By Bill Silfvast, U. C. Master Gardener

Here comes the parade! Autumn leaves are all about the beautiful palette of colors that nature presents to us, and these colors are all about sunlight.

We know from our school days that sunlight is composed of the colors of the rainbow: red, orange, yellow, green, blue and violet. But we also know that if we take a yellowish-red grape leaf into a dark closet, it has no color; it appears to be black. Thus we can conclude that all the riveting fall colors come from sunlight reflected from the leaves. So how do leaves change sunlight into the colors we so appreciate?

Before we delve into this, we need to know a little about how our eyes perceive color.  Visible light consists of many different colors. Each color is composed of little bundles of light called photons, and different colors of photons have different amounts of energy, with blue light photons having more energy than red ones.

When light falls upon any material, some photons are absorbed and others are reflected. Different materials absorb and reflect different amounts of the various colors. Mirrors reflect all colors; black materials absorb all colors; and other materials, including leaves, absorb and reflect colors depending upon which photon energies they need. What they don't need is what we see as their color.

We know that leaves are green throughout the growing season. In the autumn, they begin changing to dramatic yellows, oranges and reds. After that, they turn brown and drop.

Chlorophyll produces the energy that a plant needs to grow. Through photosynthesis, it converts sunlight and carbon dioxide into carbohydrates (simple sugars). That energy is transported throughout the plant and used as needed. Chlorophyll uses only blue and red photons. It discards, or reflects, the green photons, so we see the leaves as green.

Along with chlorophyll, carotenoids are present in leaves during the growing season. These molecules absorb the green, blue and violet photons, reflecting the yellows, oranges and reds. But the chlorophyll molecules dominate during the growing season and hence determine the leaf color.

As autumn approaches, the night length increases and the temperature drops. Photosynthesis slows down, so there is less chlorophyll to reflect the green sunlight. The carotenoids take their "place in the sun." They become dominant, reflecting the warm yellows, oranges and reds.

Fortunately for us, some leaves also produce anthocyanins in the fall in response to an excess of plant sugars. These molecules reflect the red and orange photons, enhancing the palette of fall.

As winter approaches, soil moisture and temperature change determine the proportions of chlorophyll, carotenoids and anthocyanins remaining in the leaves. Thus each fall has its own distinct color performance. 

The final demise of the leaves comes when the plant has stored all the necessary energy for winter. The color-producing pigments decay, and the dead brown leaves fall since they are of no remaining use to the plant.