Nature Snacks: Why do Leaves Change Color in the Fall?

Source: Fall Season Free Wallpaper

While some Governors in the Northern United States swear that the leaves change color in the fall to support their tourism industry and some artists swear it is Mother Nature painting, reality is more complex than the economy or color theory. In order to answer what seems like a simple question, we need to explore the role of leaves and how they work. Don't worry! This is going to be a lot more interesting than you may expect.

Working Two Full-time Jobs

While leaves serve many secondary purposes for plants, they have two primary jobs: Short order cook and traffic cop. Let's explore both of these in order to understand what leaves do for plants and what this has to do with leaves changing colors in fall.

Short order cooks work in fast paced restaurants preparing simple foods for people on the go. They collect ingredients and prepare meals as quickly as possible. The more ingredients they get, the more they can cook. Just like hungry people eat a lot a food, growing plants need sugar, which they break down to make the energy required to live. Luckily, leaves are excellent short order cooks.

Leaves take basic ingredients, sunlight and carbon dioxide, and make them into cake. Well, sugar really. In order to make as much sugar as possible, they collect every bit of sunlight that they can. Sunlight is not a single a type of light but is rather mixture of different color lights. Out of these colors (red, orange, yellow, green, blue, indigo and violet), plants use blue, green and red light to make the cake... I mean sugar. Each color of light moves at its own speed and therefore, you need a different "nets" to catch them. The nets in leaves are pigments and leaves have three kinds.

The first type of pigment, is called carotenoids because they are orangey-yellow colored, but don't be fooled. Even though they are this color, their job is to catch blue light. The second type of pigment in leaves, anthocyanins, look pinkish-red while they catch blue and green light.

Carotenoids and anthocyanins aren't the star player of the team though. That is chlorophyll, the "Hulk" of pigments. Like the Hulk, it is big, green, strong and needs a lot of energy to collect red light. Because chlorophyll is so big and plentiful it covers the other pigments so all you see when you look at leaves is green - until fall that is.

While the short order cook is slaving away in the kitchen, the traffic cop is keeping the trucks moving. Trucks? Yup. Chemicals inside plants act like trucks carrying supplies from the roots to the leaves and carrying the sugar made in the leaves to the rest of the plant. A traffic jam could be deadly! If the trucks get backed up, they can't bring the other ingredient needed to make sugar, carbon dioxide, to the short order cook. Without both ingredients, the leaves can't make sugar. No sugar equals no food for the plant. No food equals no energy. No energy means death.

So how do leaves act like traffic cops, keeping plant chemicals moving? Leaves have windows on their surfaces called stoma. Opening and closing the stoma is like putting up and down a bridge. When stomas are open, traffic flows and when they are closed, traffic backs up. Controlling the stoma so that the chemicals in the plant move at just the right speed is vital. Therefore, the leaves act like traffic cops by controlling the speed that chemicals move through plants.

Now you know that leaves already have green, orange, yellow, pink and red colors inside them and why. You also understand that, without functioning leaves, the plant can't move chemicals and energy to where it needs them. Read on to learn why both of these roles of leaves influence the changing color of fall leaves.

Changing to Fall Colors

In the fall, plants that loose their leaves, called deciduous plants, get ready to go to sleep for the winter. Since they'll be sleeping, they can survive using just the energy they stored over the summer. Therefore, the cook can take a vacation. Chemicals won't be moving around much either so the traffic cop also isn't needed. In other words, leaves aren't needed in winter. The plant will make new leaves in the spring so the current leaves won't be needed again and are dropped. Changing colors is part of the process of dropping leaves.

Notice the colors in these maple leaves. At first, the green

overshadows the other colors. As the green recedes, yellow

is the dominant color. As the yellow receded the red remains.

Finally the red gives way to brown and the leaf dries up.

The first step to dropping leaves is to close the road into the leaf, cutting off its supplies. When the supplies are cut off and the cook stops cooking, high energy users such as chlorophyll die off. Caroteniods and anthocyanins are smaller and use less energy so they can survive a little longer. With chlorophyll gone, the leaves are no longer green.

You can imagine what happens now. Without the green color hiding the orange, yellow, pink and red of the other pigments, the leaves look like they have "changed colors". In reality, these fall colors were always there waiting for their chance to shine! These pigments won't last much longer without energy. Soon the leaves die, turn brown and fall. The plant sleeps as we look forward to the green of spring.