Chapter 1: What is the main topic discussed in this episode?
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You're listening to Shortwave from NPR. Hey, short wavers. Emily Kwong here. Now, depending on where you live in the U.S., you may be enjoying some fall foliage. You know, the result of colder days, trees preparing for winter, and in some, their chlorophyll, the green pigment, is breaking down. And instead of green, we're seeing these amazing colors, yellow, orange, and red.
Colors you might tuck into a home decoration. Along with some of those mutant squash you get at the farmer's market, the really warty ones. Hey, Nell. Hey. That's NPR's Nell Greenfield-Boyce. And Nell, from what I understand, you have been getting really into autumn leaves lately. Well, some of them, some of them, particularly the red ones.
I love the red ones. They're so beautiful. People like red leaves and...
You know, when I met up with this biologist named Nikki Hughes in North Carolina, she told me my dad always was talking about his quest for the perfect red leaf. Respect.
So it's been this lifetime thing, but she's on her own quest now, and that is just to understand red leaves. Why red leaves? Why not yellow? So yellow is in the leaves all along. It's kind of hiding in there. Right.
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Chapter 2: What causes leaves to change color in the fall?
And so you mentioned chlorophyll breaking down in the fall. And when that happens, it's because the tree wants to recover some key nutrients like nitrogen. Nitrogen is really precious to a plant. It's needed for everything from photosynthesis to making proteins and DNA. And so, you know, the tree is going dormant kind of in winter, but it doesn't want to lose that nitrogen.
It wants to hang on to it. So the chlorophyll gets busted up. The green is going away. And this exposes yellow. Right. OK. And scientists agree on that. The yellow is there. It gets exposed. The red is this completely different thing. Oh, because the red is actually made brand new in the fall. Like, Nikki picked up this one maple leaf, which was this kind of mottled red on yellow.
The red was not there beforehand, or you would have seen it, because the leaf would have been purple, because green plus red equals purple. So the question is...
Why does the tree bother to make this red? Especially since the leaf is about to fall and die anyway. Why go through this? Exactly. Like within days, the leaf is doomed. So why go to the effort? Why go to the trouble of making new red colors? And I should note here that these red pigments are also responsible for the leaves we see as orange.
Okay, like Nikki Hughes picked up this other leaf from like a tupelo tree that was red and dark orange. And she said if you looked at what appeared to be orange. Under a microscope, you see lots of red freckles.
Red freckles, that's so cute. Okay, today on the show, the mystery of red leaves.
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Chapter 3: Why do some leaves turn red instead of yellow?
What are scientists' best ideas about what a tree might get from going ginger?
Join us as we rustle up some autumn leaf biology here on Shortwave, the science podcast from NPR.
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Okay, Nell, so you and I have talked about how in autumn the level of green chlorophyll goes down and the amount of red pigment goes up, at least in some trees. Right. Not all go red. What do scientists think is going on?
It really depends on who you ask, OK? I think everybody agrees that the red pigments offer some kind of protection from light. They can act kind of like sunscreen. Oh, but don't plants like light?
Don't they eat it for breakfast?
Yes, but just like with us, too much light can be harmful. And so in the fall, it's cold, you've got chlorophyll going away, and the leaves end up with kind of an excess of light. And that means you've got free radicals forming. These are these destructive, unstable molecules. And it turns out red pigments can mop those up, and they absorb some light.
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Chapter 4: What is the significance of chlorophyll breakdown?
Because beautiful as these leaves are, aren't they just going to fall off and die anyway?
So, OK, that is the question, right? I mean, the idea is that the tree is busy scavenging all this nitrogen and nutrients to store them away for the winter. And, you know, the red pigments, the chemical sunscreen may be protecting that chemical activity.
It's like a last ditch. We're going to help the tree out as it transitions. Fascinating. OK, are there studies and experiments that support this idea?
There are some. Like, there's one study that looked at yellow mutants of three deciduous species that normally turn red. Okay, so...
three tree species that are normally red, but these ones were yellow.
Yeah. And when they shed their leaves, it turns out that those leaves had more nitrogen in them. So the idea is maybe without the red pigments, the plants weren't able to recover that nitrogen and get it out as well as they would have normally.
Like the nitrogen didn't get into the tree itself. It was shed without the red pigment. Fascinating.
And people have also noted that trees that live with nitrogen fixing bacteria. So, you know, there's these, you know, species that live kind of like in relationships with bacteria that help them get nitrogen. Yeah. So they have plenty of nitrogen and they don't seem to bother to turn red. Oh, well, that's a really revealing detail. Yeah. I mean, it's it's suggestive, right?
I mean, there's a lot of kind of suggestive evidence out there. So like Suzanne Renner told me that trees in Europe tend not to be as red as in the U.S., And she did the study looking at the sunlight that comes in. Actually, you can look at it by seeing what hits solar panels. There's all this data out there on solar panels.
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