9.24: Plant Responses

So what happens to a vineyard in the middle of winter?

The vines cannot die each year. Instead, the plants go into a state of dormancy, almost as if they are taking a long nap.

Plant Responses

Like all organisms, plants detect and respond to stimuli in their environment. Unlike animals, plants can’t run, fly, or swim toward food or away from danger. They are usually rooted to the soil. Instead, a plant’s primary means of response is to change how it is growing. Plants also don’t have a nervous system to control their responses. Instead, their responses are generally controlled by hormones, which are chemical messenger molecules.

Plant Tropisms

Plant roots always grow downward because specialized cells in root caps detect and respond to gravity. This is an example of a tropism. A tropism is a turning toward or away from a stimulus in the environment. Growing toward gravity is called geotropism. Plants also exhibit phototropism, or growing toward a light source. This response is controlled by a plant growth hormone called auxin. As shown in Figure below, auxin stimulates cells on the dark side of a plant to grow longer. This causes the plant to bend toward the light.

Phototropism is controlled by the growth hormone auxin.

Daily and Seasonal Responses

Plants also detect and respond to the daily cycle of light and darkness. For example, some plants open their leaves during the day to collect sunlight and then close their leaves at night to prevent water loss. Environmental stimuli that indicate changing seasons trigger other responses. Many plants respond to the days growing shorter in the fall by going dormant. They suspend growth and development in order to survive the extreme cold and dryness of winter. Dormancy ensures that seeds will germinate and plants will grow only when conditions are favorable.

Responses to Disease

Plants don’t have immune systems, but they do respond to disease. Typically, their first line of defense is the death of cells surrounding infected tissue. This prevents the infection from spreading. Many plants also produce hormones and toxins to fight pathogens. For example, willow trees produce salicylic acid to kill bacteria. The same compound is used in many acne products for the same reason. Exciting new research suggests that plants may even produce chemicals that warn other plants of threats to their health, allowing the plants to prepare for their own defense. As these and other responses show, plants may be rooted in place, but they are far from helpless.

Summary

• Like all organisms, plants detect and respond to stimuli in their environment. Their main response is to change how they grow.
• Plant responses are controlled by hormones. Some plant responses are tropisms.
• Plants also respond to daily and seasonal cycles and to disease.

Review

1. What is the primary way that plants respond to environmental stimuli? What controls their responses?
2. Define tropism. Name one example in plants.
3. State ways that plants respond to disease.
4. Why is it adaptive for plants to detect and respond to daily and seasonal changes?