speech script

01. Why do some

Why do some leaves turn yellow or orange in the fall while others turn red or purple, even though they were all green during the summer?

02. Even if you

Even if you don't know much about plants, you see them respond to the seasons every year.

03. Fall colors give

Fall colors give us a chance to understand what's happening inside leaves as trees prepare for winter.

04. As an agricultural

As an agricultural student, I study how plants grow and respond to their environment, but this was a topic I hadn't fully understood.

05. To learn more,

To learn more, I based this speech on research from the National Weather Service, NOAA, the Smithsonian, and an article published by the American Society of Plant Biologists.

06. In the fall,

In the fall, chlorophyll breaks down, making the yellow and orange carotenoids already in the leaf more visible.

07. Some leaves also

Some leaves also make anthocyanins, which produce red and purple colors.

08. There are three

There are three parts to understanding this.

09. First, how shorter

First, how shorter days lead to the loss of chlorophyll.

10. Second, how other

Second, how other pigments become visible or are produced.

11. And third, how

And third, how sunlight, temperature, and moisture affect how bright those colors are and how long they last.

12. Let's start with

Let's start with the color we see first, and lose first: green.

13. Leaves are packed

Leaves are packed with chlorophyll, and this chart shows why that makes them green.

14. Across the bottom

Across the bottom is the color of light, measured by wavelength in nanometers.

15. Up the side

Up the side is how much of that light each pigment absorbs.

16. Chlorophyll a and

Chlorophyll a and chlorophyll b both peak in the blue and in the red.

17. In between, at

In between, at green, they absorb almost nothing.

18. That green light

That green light is reflected back to our eyes, and that's why leaves look green.

19. The light chlorophyll

The light chlorophyll absorbs drives photosynthesis.

20. The energy is

The energy is passed to reaction centers in the chloroplast, where it's used to make ATP and NADPH, which the leaf uses to build sugars.

21. Chlorophyll does this

Chlorophyll does this work all summer, so there is enough of it to mask the yellow and orange pigments in the leaf.

22. As fall arrives,

As fall arrives, days get shorter and nights get cooler.

23. These are the

These are the signals trees use to prepare for winter.

24. Chlorophyll production slows

Chlorophyll production slows while the existing chlorophyll keeps breaking down, and the green fades.

25. So what's left

So what's left behind once the green is gone?

26. Hidden under the

Hidden under the green are the carotenoids, the yellow and orange pigments.

27. They've been in

They've been in the leaf the entire time, and they're the same kind of pigment that gives carrots their orange color.

28. In summer they

In summer they protect chlorophyll.

29. In strong light,

In strong light, excess energy can create molecules that damage chlorophyll, and carotenoids take up that energy and release it as heat.

30. Once the chlorophyll

Once the chlorophyll is gone, the carotenoids become visible.

31. Red and purple

Red and purple are a different story.

32. Some leaves make

Some leaves make a pigment called anthocyanin, and they make it new in the fall.

33. As sugars build

As sugars build up in aging leaves, they can help stimulate anthocyanin production, and bright light can promote it too.

34. You can find

You can find the same pigments in cranberries and red apples.

35. This raises a

This raises a question.

36. Why would a

Why would a tree make a brand new pigment in a leaf that's about to fall off?

37. Researchers led by

Researchers led by Taylor Feild studied red-osier dogwood leaves as they aged in the fall, in a study published in Plant Physiology.

38. Red leaves recovered

Red leaves recovered from high-light stress, while yellow leaves did not.

39. The researchers suggest

The researchers suggest that in red-osier dogwood, anthocyanins work like a sunscreen.

40. They shield the

They shield the leaf from damaging light while the tree pulls valuable nutrients back out of it.

41. So why does

So why does the same tree look different from one year to the next?

42. A big part

A big part of the answer is the weather.

43. The most vivid

The most vivid fall color usually happens when sunny days are followed by cool nights, as long as those nights stay above freezing.

44. Sunny days help

Sunny days help leaves make anthocyanins, and that combination gives us the strongest reds and purples.

45. When the weather

When the weather stays cloudy for long stretches, the colors tend to be duller.

46. A hard frost

A hard frost can do the opposite.

47. If it hits

If it hits early, it can damage leaves before their color has fully developed.

48. And a severe

And a severe drought can cause trees to drop their leaves early.

49. Now that I've

Now that I've explained how weather affects fall colors, let's bring it together.

50. In the fall,

In the fall, chlorophyll breaks down, making the yellow and orange carotenoids already in the leaf more visible.

51. Some leaves also

Some leaves also make anthocyanins, which produce red and purple colors.

52. Again, there are

Again, there are three parts: how shorter days lead to the loss of chlorophyll, how other pigments become visible or are produced, and how sunlight, temperature, and moisture affect the brightness and duration of those colors.

53. The next time

The next time you notice the leaves changing, you'll have a better understanding of what's happening inside them and why the colors vary.

54. What we see

What we see every autumn is the result of changes happening inside leaves as trees respond to the changing seasons.