Chemistry of Changing Leaves

The Chemistry of Changing Leaves

BETH NISSEN, reporting:

Ask kids to draw trees in the summer, and almost all will use the same color crayon: green. Ask them to draw trees in the autumn, and most will fill the page with blazing yellow; burning orange; fiery red.

Dr. NATE LEWIS (California Institute of Technology): One of the best places to see leaves changing in the fall foliage is in New England, when you have this brilliant rainbow of colors in October. That’s what our image of seasons is all about.

NISSEN: Why do leaves in northern areas change out of their summer green and into autumn colors? Like bright clothes covered by a green coat, those golden yellows and oranges have been there all along.

Dr. NANCY GOROFF (Stony Brook University): We just see it as green, and we can’t see the orange color in the leaves because it’s masked by the green.

NISSEN: Chlorophyll, as you probably know, is the green pigment molecule in plants. In fact, the name comes from the Greek word khloros, meaning “green” and the Greek word phyllon, meaning “leaf.”

In summer, almost all leaves are green; or rather, we see them as green. Those chlorophyll pigments absorb all the colors in the visible ‘rainbow’ light spectrum except green, which is reflected back into our eyes.

In the warmth and light of long summer days, plants synthesize chlorophyll continuously. They have to: Chlorophyll is not a stable compound. It breaks down, or decomposes, at a steady rate in bright sunlight and must be constantly replenished.

So what happens when the seasons change?

As you know, the earth orbits the sun. Because of the tilt in the earth’s axis of rotation (that’s the imaginary line running between the north and south poles), the top half of the earth, the northern hemisphere, is angled toward the sun in summer. In winter it’s angled away from the sun; it gets less direct sunlight, which is why the days get shorter and temperatures cool.

Less light means less sun energy for photosynthesis. Plant food production shuts down. Chlorophyll is no longer replenished. It decays and “drains away.”

GOROFF: And so the leaves will look orange or red because there is no longer any chlorophyll absorbing that part of the spectrum.

NISSEN: So, which pigments are now showing their colors? Carotenoids, like carotene and xanthophyll, and flavonoids, like quercetin and anthocyanin. By themselves, or in combination, these pigments contribute particular autumn colors.

Yellow leaves? That’s usually because of carotenoids, the same pigments in, yes, carrots and pumpkins and sunflowers.

Orange leaves? They contain good amounts of both carotenoids and anthocyanin, a pigment also found in apples and grapes.

And the bright red leaves? Those are mostly anthocyanin pigments.

Actually, most anthocyanins are produced in the fall. In some leaves, like red maples, anthocyanins are produced with plant sugars that are trapped in the leaves. Sunny days promote sugar production in the leaves, and the cool, crisp temperatures prevent the sugar from flowing out of the leaf. The sugar builds up, and at high concentrations, reacts with the leaf sap to produce anthocyanin.

Although genetics determine which pigments a plant makes, the autumn color display will change year to year because of other factors.

GOROFF: Plants are very sensitive to their conditions, and they’ll produce different amount of pigment under different conditions. So the amount of sunlight that they are getting, the temperature, all of these can affect the amount of pigment that’s produced.

These changing leaf colors are brought to you by the changing seasons…and chemistry!

• CHEMISTRY NOW: Chemistry of Changing Leaves (Grades 5-8)
• CHEMISTRY NOW: Chemistry of Changing Leaves (Grades 9-12)

K-5 Activity:

Use a leaf template and cut out 24 green leaves (approx. 4”). Use the same template and cut out 8 orange, 8 red, and 8 yellow leaves. Staple a green leaf on top of the orange leaf and a green leaf over the red leaf and a green leaf over the yellow leaves. Randomly hand out the leaves to all students. Everyone has a green leaf because the season is summer. Have the students remove the green leaf to reveal the color underneath the green leaf. Tell students that the color of the fall leaf is dependent upon the next most abundant pigment in their leaf. Explain that the color (orange, yellow, or red,) is always there, we just can’t see it until the days become shorter and colder and the chlorophyll stops being made.

Big Questions: Why do leaves change colors?

It wouldn't feel like fall without the sight, smell and oh-so-familiar crunch of fallen leaves. But why do some trees change their colors — and then give up their leaves — as autumn sets in?

Read more on Newsela

Leaf, Leaves, Autumn, Color, Chlorophyll, Carotenoid, Flavonoid, Anthocyanin, Xanthophyll, Quercetin, Pigment, Yellow, Orange, Red, Green, Molecule, Summer, Light, Sun, Sunlight, Visible Light, Light Spectrum, Photosynthesis, Tree, Plant, Red Maple, Plant Genetics, Planet, Axis of Rotation, Compound, Chemistry Now

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