Initiation of Fall colors

Bob Wise wise at
Thu Sep 28 10:00:18 EST 2000

But this begs the question of why would a plant merely replace one pigment
(chlorophyll) with another (carotenoid or anthocyanin)?

Carotenoids and anthocyanins are completely C, H, and O, which are "cheap"
in terms of acquisition because the plant can make carbon compounds out of
the air, given a little water and light.  However, chlorophyll contains N
and Mg, which are "expensive" because the plant has to take them up from
the soil, metabolize them, transport them, etc.  Warm fall days promote the
photosynthesis that is needed to fuel the senescence of the photosynthetic
apparatus itself (gotta get all the protein and minerals out of that leaf
before it is shed).  But cool fall nights inhibit phloem transport so the
leaf gets constipated with carbon, which, through various feedback loops,
is inhibitory to photosynthesis.  The leaf then shuttles the excess carbon
into carotenoids and/or anthocyanins, and gets on with the business of
using the energy derived from photosynthesis to dismantle the nutrient rich
photosynthetic apparatus, including chlorophyll itself.  In the end, the
tree sheds a leaf that is high in carbon, hydrocarbons, and carbohydrates
and low in everything else.

I know this diverted from the original question of timing, but I thought I
would add it to the conversation.


>A combination of changes in hours of light/dark and temperature triggers
>changes, both the changing of colour and loss of leaves going into winter
>(short, bright, cool days) and the resumption of leaf growth in spring.  Each
>year is different in these climatic features so the timing of colour change
>will vary.
>As for the intensity of colour: leaves change from green to orange/red as
>chlorophyll is lost and carotenoid (yellow/orange) and anthocyanin
>(red) pigments are produced or rather they exist in a higher ratio to the
>Chlorophyll and thus the leaves change colour.  As the chlorophyll decreases,
>daylight will promote the synthesis of the other pigments so as to maintain
>some photosynthesis. Variation in colour is merely a reflection of
>variation in
>the pigment concentration.
>hope this helps
>In article <B5EA9FF1.BE%jclausz at>, jclausz at says...
>>I've been asked about the initiation of fall colors, why it comes earlier
>>one year or another, and why the intensity of color varies from year to
>>year. My recollection is that color is initiated by changes in daylength.
>>But what about advances or delays in color and intensity? Thanks for your
>>John Clausz

Robert R. Wise, Ph.D.
Associate Professor of Plant Physiology
Department of Biology and Microbiology
University of Wisconsin Oshkosh
Oshkosh, WI 54901
tele:  (920) 424-3404
fax:  (920) 424-1101
wise at


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