Professor Herb Underwood calls himself a “clock-biologist.” Unlike many of his colleagues in the biology department, Underwood is looking at the indicators to predict the future of biological seasonal patterns. But he doesn’t have to look too far for some of these indicators—he just has to look down at the autumn ground.

The seasonal transition of plants during the fall indicates many other changes going on in the natural world and are directly related to changes in the daily routine of plants.

“I’m interested in daily rhythms and cycles,” Underwood said. He focuses on circadian rhythms, stemming from Latin, meaning around the rhythms around the day.

According to Underwood, Erwin Bünning , a German botanist, discovered the relation between daily rhythms in organisms and seasonal patterns.

“[Organisms] are using their biological clock to measure day length,” Underwood said. “So a daily rhythm is actually partly involved in initiating these yearly cycles.”

An example of one of these yearly cycles due to length-of-day change is the process of leaves changing colors in the fall. Additionally, the falling of the leaves is a photoperiodic cycle. This cycle deals with light periods. According to Underwood, day lengths are longer in the summer and shorter in the winter, and plants can sense this daily change in light intensity and therefore react to it.

“Basically, there’s a rhythm of light sensitivity that occurs every day, and if the light is long enough it hits the sensitive portion of the leaf that triggers the photoperiodic response,” Underwood said.

In this case, that response would be the color change brought on by shorter days.

Other factors that accompany winter, including the cold and lack of water, would seem to be factors that trigger reactionary responses in plants, but Underwood said day length was the most precise trigger.

“Day length change is the same year after year after year; it doesn’t vary like temperature,” Underwood said. “So it’s very reliable, it’s consistent; it’s called noise-free. It doesn’t go up and down depending on the weather conditions.”

The timing of this color change has been built into the plant through evolution. The plants that survive are the ones that can anticipate weather changes and prepare by losing their leaves, and so natural selection weeds out the ones who do it poorly.

“You can imagine there are very strong selection pressures on organisms to be able to anticipate these conditions that they prepare for because it’s crucial”, Underwood said, “I mean if you don’t flower at the right time of year, if you don’t breed at the right time of year, you’re in trouble and your species is in trouble.”

Circadian rhythms explain why and when plants begin to prepare for winter. But they have no inner control of their color change.

This, however, is Chad Jordan’s forte. Jordan, is the undergraduate programs coordinator and a professor of plant biology. His interest focuses on the processes going on inside the plants that produce their brilliant and impressive colorings around this time of year.   

Normally leaves appear green due to the presence of chlorophyll, a pigment present in organelles in plant leaf cells that photosynthesize to produce sugar for energy and growth. But with less light in the winter, chlorophyll is less useful to the plant. Cold weather poses danger to plant leaves because the water inside them could freeze and damage the organisms’ tissues, according to Jordan. Therefore, the plant begins to go through processes to drop its leaves.

“It’s sort of like an insurance or protection against damage that would be brought about by freezing,” Jordan said.

The plant is cutting its losses and manages to hold on to some of the energy it put into creating the chlorophyll. The next year, the plant will break the pigment down to recycle and reabsorb it, according to Jordan.

“The chlorophyll are broken down into any number of different compounds, mostly in the form of sugar…,” Jordan said. “It can be stored so that during the next growth season there is at least a little bit of energy reserved in addition to what is already there to help initial growth.”

This re-absorption of sugar is a process called glueconeogenosis.

While leaves are gradually withering away, plants are reabsorbing their chlorophyll for storage, prompting the drastic color change.

“Now what that does is to unmask certain types of pigments that were always there during the whole growth season,” Jordan said.

As chlorophyll becomes less concentrated, we begin to see the colors of the leaves. Examples of these residual colors come from carotenoids, a bright orange pigment, and xanthophylls, which are a pale yellow.

There are also some pigments that are produced by the plant. These are called anthocynins, and are created from sugars.

“Some of the sugars that are in the leaf remain in the leaf,” Jordan said, though the plant is attempting to remove them. “And part of that is because when it’s cooler at night they tend to stay where they are more readily, and as a result of that what happens is this conversion of some of the sugars inside over to those anthocynin compounds.”

These anthocynin compounds cause the leaf to be colored bright red or purple.

This means that certain factors, temperature being the most pertinent, can be optimized to create more brilliant leaf colorings.

“So the intensity of color change, really in some cases, will be dictated between those temperature differences between day and night,” Jordan said. “So warm days and cool nights tend to make the color change more vibrant.”

He said nights of about 40 to 45 degrees would be the optimum for leaf color change, but that anything below freezing would kill the leaves.

North Carolina is lucky enough to often have almost perfect conditions for leaf coloring in the fall. According to Jordan, this climate is unique.

“There are only certain parts of the world in which color change is vibrant like it is here,” Jordan said.

According to Jordan, the North Carolinian Southern Appalachians leaf displays are considered as excellent as those in the North East and the Rockies, as they have some of the most vibrant leaves in the country, something everyone will be pleased to take advantage of this season.