Part XLVII: Of plant pigments, fall colors, and the chemistry and physics behind color

For those of us who live in a temperate climate zone (four seasons) we look forward every fall to the changing colors. Indeed, many people plan short trips or plan vacations around the color changes. But what makes the trees turn the bright yellows, oranges, reds, or rustic browns? What environmental factors and anatomical/physiological influences are responsible for these changes? Well, first of all, the lowering of the high and low temperature is a major factor as well as the deceasing number of daylight hours. But why does the duration of daylight hours decrease in the fall? Good question, right? Indirectly, the tilt of the earth on its axis is the root cause of changes in light duration. As the process continues to occur, leaves gradually cease to make chlorophyll and then the other pigments that were already present but masked by chlorophyll become visible. With decreasing daylight hours comes lower temperatures which also contributes to the process. It all starts inside the leaf. Leaves have color because of chemicals called pigments, and there are four main types of pigment in most leaves:

Biology and chemistry

So, now that we have determined the conditions that bring about the degradation of chlorophyll, the question becomes why must the leaves change color and fall to the ground and why does chlorophyll production have to cease? The simple answer is that chlorophyll production takes a lot of energy and there just isn’t enough sunlight available in the winter to maintain production. Furthermore, there isn’t enough liquid water available to keep the deciduous trees alive and produce chlorophyll too. So they go dormant. So why don’t evergreens drop their leaves (needles). I just gave a clue. First of all, they do drop them or at least some of them periodically. Remember the relationship of surface area vs. volume from essay XIV and the anatomy of the leaf, especially the epidermis and most especially the lower epidermis? Like our skin the leaf has small (microscopic) pores called stoma (stomata) that allow CO₂ in and O₂ out as well as excess H₂O (transpiration) Therefore, broadleaf trees lose their leaves to conserve both energy and water.

Physics of light

When I used to tell my students that the wall of the classroom looked blue or yellow because those were the colors of light that reflected back to our eyes and absorbed the other colors of the rainbow (spectrum), I usually got strange looks of astonishment, confusion, disbelief, or all of the above. Therefore, trees, grass, etc. appear green because chlorophyll a absorbs light in the range of 450 nm and again in the 680 nm (violet) and chlorophyll b in the 490 nm and 650 nm range. Note that this is in complete agreement with my earlier statement that we see the color of light that is being reflected back to our eyes.

Although the fall colors may be gone in many parts of the Northern Hemisphere, their beauty remains etched in our photographs and our memories. Next up in my part of the world is the beauty of winter and for those in the Southern Hemisphere temperate zone, the joy of summer.