Fall is in full swing. The days are getting shorter, the air is getting cooler, and the leaves - they are a changin'. Here's a crash course on why deciduous leaves change color and fall off this time of year.
Three main pigments determine leaf color. Chlorophyll is the most abundant pigment. It's present in plant cell structures called chloroplasts and gives plants their green color. Chlorophyll is necessary for photosynthesis, a chemical reaction that uses light to make sugar. During the growing season (typically spring and summer), plants are continually making and using chlorophyll. As the days get shorter, chlorphyll production slows and eventually stops.
Here are tiny round molecules of chlorophyll contained in the chloroplasts of this plant.
Carotenoid is the second type of pigment that plays a role in leaf color. This pigment gives rise to yellow, orange, and brown leaves. It's also present in bananas, buttercups, and carrots. Carotenoids are made and reside in chloroplasts along with chlorophyll. These yellow and orange pigments are always present, however the number of chlorophyll molecules far outnumbers the amount of carotenoid molecules. Therefore, the green chlorophyll color masks the yellow and orange pigments during the growing seasons. As the nights get longer, chlorophyll production comes to a halt and all of the residual chlorophyll in the leaf is broken down. When this happens, the underlying carotenoid pigments become visible.
Anthyocyanin is the last major type of pigment present in leaves. In contrast to chlorophyll and carotenoids that are produced during the growing season, anthyocyanin pigments are only produced in autumn in response to excess sugar trapped in leaves. Cooler temperatures, especially at night, cause veins in the leaves to contract and eventually seal themselves off causing sugar to become trapped in the leaves. Anthocyanins produce red and purplish colors and are also present in blueberries, cranberries, cherries, red apples, and strawberries.
Leaves not only change color, but also fall off during autumn. This is a molecularly regulated process as well. During the growing season, leaves remain firmly attached the branches. A plant hormone called auxin prevents leaves from sealing themselves off from the rest of the tree. However, as the days grow shorter, veins within the leaf begin to constrict and auxin production decreases. Less auxin causes cells at the base of the leaf to multiply and seal the leaf off from the branch. Once the seal is complete, the leaf is ready to fall off via wind or due to it's own weight.
You may be asking why all of the color changing and falling is necessary. It's all part of a protective mechanism. Trees need to protect themselves from freezing during winter. Trunks and branches can withstand freezing temperatures because they have a tough outer covering (bark) and the wood itself will become dormant until spring. Tender leaves either need to toughen up (like how pine needles have evolved) or fall off and start fresh in the spring. Leaves that fall off are actually an essential part of the surrounding ecosystem. They become food for soil-dwelling organisms. In addition, decaying leaves help to replenish nutrients and absorb rainwater. Therefore, falling leaves not only benefit the tree from which they came, but also their environment.
