What Makes Leaves Change Color?
 The
bright yellow, orange and red colors of autumn leave are one of
nature's most beautiful spectacles. However, until recently the
explanation for this familiar annual event has been based primaril
on random observation and speculation.
Recent research has
answered many questions about his phenomenon and members of the
University of Wisconsin - Extension horticulture team recently
published the answers as a UW-Extension Garden Facts Sheet.
During summer, leaves
hold most of a plant's nutrients. These nutrients, especially
nitrogen and phosphorus, are components of proteins, pigments
and other compounds the plant needs for using the sun's energy
to make sugars from water and carbon dioxide - a process called
photosynthesis. Before the leaves fall, plants need to store those
nutrients in stems and roots so they can be reused in the next
season.
Chlorophyll is the pigment
that gives plants their green color. Plants use chlorophyll to
capture the sun's energy. Throughout autumn, plants break down
chlorophyll and the many other photosynthetic components. As levels
of chlorophyll decline, the brightly colored pigments we associate
with autumn leaves become visible. Two types of pigments, carotenoids
and anthocyanins, give leaves their bright autumn colors.
Carotenoids protect
leaves from a variety of harmful compounds that are normal by-products
of photosynthesis. Without carotenoids, leaf photosynthetic components
literally destroy themselves. Interesingly, a recently developed
corn herbicide actually prevents carotenoid production, which
results in the plant self destructing from these harmful by-products.
Anthocyanins are responsible
for many of the reds, oranges, pinks, purples and blues in flowers
and fruits, but they also produce the brilliant red and orange
autumn foliage. Plants manufacture anthocyanins midway through
autumn when leaves still have a significant amount of chlorophyll.
As this occurs, the presence of both the red pigments and chlorophyll
results in a dark burgundy color. The brightly colored anthocyanins
become increasingly visible as chlorophyll levels continue to
decline. Red leaves contain similar amounts of carotenoids as
in yellow leaves, but in red leaves, the yellow carotenoids are
masked by the intensely colored anthocyanins.
Plants produce red pigments
during autumn to shade leaf photosynthetic systems from bright
sunlight. Protection from bright light during autumn is important
because damage to the photosynthetic components during this time
will reduce a plant's capacity to recover nutrietns from leaves.
The shading function of the red pigments explains why leaves exposed
to direct sunlight are the brightest red, while leaves shaded
within the canopy of a plant often show less intense color.
Many factors affect
the quality and duration of autumn leaf colors, but weather is
the most important. Cool, sunny weather results in the brightest
foliage colors because plants use the red pigments to shade leaves
from bright light. While cool weather is best for the development
of brightly colored foliage, hard freezes may irreparably damage
leaves before the brightest colors have emerged. Very warm autumn
temperatures accelerate the processes within leaves and shorten
the length of time that the colorful leaves remain on plants.
Warm temperatures will also reduce the amount of red pigments
produced in leaves.
Stresses such as drought,
nutrient deficiency and even diseases can increase the production
of red pigments and may improve the color display of autumn leaves.
Drought is a common stress in many environments. Unless the water
deficiency is so severe that leaves are killed before autumn -
a very rare occurrence - drought conditionse will generally improve
the quality of fall leaf colors.
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