Autumn leaves red for a reason

· Oct 8, 2001 Tweet

Sunscreen is a vital tool used by humans to stop the sun’s harsh ultraviolet rays from damaging skin tissue. However, a recent study by a team of UW-Madison researchers found that humans are not the only ones who are taking advantage of sunscreen.

Tree leaves have joined the crew.

“The red pigment in trees’ leaves is produced to act as a natural sunscreen,” said Bill Hoch, a UW graduate student in the College of Agriculture and Life Sciences.
Hoch, along with co-researchers Eric Zeldin and Brent McCown, said their findings were derived from a string of previous experiments within their department.

“We arrived at the theory after culminating a lot of previous information and studies on leaf pigmentation,” Hoch said. “Most of the previous theories we looked at focused mostly on the biochemistry aspect of leaves being red. We researched the biological perspective to figure out exactly why the red pigment was being produced in leaves.”

In doing this, the researchers discovered the red pigments in leaves, called anthocyanins, are carefully developed to aid the leaves’ natural processes.

“Each fall, leaves break down their photosynthetic tissues while trees reabsorb the nutrients from [the leaves],” Hoch said. “When this breakdown occurs, the leaves are then more likely to [succumb to] environmental stress, including frost and high light exposure.”

When stressed, the leaves manufacture their own sunscreen.

“The leaf produces the anthocyanins near the leaf surface to shade the sensitive photosynthetic tissue and acts as a sunscreen and protectant from the harsh environmental conditions,” Hoch said. “This allows the internal machinery of the leaf to continue to work and use a lot of energy without being harmed.”

Some other stressors that can lead to increased anthocyanin production are drought or low nutrition levels.

The theory also contests that the best weather for vibrant autumn colors in trees like oaks and maple is when conditions are very dry and very sunny.

The theory also explains why inner leaves tend to not be as colorful as leaves on the outer parts of the tree. They simply are not exposed to as much sunlight, and therefore do not need as much protection from environmental conditions. This means less anthocyanin production.

Hoch and his fellow researchers recently contributed to an article detailing their new theory that was featured on the cover of the scientific journal Tree Physiology.

Now, the team’s biggest problem is the approaching winter.

“We are throwing around a couple of new ideas for future researching,” Hoch said. “However, our field research will be stalled until next fall when the leaves begin to change color again.”


This article was published Oct 8, 2001 at 12:00 am and last updated Oct 8, 2001 at 12:00 am


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