Ask A Scientist© Biology Archive

Author:      macmillan   
Text:        Why do leaves change color in the fall?  Is the cold a 
contributing factor?  What determines the ultimate color of a leaf?  Is the 
change due to an absence of chlorophyll or the presence of something else?
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Response #:  1 of 1
Author:      Sam Bowen
Text:        This will be a partial answer.  The colors are due to chemicals 
called carotenes, the same chemicals that give rise to color of carrots.  
There are several and they have different colors.  They are present in the 
leaves all of the time.  We see them in the fall because the chlorophyll 
production in the leaves stops.  I think it is due to the cooling, not 
directly the presence of freezing temperatures, it seems to depend on the 
plant.  It would be a good experiment to see what events contribute to the 
changes.  Clearly the amount of water in the plant contributes to the quality 
of the color.
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Up-date:  January, 2001

The reason that leaves are shed in the fall is to help the plant preserve water 
when the ground is frozen and water is not readily available.  Stored water is 
primarily used in the winter months.  The loss of leaves and consequently the 
leave's color change is due to plant's ethylene gas and auxin balance changes.  
Ethylene causes aging in plants and auxin levels stimulate growth.  Their 
levels apparently determine the where the plant grows, where the plant just 
maintains itself and helps explain dying of selective parts of the plant.  
This varies widely between species.  Consequently, with most deciduous leaves, 
when auxin levels decrease and ethylene increases, enzymes begin to digest the 
cellulose of the abscission layer (the point the leave stem separates from the 
plant.)  This change of balance is related to reduced sunlight periods 
(photo periods), but apparently, other facts such as moisture available to the 
roots and amount of tree water storage, drying winds, and yes, cold stress can 
be a factor, among other factors as well.

This change of auxin and ethylene balance apparent triggers the reabsorption of 
nutrients from the leaf that will be used in the spring to grow new leaves.  
This process also stops the production of the dark green chlorophyll to replace 
the chlorophyll that deteriorates naturally in sunlight.  Many people do not 
know that the many plants actually produce some of the colors that show after 
the green chlorophyll deteriorates at the end of the growing season.  This is 
initiated during the time of the change of balance between auxins and ethylene 
production just prior to leave loss.  However, many of the colors were there all 
along.

I have simplified this explanation somewhat (maybe too much), and not all of 
this is completely understood at this writing according to my sources.

Steve  Sample
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