READ: Plants and Energy

Photoperiodism

How does this plant know it's Christmas?

This plant is known as a Christmas cactus because it only blooms once a year, during the Christmas season. But plants can't read a calendar. How can a houseplant know the time of the year?

Seasonal Changes

Have you seen the leaves of plants change colors? During what time of year does this happen? What causes it to happen? Plants can sense changes in the seasons. Leaves change color and drop each autumn in some climates (figure below).

Certain flowers, like poinsettias, only bloom during the winter. And, in the spring, the winter buds on the trees break open, and the leaves start to grow. How do plants detect time of year?

Although you might detect seasonal changes by the change in temperature, this is not the way in which plants know the seasons are changing. Plants determine the time of year by the length of daylight, known as the photoperiod. Because of the tilt of the Earth, during winter days, there are less hours of light than during summer days. That’s why, in the winter, it starts getting dark very early in the evening, and then stays dark while you’re getting ready for school the next morning. But in the summer it will be bright early in the morning, and the sun will not set until late that night. With a light-sensitive chemical, plants can sense the differences in day length.

For example, in the fall, when the days start to get shorter, the trees sense that there is less sunlight. The plant is stimulated, and it sends messages telling the leaves to change colors and fall. This is an example of photoperiodism, the reaction of organisms, such as plants, to the length of day or night. Photoperiodism is also the reaction of plants to the length of light and dark periods. Many flowering plants sense the length of night, a dark period, as a signal to flower. Each plant has a different photoperiod, or night length. When the plant senses the appropriate length of darkness, resulting in an appropriate length of daylight, it flowers. Flowering plants are classified as long-day plants or short-day plants. Long-day plants flower when the length of daylight exceeds the necessary photoperiod, and short-day plants flower when the day length is shorter than the necessary photoperiod. Long-day plants include carnations, clover, lettuce, wheat, and turnips. Short-day plants include cotton, rice, and sugar cane.

http://www.ck12.org/life-science/Seasonal-Changes-in-Plants-in-Life-Science/lesson/Seasonal-Changes-in-Plants/r23/

Phototropism

Why are these plants turning sideways?

Plants respond to their environment in how they grow. In this picture, the light source is probably off to the left side. As a result, the plants grow in this direction to get more light.

Tropisms

Plants may not be able to move, but they are able to change how they grow in response to their environment. Growth toward or away from a stimulus is known as a tropism ( table below). Auxins, a class of plant hormones, allow plants to curve in specific directions as they grow. The auxin moves to one side of the stem, where it starts a chain of events that cause rapid cell growth on just that one side of the stem. With one side of the stem growing faster than the other, the plant begins to bend.

Phototropism

You might have noticed that plants bend toward the light. This is an example of a tropism where light is the stimulus, known as phototropism (figure below). To obtain more light for photosynthesis, leaves and stems grow toward the light. On the other hand, roots grow away from light. This is beneficial for the roots, because they need to obtain water and nutrients from deep within the ground.

http://www.ck12.org/life-science/Tropisms-in-Life-Science/lesson/Tropisms/r23/

Summary and Vocabulary

Vocabulary

• photoperiodism: The reaction of organisms to the length of day or night.
• season: Subdivision of the year due to a change in weather, ecology, and hours of daylight.
• phototropism: Growth in response to light.
• tropism: Growth toward or away from a stimulus.

Summary

• Plants can respond to the change of season by losing their leaves, flowering, or breaking dormancy.
• Plants go through seasonal changes after detecting differences in day length.
• Tropisms are growth toward or away from a stimulus.

http://www.ck12.org/life-science/Tropisms-in-Life-Science/lesson/Tropisms/r23/