Posts Tagged ‘photosynthesis’

Fall Color

October 28, 2010

We are lucky to live in Michigan, where there is a definite change in seasons. One of my favorite times of the year is fall, when it cools down and the leaves change to yellow, orange and red. We enjoy the beautiful colors, but mostly take them for granted and don’t think too much about why it happens. I’m sure we all learned about the leaf changing process in school, but I can’t say I remember. In case you don’t either, keep reading.

Plants take water from the ground and carbon dioxide from the air. A chemical called chlorophyll absorbs sunlight to create energy to change that water and carbon dioxide into oxygen and glucose. This changing process is called photosynthesis. The oxygen is released into the atmosphere. Glucose is used by the plant for food and as fuel for growth. There is not enough light in the winter for photosynthesis, so the tree stores glucose during the summer to help it survive during the winter.

Chlorphyll happens to be green, and when the leaves are full of it, they are also green. As fall nears, the days get shorter and this signals the tree that winter is coming. The tree produces less chlorophyll and then stops completely. Once the chlorophyll is gone, other colors that were hidden by the green chlorophyll can show. These are called carotenoids and they are pigments that are always present in the leaves, but are usually covered by the chlorophyll. Leaves containing carotenoids are yellow, gold, orange and brown. These colors are not affected by temperature or weather, so they remain fairly similar from year to year.

Red leaves are created by a different process. When the tree starts to shut down chlorophyll production for the winter, the veins that connect the leaves to the tree start to narrow. Glucose that is trapped in the leaves is broken down by sunlight to create another group of pigments called anthocyanins. The more anthocyanins are present, the more red, crimson and purple color in the leaves.

As the tree is shutting down for the winter, a layer of cells grows between the leaf stem and the tree. When it is finished, the leaf is barely attached to the tree and will eventually fall to the ground.( Unless it is an oak tree, whose leaves do not totally detach and often remain through the winter.) Leaves are dropped by the tree because they are the most vulnerable portion of the tree. The trunk and branches can withstand freezing temperatures without damage, but leaves could not. They would freeze and in turn cause damage to the tree, so the tree protects itself by losing the leaves.

Yellow and orange leaves are pretty, but I don’t think you can argue that the more red leaves, the better. Some weather and temperature conditions can affect the colorful display. If there is a drought in the late spring or summer, the fall color may be delayed. A very warm fall can also delay the colors. For the most spectacular color, you need a warm, wet spring, a normal summer, sunny fall days and cool fall nights.