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Notes to Instructors
Chapter 39 Plant Responses to Internal and External Signals
What is the focus of this activity?
In Activity 37.1, students looked at some of the nutrient conditions required to develop
What is this particular activity designed to do?
Activity 39.1 How do gravity and light affect plant growth responses?
Students apply what they have learned about gravitropism, phototropism, and
What misconceptions or difficulties can this activity reveal?
Activity 39.1
Many students have difficulty understanding how higher concentrations of auxin can
258 Notes to Instructors
Answers
Activity 39.1 How do gravity and light affect plant growth
responses?
Review Chapter 39 of Campbell Biology, 9th edition, and your answers to Activity 37.1.
Then answer the questions.
Activity 39.1 259
1. One of the problems associated with growing plants in space is lack of gravity.
a. How does gravity affect the
normal growth of a plant’s
roots, stems, and other
parts? Explain the
mechanisms involved.
Under the influence of gravity,
auxin accumulates on the
b. How would a lack of
gravity affect normal plant
growth?
Normal seed germination and
seedling growth begin
underground in the absence of
light. Under these conditions,
c. Propose mechanisms to
overcome the problems
associated with a lack of
gravity.
Plant orientation is also
affected by light. (See question
2.) The response to light can
(Continued on next page)
2. Another problem with growing plants in space relates to a plant’s light requirements and phototropic responses versus the photoperiods
required for the plant to flower and produce fruit.
a. How do phototropism and photoperiodism differ?
Phototropism is the growth of a plant toward or away from light. In general, shoots grow toward light (are positively phototropic)
some species of plants are triggered by changes in day length.
b. What light characteristics would you use to maximize plant growth per unit time?
In photosynthesis, the chlorophyll molecules in photosystems I and II respond to or absorb light in the red and blue wavelengths
c. What kind of physical environment would you need to maintain appropriate phototropic responses among plants?
In space or in the lab, you would need to set up banks of overhead lights. Light bulbs that are rich in red and blue wavelengths
d. What design modifications would you need to make to support plants with different photoperiods—for example, long-day versus
short-day plants?
If the different plant species you were growing had different photoperiods, you would need to grow them in separate chambers. You
Activity 39.1 261
1. Compared to the wild type, a mutant plant that overproduces cytokinin will probably
have ______ lateral branches. Fill in the blank with the best answer and explain your
answer.
a. more
b. fewer
c. the same number of
2. Combining the mutation in question 1 above with one that causes the overproduction
of auxin would ______ the effect you chose above. Fill in the blank with the best
answer and explain your answer.
a. increase
b. decrease
c. not change
3 and 4. A species of long-day plant will flower when the days are over 14 hours long.
Which of the following light cycles would cause this plant species to flower?
Answer True if the cycle causes flowering and False if it does not. Explain your answers.
T/F 3. Continuous white light day and night, interrupted every 4 hours by flashes of
red light.
T/F 4. A daily cycle of 9 hours of light followed by three periods of 4 hours and
50 minutes of uninterrupted dark, and 10 minutes each of red light and
white light.
39.1 Test Your Understanding
