Leaf colour in autumn

Gunnar Fridborg Gunnar.Fridborg at fysbot.uu.se
Tue Oct 8 07:40:28 EST 1996


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



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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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