Hi, my name is Aubrey and I want to know
why leaves change color in the fall?
My name is Ray Guries, I'm a retired
professor from UW-Madison. I worked for
almost 40 years in the department of
forest ecology. People do admire fall
foliage, the oranges and yellows and reds in September, October and other really peak
months. But a beginning point to
understand it is is the whole process of
photosynthesis. All plants, including
trees, have green leaves and the green
color that we see is simply the
reflected light that isn't absorbed as
part of the photosynthetic process, but
perennial plants like trees in northern
climates like Wisconsin are seasonal in
terms of their productivity. The
leaves do senesce in the fall as an
adaptation to dealing with winter, cold
climates, those leaves would freeze
in the winter, so the plant's
adaptation is to simply preserve the
food that they produce, transport it out
of the leaves at the end of the summer,
and lose those leaves. Part of
that senescence involves the breakdown
of chlorophyll, what we think of as green
leaves. Those green colors we see masks
the yellows and the oranges and the Reds
that are produced by carotenoids and
xanthophylls that are also plant
pigments. Chlorophyll isn't the only
plant pigment that occurs. Leaves are
filled with pigments as the chlorophyll
breaks down, it reveals the underlying
yellow and orange pigments, the
carotenoids and xanthophylls that are
also involved in photosynthesis but
also involved in protecting the plants
from the leaves from very high levels of
light. Light can be damaging to leaves in
the same way that sunlight
can be damaging to human skin. When those
chlorophyll molecules break down and
the end of summer has occurred it's
revealed as yellow pigments and orange
pigments that have been there all along
but are now revealed to us because of
the loss of chlorophyll. What sets the
trigger for all of this is day length
or more exactly the the length of nights.
When nights get too long from the
standpoint of the plant, it sets in
motion that set of processes that leads
to leaf senescence and the breakdown of
chlorophyll and and ultimately the
revelation of those other plant pigments.
