Nell Greenfield Boyce

Oh, but don't plants like light?

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.

I was talking to Suzanne Renner at Washington University in St.

Louis, and she told me that for decades, we have known that red pigments can protect against this extra light.

So photo protection, protection from light.

She told me all the plant physiologists are convinced this is a key thing.

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.

There are some.

Like, there's one study that looked at yellow mutants of three deciduous species that normally turn red.

Okay, so...

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.

And people have also noted that trees that live with nitrogen fixing bacteria.