Leaf colour in autumn

David L. Robinson dlrobi02 at HOMER.LOUISVILLE.EDU
Wed Oct 9 20:08:34 EST 1996

There is yet another context in which we could look at the 
phenomenon of why and how deciduous plants lose their leaves 
in the fall......excretion.

I'm sure there are other references on it, but for starters see Journal of 
Biological Education (1986) 20:251 for a short essay by Brian 
Ford.  We usually don't associate the ability to excrete with 
plants, but maybe that is what these plants are doing in the fall....
getting rid of all of the wastes that accumulate in the plant during the 
growing season. Don't forget that evergreens also drop their leaves, its 
just done more slowly and continuously during the growing season.

Leaves don't just change color in the autumn because the chlorophyll 
disappears....there is active synthesis of yellow and red pigments, plus 
oxalate crystal formation and tannin accumulation. Oxalate and 
tannin levels increase in the leaves in the autumn just prior to leaf 
drop. Plus, the process of abscission is dynamic and highly regulated. 

The end result is that leaves drop to the ground and blow away to some 
other site far away from the source....leaving the soil beneath each tree
relatively clear of its own waste products....(or at least one might 
speculate so).

So next time you take a walk in a deciduous forest in the fall wear a hat 
and watch where you step. The trees are excreting!

Dave Robinson
Biology Department
Bellarmine College

On 8 Oct 1996, Gunnar Fridborg wrote:

> Bio-Edders!
> Carol Auer bring to the fore a topical question: leaf color. I havn=B4t a
> direct answer but I will contribute with some reflections and questions of
> my own.
> Please correct me if I=B4m wrong but I think modern textbooks on a basic
> level don=B4t pay very much attention to the senescence, colouring and fall
> of leaves; at least they don=B4t give a concise picture of the problem as a
> whole. And yet this in one of the most spectacular events in the northern
> hemisphere, giving rise to many questions from students, teatchers and
> "ordinary" people!
> The development of the abscission layer and its hormonal control is
> described in most textbooks.  The breakdown of leaf proteins and the
> withdrawal of  valuable nitrogenous  and other  compounds into the shoot is
> mostly mentioned, also the appearance of carotenoids when chlorophyll
> disappears.- However, the interested teacher and student should like to
> have some more details: changes in chloroplast structure, membrane function
> and solute composition;the remobilization of mineral nutrients before leaf
> drop etc.- Anyone can also see that some leaves become intensely red in
> fall, due to anthocyanin synthesis. Have this to do with an increase in the
> concentrations of soluble carbohydrates, which are parts of the anthocyanin
> molecules? I think so, especially when I have seen that girdling of trunks
> in  spring results in a very deep and early red colour of mountain ash in
> late summer =3D sugars can=B4t be transported to the root because of the
> girdling, so they remain and are used in the leaves and shoots. What do you
> know or think about this?  A common observation is also that a night frost
> is followed by an increase in red colours. Does this also depend on an
> increase in soluble sugars? Can you adopt a functional attitude towards the
> anthocyanin synthesis or is it merely a stress reaction? - The abscission
> in some trees (In Sweden oak and hornbeam) is interrupted by  the winter so
> the leaves remain on the trees, dry and brownish, and don=B4t fall until the
> leafing of next spring. However, this is only true for young oaks and for
> the juvenile parts of older ones. I have not found a physiological
> explanation. However I have a suggestion: Cytokinins are produced in roots
> and transported up to the leaves. They are known to delay senescence in
> leaves. When cytokinin production and transport decreases it=B4s logical to
> believe that senescence will start at the top of the tree and go downwards.
> Perhaps oak and hornbeam are not fully adapted to northern climate, so the
> winter  comes surprisingly and abscission is interrupted in the lower part
> of the tree?? - You can also ask yorself: have the remanining leaves any
> function? Shielding from strong light (heat)  especially the buds in the
> lower part af the tree which otherwise could burst to early in spring,
> close as they are to the goodies of the root??. I=B4m sorry my imagination i=
> s
> running away..
> Dr Gunnar Fridborg  Uppsala
> -------------------------------------------------------------------
> Gunnar Fridborg
> Associate Prof, Lecturer, Director      Docent, Univ lektor, Studierektor
> of Education
> Department of Physiological Botany      Inst for fysiologisk botanik
> Uppsala University                      Uppsala universitet
> Villav=E6gen 6  S 752 36 Uppsala Sweden   Villav=E6gen 6  752 36 Uppsala
> Phone +46 18 182820                     Tel 018 182820
> =46ax   +46 18 559885                     Fax 018 559885
> E-mail  Gunnar.Fridborg at fysbot.uu.se
> --------------------------------------------------------------------

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