Monday, November 30, 2015

The Beauty of Autumn – and the Science Behind It

By Ashley Wong (T'16)

Culberson Asiatic Arboretum

Anyone who has taken a stroll through Duke Gardens within the past few weeks can resoundingly attest that fall is in full swing.

Now that the foliage has taken an autumnal turn and the air holds a pre-winter crispness, you may have wondered about the processes governing nature’s lovely color transformation that we witness at this time every year. It’s a phenomenon poignant enough to have inspired countless works of literature by writers and philosophers -- so poignant that renowned autumn-enthusiast Henry David Thoreau notably wrote an entire journal chronicling his observations of autumnal beauty as an homage to the season.

The Spring Woodland Garden

Unsurprisingly, the transformation of green leaves into sundry shades of red, orange, yellow, and brown has evolved into a particularly lucrative tourist attraction over the years. Autumn-inspired tourism, an activity so prominent that it has earned the name “leaf peeping tourism,” brings in approximately $1 billion dollars in revenue to North Carolina each year, a sizable chunk of its more than $20 billion dollar annual tourism income. Luckily for us, North Carolina seems to be a prime location for admiring fall foliage.

Blomquist Garden. Photo by Micaela Unda.
So what are the biochemical processes that lead to this revenue-boosting spectacle? Stefan Bloodworth, curator of the Blomquist Garden, recently presented a lecture on the biochemistry of fall color for Duke Gardens volunteers. He says that the onset of fall color is essentially a signal that photosynthesis has ended for the year.

As the number of daylight hours dwindles, the energy cost of photosynthesizing begins to climb, since the inputs of sunlight and warmth that are necessary for photosynthesis are no longer steadily coming in. Thus, plants are triggered to stop producing chlorophyll, which absorbs red and blue light and reflects the leafy green for us to see. In addition, the plant grows a corky membrane between the leaf and the tree, essentially cutting off support of the leaf. At this point, we begin to see pigments that have been there throughout the growing season but were masked by chlorophyll the rest of the year.

One class of pigments, carotenoids, helps enhance the annual photosynthetic process by allowing the leaf to absorb light from the blue spectrum. Carotenoids have an additional part to play as well: acting in a photoprotective role throughout the growing season. Carotenoids protect leaves by cleaning up free oxygen radicals within the leaf. Also, as the level of sunlight declines, excess energy can build up and harm the leaf; carotenoids help in dissipating that energy as heat. This class of pigments is predominantly responsible for the vibrant orange and yellow hues associated with autumn foliage. 

But what about the purple and red colors? Anthocyanins, another class of pigments, are responsible for those hues, and they also emerge after photosynthesis ends and chlorophyll is no longer present. These pigments absorb blue-green light wavelengths, allowing the red wavelengths to be visible to us as red. Anthocyanins are also integral to photoprotection, acting as light screens to modulate light absorption and minimize the damage from too much sunlight. We have these molecules to thank for our visual and emotional pleasure every autumn. 

So even as the end of fall semester approaches and work inevitably begins to pile up (mercilessly so), take a minute to step outside and absorb the brilliant autumn coloration surrounds you – believe me, it’ll be worth it.
Japanese maple in the Historic Gardens. Photo by Lori Sullivan.
Blogger Ashley Wong is a Duke senior majoring in environmental studies, with minors in biology and visual & media studies. She is also a Duke Gardens work-study marketing assistant. All photos by Ashley Wong except where noted.

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