasking of the carotenoids.

Leaves of some plants, however, not only loose the green pigments but also
synthesize red pigments called anthocyanins.  The intensity of red
coloration varies from year to year and from tree to tree.  Environmental
conditions such as temperature and moisture, and also the condition of the
plant, certainly influence the ability of the plant to make anthocyanins
and this may be a key to understanding their function (hint hint).

Anthocyanins belong to a family of phenolic compounds called flavonoids.
The red and purple anthocyanins that accumulate in many flowers and fruits
are there to attract animals for pollination or seed dispersal.
Anthocyanins also accumulate in young leaves and in leaves of many tropical
plants growing in extreme shade.  In both situations anthocyanins appear to
protect the photosynthetic apparatus from photoinhibition.  Plants growing
in extreme shade are particularly sensitive to photoinhibition when a
sunfleck hits them because they are enriched in chlorophyll b and
photosystem II, the main site of photoinhibition (see Gould et al. 1995,
Nature 378: 242-243 - thanks Kevin Gould for pointing your paper out to
me!).  Gould demonstrated that the anthocyanins serve a protective role by
absorbing some of the excess light energy that could excite chlorophyll b.

Young leaves exposed to full sun are also sensitive to photoinhibition
because (as I understand it) they not fully able to repair the damage until
they are mature.  None of this, however, is relevant to anthocyanin
accumulation in leaves that are about to fall off because the anthocyanins
accumulate after the photosynthetic lifetime of the leaf is over or nearly
over.  Because anthocyanin accumulation requires energy there must be a
reason for it.

Mary Barkworth (Intermountain Herbarium, Utah State University) and Thomas
Bj=96rkman (Dept. of Horticultural Sciences, Cornell University) both brough=
to my attention a recent idea on the role of red colors in senescing
leaves.  I don't have the original source so I hesitate to try to explain
it here.  If you know the source please let us know.  The story was
mentioned in "The Economist" on p. 76 of the Oct 5 1996 issue.  Bill
Hamilton (noted evolutionary biologist) and a graduate student, Sam Brown
proposed that trees with red leaves were signaling to some species of
aphids (that can distinguish colors) not to lay eggs on them because they
had the resources to mount a significant defense in the spring when the
eggs hatched.  As I said, I have not read the original paper and will not
try to explain further since essential details may have been omitted from
the news story that I read.  It sounds quite interesting to me and I look
forward to seeing the results of future experiments testing this

Again, thanks for your help.  Don't worry, I'm not going to try to explain
all of this to the local TV reporter...


  Jonathan Monroe                voice:  540-568-6649 (office)
  Department of Biology                  540-568-6045 (lab)
  James Madison University       fax:    540-568-3333
  Harrisonburg, VA 22807-0001    e-mail: monroejd at jmu.edu

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