Hello Asteemed researchers...

My 7 year old daughter and I observed her sunflower plants today and
noticed how they turn toward the sun, and we rotated the planter and
after a short period of time the plants again faced the sun.

I found information about how cells elongate in seedlings, but these
are mature sunflower plants.

So my question is "what mechanism makes mature plants turn toward a
light source?"

cimmerianike,

The act of plants turning towards a light source is known as
phototropism. This trait is thought to evolved in plants over
thousands of years, as growing towards a light source, the provider of
energy though photosynethsis, improved the survival rate of the
plants.

For a detailed explanation of phototropism and other growth traits of
plants such as geotropism, please see the Botany Online site at
http://www.biologie.uni-hamburg.de/b-online/e32/32b.htm

For a controlled experiment you can perform to illustrate the
properties of phototropism, please see the Canadian Space Agency site
on Plants and Light at
http://www.space.gc.ca/kidspace/1-edu_res/resources/all/mars/grade3/light/default.asp

Goggle search terms used:
  plants grow towards light source
  what is phototropism?

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Clarification of Answer by hailstorm-ga on 04 Nov 2002 17:56 PST

Many thanks to fellow researcher pinkfreud, who pointed me out to a
more detailed site which includes the following information:

"......why do sunflowers always appear to be reaching toward the sun?
The technical answer is that the flower's orientation to the sun in
due to differential growth of the stem. A plant-growth regulator
builds up on the shaded side of the plant when unequal light
conditions exist. Because of this, the darker side of the plant grows
faster than the sunlit side, causing the stem to bend towards the
sun."

For more, please visit http://www.barbcollins.com/sunflowers.html

Heliotropism (also called "solar tracking") is the mechanism that
makes a plant follow a light source throughout the day.  This is
possible by the action of motor cells increasing or decreasing water
pressure (turgor) within the flower's "stem".  Therefore, increasing
turgor on one side of the stem will cause the flower to face opposite
the side of increased turgor, and vice versa.  As stated above by
hailstorm, following the sun increases the amount of light received by
the leaves/flowers during the day, thus improving the odds for
successful reproduction.

Sunflowers typically only display heliotropism while developing, and
cease when the flower reaches maturity.
( http://www.livingfoods.com/articles/rawseeds.html )

Tropism refers to the involuntary movements of an organism either to
or away from external stimuli.  The term is usually applied to growth
and turgor movements in plants.  Movements caused by turgor are
generally faster and less permanent.  There are several tropisms. 
Thigmotropism refers to a plant's response to physical contact (such
as the sensitive plant), while phototropism refers to a plant's growth
toward a light source.

For more information on "tropisms":  
http://www.slider.com/enc/53000/tropism.htm
http://www.wiu.edu/users/mfer1/teaching/bio102/ch37/ch37.htm  (half
way down the page)

Search terms used:
sunflower heliotropic 
heliotropism  
tropism

Just to add my .02 worth and a little more detail...

The actual mechanism that moves the plant's leaves or flower head is
changes in turgor (think water pressure) in an organ called the
pulvinus located at the base of the leaf cluster or flower head.
Pressure is increased on one side and decreased on the other causing
the leaf or flower to bend or sag towards the sunlight.

You can see this in action with animated GIF's on the web page of Dr.
Irwin Forseth, a professor of biology at the University of Maryland...

http://www.wam.umd.edu/~irv/leafmovements.htm

Not all plants use this to turn towards the sun, called
diaheliotropism, some plants, especially in desert environments, use
it to turn away from the sun to reduce leaf temperature and water
loss. This turning away from the sun is called paraheliotropism.

wayga-ga

The response of a plant to light is called phototropism.  When a plant
grows toward the light, this is called (+) phototropism.  When the
plant grows away from the light, this is called (-) phototropism.

When does this happen?

On the unexposed part of the plant, auxin, is indole-3-acetic acid
stimulate cellular mitosis.  This increase in cell number results in
an increase in cell mass, thus causing the plant leaf to drop down or
appear to grow toward the light.  Auxin activity is reduced on the
illuminated part of the plant. Thus a differential rate of mitosis
occurs on the plant between the exposed vs the nonexposed parts of the
plant leaf.