We all love to watch the tree leaves turn from bright verdant green to the rich colors of autumn. The trees painted with red, gold, and brown entrance us as the days get shorter and crisp winds stir the early mornings and evenings. Everyone appreciates nature’s artistry, but for those who quest for knowledge, the reasons why the leaves change color is not as obvious.
Leaves change color in the fall as a result of changing hormones and chemicals in the plant. The hormonal and chemical changes are triggered by reduced daylight hours and dropping temperatures. Chlorophyll production slows down, allowing other pigments in the leaf to be seen.
Not many people know that plants have hormones, and leaves contain other pigments besides green. For many years, trees kept their secrets. Scientists eventually discovered the processes, chemicals, and hormones which cause these magnificent changes. This article will explain leaf pigments and the changes in the trees that give us the glorious fall hues.
Colors In A Leaf
Pigments are responsible for creating color in a leaf. The presence of pigments is determined by various chemicals and the needs of the tree. When the tree requires sugars and carbohydrates for growth and cell replacement, then the leaves will be green. When the demand for nutrients declines, then hormones produced by the tree will result in fall colors.
A tree has green leaves because of the presence of chlorophyll. The tree produces chlorophyll and directs it to the leaves. Chlorophyll assists the tree in making food for itself. Chlorophyll has small structures called chloroplasts which are capable of absorbing sunlight.
Sunlight is the energy source used for the process of photosynthesis. Photosynthesis is when carbon dioxide and water are transformed into carbohydrates (sugars and starches) that the tree can use for growing. Photosynthesis releases oxygen into the air. Oxygen is a byproduct of photosynthesis and essential for the earth’s health.
Other Pigments In A Leaf
Other pigments exist in leaves, even though they are not always visible. Carotene in a leaf causes orange colors. Flavanols, specifically xanthophyll, give yellow colors, and anthocyanin gives red, pink, and purple hues. Carotene and xanthophyll are present in the tree leaves during spring, summer, and autumn. The over-abundance of green chlorophyll masks the yellow and orange. Anthocyanins are only produced in the leaf in autumn.
The Function Of Carotene
Carotene is a pigment that occurs naturally in plants. The effect of carotene can be seen in the orange color of carrots and cantaloupes. Carotene plays an integral part in photosynthesis. It absorbs ultraviolet, blue, and violet light and transmits the light to chlorophyll.
It scatters orange and red light which is why we perceive the leaves as red or orange. Yellow light is also diffracted but to a much lesser extent than red and orange light. Carotene aids the plant by absorbing free single oxygen molecules. If left un-managed, these oxygen molecules could cause damage to the plant.
The Function Of Xanthophyll
Xanthophyll is a powerful antioxidant in plants. It assists carotene with the removal of single oxygen molecules, sometimes known as free radicals. Xanthophyll absorbs the single free oxygen, protecting the plant from damage. Single oxygen molecules have unpaired electrons which try to bond with plant cells. The bonding causes stress to the cells and ultimately results in disease.
The Function Of Anthocyanin
Anthocyanin is only produced in autumn. Scientists believe that it functions to keep the leaf alive for an extended period as the chlorophyll declines. Anthocyanins have anti-inflammatory and antimicrobial properties which extend the life of the leaf. The longer a leaf is alive, the more molecules the tree can reabsorb from it. This process saves energy in spring when these molecules must be used to create chlorophyll, carotene, and xanthophyll to grow new leaves.
How Does A Leaf Change Color?
In autumn, the days shorten, and the temperature drops. These changes cause the plant to decrease the production of chlorophyll. Chlorophyll molecules in the leaves are broken down into smaller molecules and reabsorbed into the tree tissues. The reduction in chlorophyll allows the carotene and xanthophyll colors to show, resulting in yellow or orange leaves.
Environmental changes stimulate the production of anthocyanins in the leaves. Anthocyanins are responsible for leaves becoming red, pink, or purple. The three pigments interact to give other colors such as brown or differing shades of colors.
Why Are Colors Different Each Fall?
The intensity of the fall colors is affected by the weather conditions. Sunny, cold days in autumn result in greater anthocyanin production and more intense red-colored leaves. The temperatures should be cold but must not drop below freezing for there to be good anthocyanin production.
Short, dry spells result in a concentration of sugars in the leaves, allowing more anthocyanin to be produced. A drought will cause the leaves to dry up and fall. Sufficient water allows the leaves to stay on the tree for longer, and the colors become more intense.
Which Trees Change Color?
Trees with broad leaves change color, especially in colder northern climates. Trees with broad leaves that change color are hickory, maple, birch, cherry, red oak, aspen, sassafras, and dogwood, amongst others. Broad-leafed trees in warmer southern climates may not change color in fall or lose their leaves as the winters are too mild to cause any change. Southern areas also maintain longer daylight hours in their autumn and winters. Therefore there is no trigger for the change in the leaves.
Why Do Leaves Fall Off A Tree?
Leaves change color and fall off a tree to preserve energy in the cold winter months. The tree absorbs chlorophyll molecules and other nutrients from leaves, breaking them down into smaller molecules for storage. Chlorophyll takes a lot of energy and nutrients to produce. The tree conserves energy by storing as many chlorophyll components as possible to make it easier and quicker to produce chlorophyll in spring.
What Makes Leaves Fall Off A Tree?
When a leaf forms, there is a particular cell layer at the point where the leaf attaches to the tree. This layer of cells is named the abscission layer. The shortened hours of daylight cause the tree to produce a hormone called auxin, which triggers cellular elongation in the abscission layer. The elongation causes the cells to fracture, and the leaf falls off. The tree seals the point where the leaf breaks off, and a leaf scar is left on the branch.
Is It Beneficial For A Tree To Lose Its Leaves?
Broad-leafed trees that grow in cold climates need to conserve energy in winter. Broad leaves collect snow and have a large surface area for heat loss. Strong winds catch in broad leaves and could potentially damage the tree. It is, therefore, in the tree’s best interest for the leaves to fall off.
Some trees do not change color in fall. These are either broadleaf trees that grow in areas where winters are very mild or trees with needle-type leaves. Trees with needle leaves are evergreen, even in cold climates. The needles do not cause the tree to lose heat or moisture because they have a small surface area. They can also stand up to strong winds and heavy snowfalls due to the shape of the needles.
Trees change color in fall as a result of shortening daylight hours and declining temperature. The process is governed by hormones produced by the tree. The pigments which cause the various leaf colors are responsible for essential functions in the tree. The pigments which produce the fall colors are carotene, xanthophyll, and anthocyanin. For these colors to be seen, the chlorophyll must decrease. The intensity of the colors is influenced by temperature, humidity, and the number of daylight hours.
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