Why Do Leaves Change Color in the Fall?
Posted by Greg MacDonald on
We know that leaves are food factories for the plants, and that they use a process called photosynthesis. Leaves use water, carbon dioxide and the sun’s energy to make the plant’s food, a sugar called glucose.
A green chemical called chlorophyll is very important to the process of photosynthesis. Chlorophyll is the chemical responsible for giving leaves and other parts of plants their green color.
As Fall (or Autumn) brings increasingly cooler days, and shorter days (so there is not so much sunlight each day), many plants begin to prepare for the winter, when there will not be enough sunlight or water for photosynthesis. These plants begin to close their food factories (their leaves) down. At the base of each leaf's petiole, a layer of cork cells forms, shutting down each leaf’s veins. Gradually, each leaf factory’s work slows, then it ends. Eventually, this weak cork layer breaks, and the leaf falls to the ground.
While all of this is happening, cholorophyll is broken down into smaller molecules by the leaves and these particles are stored by the plant. As the chlorophyll disappears, orange and yellow pigments that had always been present in the leaves (but masked by the abundance of green chlorophyll) are revealed. These pigments are called carotene (orange) and xanthophyll (yellow).
We also see red, pink, and purple colors, which are caused by other plant pigments called anthocyanins. These chemicals are produced in the leaf as phosphate and other nutrients move out of the leaf into the plant’s stem, so that the plant can retain them before each leaf falls. Anthocyanins help the plant to recover nutrients left in the leaf before the leaves fall.
Brown leaves are caused by a chemical called tannin, and also as the leaves die completely, drying out and leaving behind a mixture of various pigments
Different combinations of these various pigments bring us all of the wonderful colors that we see in fall leaves.
Scientists have not yet agreed on how these color changes might serve the plants.
Some scientists believe that some of the pigments protect the leaves against the sun’s harmful effects at low temperatures, and that they may also help the tree to more efficiently reabsorb nutrients such as nitrogen.
Other scientists think that the colors warn insects not to use the plant as a host, as their leaves contain chemicals that these creatures find unpleasant.
It has even been suggested that the plants change their colors in order to reveal herbivores to predators, thus reducing the number of these animals that will eat plants over the Fall, and in the following year!
Etymology
photosynthesis From Greek: phõs= light, and suntithenai= to put together).
glucose From Greek: gleukos= sweet wine
chlorophyll From Greek: khlõros= pale green, and phyllon= leaf)
carotene From Greek: karõton= carrot
xanthophyll From Greek: xanthos= yellow and phyllon= leaf
anthocyanin From Greek: anthos = flower and kuanos = dark blue
tannin From Latin: tannum= tanbark
© Greg MacDonald 2019