BIO 201: Botany

WHY LEAVES CHANGE COLOR

During summer almost all leaves are green

Where do the colors come from?

Leaves are loaded with a number of pigments all through the summer!

• Green of chlorophylls
• Orange of carotenoids
• Yellow of xanthophylls
• Red of anthocyanins

Why are all these pigments not visible throughout the summer?
During spring & summer chlorophyll is constantly being produced
The chlorophyll becomes the most abundant pigment in the leaves
The other pigments are present, they are just covered up by the chlorophyll

PHYSIOLOGY BEHIND COLOR CHANGE

As autumn approaches, we notice shorter days and cooler weather

Shorter days lead to in less daylight and less light intensity, resulting  in:

• The slowing of quick growth
• Reduction of chlorophyll production
• Leaf senescence (slower water and nutrient transport to and from leaf)

Cooler nights signal a further change

• Less chlorophyll production (cooler = slower metabolism)
• Slower transport of sugar from leaf to stem
• Commonly the extra sugar is used to produce anthocyanins

Anthocyanins are produced for several reasons

• Absorb harmful light rays (inc UV)
• Act as an antifreeze
• They are not a photosynthetic pigment

Fall colors are greatly enhanced by:

• Warm sunny days and cool evenings (enhances production of anthocyanin)
• The lack of a hard freeze
• Big debate over the effects of drought!
  (the best idea here is that during a drought it is more sunny, hence more production of anthocyanin to protect leaves)

Variation on a leaf

• Exposed leaf surfaces warm, enhance anthocyanin production during daylight
• Shaded leaf surfaces stay green longer
• Top or west side of tree may changecolor first
• Genetics (red maples may be red, orange or yellow)