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Autumn Color

In this episode – Autumn Color . . .

I'm sure you've noticed the leaves on the trees are changing. This science update from howstuffworks.com by Jennifer Horton gives us some insight as to why - join me.

For most of the year, leaves are a tree's workhorses, constantly converting carbon dioxide, water, and sunlight into energy in a process called photosynthesis. The special ingredient for this process, the pigment chlorophyll, gives leaves their bright, green color. But while chlorophyll is the star of the show, it has some help in the pigment's carotene and xanthophyll. Xantho is Greek for "yellow," and carotene gives carrots and egg yolks their orangish color. These two pigments are always present in leaves and help absorb sunlight, which they transfer to chlorophyll for photosynthesis.

As the days get shorter, the increased amount of darkness stimulates trees to prepare for hibernation. Leaves won't continue photosynthesizing during winter due to the dry air and lack of sunlight, so the tree does two things.

First, it forms a separation layer made of cork-like cells at the base of each leaf to seal it off from the tree.

Second, it stops producing chlorophyll since it won't need this pigment until the days start to lengthen once again in the spring. With chlorophyll out of the picture, the yellow and orange stains get a chance to shine.

The red hues, which come from pigments called anthocyanins, are slightly more complicated. Whereas all trees contain chlorophyll, carotene, and xanthophyll, not all of them produce anthocyanins. Even the ones that do have anthocyanins only make it under certain circumstances.

Remember that layer of cells at the base of the leaf? Its purpose is to protect the tree during the colder winter and prevent it from drying out. When the separation layer is complete, the leaves fall off in the tree's attempt to conserve energy. But before the leaves fall off and the tree closes shop, it wants to pull in as much sugar and nutrients as possible from its leaves, which is where the anthocyanin comes in.

Although scientists offer several reasons why some trees produce anthocyanins and autumn leaves change color, the prevailing theory is that anthocyanins protect the leaves from excess sunlight and enable the trees to recover any remaining nutrients.

The reason you'll see more vibrant reds during some years is that lots of sunlight and dry weather increase the sugar concentration in tree sap, triggering the tree to release more anthocyanins in a last-ditch effort to gather up the energy to get through the winter. In addition, near-freezing weather, low nutrient levels, and other plant stressors seem to trigger increased levels of anthocyanins.

If it's been stormy and overcast, you won't see much red foliage. Without bright sunlight, the trees don't need the added protection that the red pigments provide, so they don't bother producing them.

So, if autumn just isn't the same for you without the occasional splash of red, hopefully, the weather will cooperate. If not, you'll just have to make do with the more reliable yellows and oranges. Or you could always join the leaf-peepers in New England in their endless search for the ultimate display of fall color.

So, won't you be my virtual neighbor? If you enjoy our weekly visits, please share them with a friend.

I'm Patrick Ball; thanks for listening. See you in the next episode.

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