Autumn. It’s the season where we break out our sweaters and
flannels, drink hot cocoa and apple cider, carve pumpkins, go apple picking, and
enjoy the chillier weather. But, in my opinion, the best part about autumn is
the changing colors of the leaves. They change from their normal green color to
varying shades of yellow, orange, red, and brown. Fall foliage is beautiful and
breathtaking. But how and why do the leaves change their colors during this
season?
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First, we need to understand what makes leaves green in the
first place and why this coloring disappears. Leaves contain structures in
their cells called chloroplasts, which is where photosynthesis takes place.
Photosynthesis is the process where light energy from the sun is captured and
is used to convert carbon dioxide and water into oxygen and sugars. The light
from the sun is absorbed by chlorophyll found in the chloroplasts. Chlorophyll
is also responsible for the green in leaves because it acts a dominant pigment,
overshadowing the others. As the hours of sunlight decrease in autumn, the chlorophyll
present struggle to keep the photosynthesis process going and the trees stop
producing new chlorophyll. Eventually there are no chlorophyll left and the green
color of leaves goes away.
Photosynthesis Chemical Reaction
This allows the other pigments to show themselves. Carotene
and xanthophyll are responsible for the yellow and orange colors in leaves. The
vivid and scarlet red colorings come from anthocyanins. These are produced in the
fall from built up sugars in the leaf. They can combine with the leftover green
pigments of chlorophyll to create varying shades of red.
Beta-carotene
Xanthophyll
Anthocyanin
According
to the United States National Arboretum, the quality of the leaf colors seen in
autumn depend on the sunlight, weather, and soil moisture. To get the brightest
colors, a sufficient amount of moisture is needed in the growing season,
followed by a dry, cool, and sunny autumn that has warm days and cool nights,
but no frost. Wind and heavy rain can cause leaves to fall before they fully
show all their colors, while a lack of rain and wind will cause the autumnal
color display to flourish.
Tannin
Leaves eventually turn brown from the tannin pigment as the
sunlight available continues to decrease, the temperature drops, and the frost
kills the leaves. In spring the process begins anew!
Sources:
Good title and graphics. Interesting topic and good outline of the chemistry and good use of the chemical structures. It would be helpful to include links to the original general interest sources in the text.
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