Why do only some leaves turn red in the fall? Scientists can't agree on an answer

"This is the time of year when Nicole Hughes gets constantly distracted by the fall foliage as she drives around North Carolina. "I'm always looking," says Hughes, a biologist at High Point University. "You know, just seeing what's turned colors when, and what hasn't turned." Part of her brain is endlessly searching for clues about one of the biggest mysteries of autumn leaves: why the leaves of only some trees turn red."

"She's made a career out of studying this, even though her interest in red leaves goes back even further. Growing up, she says, "my dad was always talking about his quest for the perfect red leaf." Every autumn, trees break down the green chlorophyll in their leaves, so that they can recover precious nitrogen before the leaves fall. (That nitrogen is necessary for photosynthesis, as well as building proteins and DNA.)"

"The loss of this chlorophyll exposes yellow pigments that have been there in the leaves all along. That makes yellow leaves easy to explain. But the red color is different. It comes from brand-new chemicals that are generated just days before a leaf plunges to the ground. "The red was not there beforehand, or you would have seen it, because the leaf would have been purple. Because green plus red equals purple," says Hughes."

"Low levels of these same new pigments are also responsible for leaves that look orange, she explains: "Under a microscope, you see lots of red freckles." So why are trees bothering to create costly red pigments in leaves that are almost done for? Hughes says scientists don't have a lot of answers. Protecting leaves from light One possible explanation says that the red pigments basically act as a kind of sunscreen"

Some trees produce red pigments in their leaves just days before leaf drop. During autumn, leaves break down green chlorophyll to recover nitrogen needed for photosynthesis and to build proteins and DNA. Chlorophyll loss unmasks existing yellow pigments, explaining yellow leaves. Red pigments (anthocyanins) are newly synthesized and can appear as orange at low concentrations. The adaptive function of red pigment production is uncertain. One leading idea is that red pigments act as a sunscreen, protecting leaves from excess light while chlorophyll activity declines, a concept known as the photoprotection hypothesis.

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