The Sun: Man's Friend & Foe - The Sun & Our Garden

The Sun has numerous effects on plants and gardens in particular. These effects can be the life giving energy it provides, as well as the devastation it brings when there is either too little or too much of it. It can be the cause of feast or famine for many a farmer. This page will cover only the positive influence of the Sun on plants.

Plant Growth

[Picture of a woman working in her garden.] Anything that effects the direction a plant moves is called tropism. When plants move toward light it is called phototropism. When plants are exposed to light they will always turn toward the light and away from the shade. In this same way leaves will always tilt toward the Sun in order to get more light.

Phototropism is the result of a plant hormone called auxin. What auxin does is cause plants to grow more rapidly wherever it is present. It is strange in that it always moves away from the lighted side of plants to the shaded side of the plant.

High concentrations of auxin cause stems to grow more rapidly and get longer. When the auxin is all on one side this causes the stem of a plant to grow unevenly. The side away from the light grows faster than the side toward the light. As a result, the stem bends towards the light.

Auxin can also be found in the roots of plants. Auxin not only is sensitive to light, it is also sensitive to gravity. This sensitivity causes plant roots to grow in the direction of gravity, making plants roots grow downward.

The Sun Sets Plants Schedules

In the Northern Hemisphere, the length of time it is light constantly varies, in the summer the days are long and it is light sometimes until 3 in the morning. In the winter it sometimes is dark at 4 in the afternoon! The only time we have the same amount of daylight and darkness is on March 21^st and September 23^rd. Each day the amount of sunlight plants get varies. After each change the plants are triggered to do certain things. If the change is one where the plant gets more light it can trigger the plant to flower. If the change is one where it means this particular plant will be getting less sunlight it might signal to the plant that it is time to drop leaves, or prepare for winter. These responses are called photoperiodism.

The reason the plants are triggered by the changing amounts of light is because of another substance in most plants called photochrome. Photochrome measures the length of a day by absorbing the light. It causes "short-day" plants such as chrysanthemums to flower during the spring and fall, as well as causing "long-day" plants such as clover to bloom in the late spring and early summer when the days are longer.

Color Changing

As the number of hours of sunlight decreases in the fall, photochrome tells plants to form winter-resistant buds as well as seal off the flow of water to their leaves. Each leaf stem forms a cork-like plug called an abscission zone at the base. This plug thickens until gradually the leaf will die of strangulation. As the leaf dies the color-giving chlorophyll in the leaf begins to fade. This fading unmasks the plants true colors causing the plant to turn a bright red eventually fading into orange and then a yellowish color. Eventually the leaf breaks off at its base and falls to the ground.

The Sun Provides Food and Oxygen Through Plants

"To the best of our knowledge, our Sun is the only star proven to grow vegetables."

- Philip Scherrer, 1973

Any living thing that makes its own food is called an autotroph. Plants are autotrophs, but they couldn't be without the Sun. Most plants convert sunlight to food through a process called photosynthesis. Every year plants make more than 125,000,000,000 tons of carbon 30,000,000,000 tons of hydrogen, as 200,000,000,000 tons of oxygen. Photosynthesis involves four major ingredients: water, carbon dioxide, chlorophyll, and sunlight. When light strikes a molecule of chlorophyll, its electrons absorb energy. This extra energy excites the chlorophyll electrons so that they bounce from one chlorophyll molecule to another.

As the excited electrons bounce about they split apart the molecules of water that they bump into, producing both hydrogen and oxygen. The oxygen is then released into the air. The hydrogen however combines with carbon dioxide to form the basic building blocks of food. The hydrogen and carbon dioxide combination then reacts with 8 other molecules to form glucose. Glucose is what plants store energy in.

Located in the cells of plants there are tiny sacks called chloroplasts. Choloroplasts contain from 200 to 400 molecules of chlorophyll. Chlorophyll absorbs all colors of visible sunlight except yellow and green. Because yellow and green light is reflected instead of absorbed, chlorophyll makes plants look green.

The chemical reactions of photosynthesis provide oxygen for animals and humans to breathe. Without this production of oxygen, human life would be non-existent.

Garden Activity