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Colorful fall mystery solved by Wisconsin scientist

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Catherine W. Idzerda
November 1, 2008
— We’ve been to the moon, found a vaccine for polio and invented a flame-resistant material for children’s pajamas.

But it wasn’t until recently that scientists understood why leaves turn color.


So while you’re raking up the leaves for Janesville’s annual collection, consider this: It was Beaver Dam native William Hoch who finally solved the mystery of trees’ fall transformation.


Hoch worked on the problem during his graduate school years at UW-Madison, and his work was published in 2003.


Before Hoch’s work, scientists had a variety of theories about why leaves change colors.


“When I was doing my research, I became interested in the history and looked through a lot of the old journals in the university’s libraries,” said Hoch in an interview from Montana State University in Bozeman, where he is assistant professor of plant sciences and plant pathology.


A work in one of the journals from the 1940s speculated that the phenomenon was “botanical art for art’s sake,” Hoch said.


Another, more common theory, was that the sugars produced by photosynthesis get “trapped” in the leaves. It’s a theory sometimes still found in children’s textbooks or botanical handouts.


Here’s how it really works:


Chlorophyll is the pigment that gives plants their green color and is used by plants to capture the sun’s energy through photosynthesis.


Shorter days and dropping temperatures tell trees that it’s time to disassemble and gather the nutrients from leaves and transfer them to roots and branches for future use.


It’s like canning tomatoes for future use, or, more crudely, getting everything you possibly can from the buffet before it closes for the night.


“It’s trying to get out as many of those nutrients as possible,” Hoch explained. “It would like to drop nothing but a carbon skeleton, but it can’t get a hold of f everything.


The leaves even take apart their own DNA to get at the phosphorus, which is one of the major “marco nutrients” plants need. Phosphorus is the P in the NPK that gardeners see on every fertilizer bag.


Carotenoids, which are present in the leaves all summer, are responsible for the bright yellow colors in the fall. During the summer, carotenoids work as antioxidants, protecting leaves from the damaging byproducts of photosynthesis.


Beta-carotene, the antioxidant found in carrots, is also a carotenoid.


When chlorophyll levels drop in autumn, the yellow carotenoids show through.


Another substance, anthocyanins, is responsible for the stunning red and orange shades. Anthocyanins are made when the trees break down nutrients in the leaves but are not found in all species.


As the chlorophyll breaks down, the anthocyanins work as a sun block, protecting the leaves from sun damage.


Trees that don’t turn red in autumn are more resistant to sun damage.


Click here [PDF] to view the City of Janesville leaf collection map.

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