Evaluate the following assertions about phototropism. Select True or False for each statement.

T/F Cells on the illuminated side of a stem elongate more than cells on the shaded side.

T/F Phototropism is triggered by blue light.

T/F Phototropins play a significant role in phototropism.

T/F The bending of the plant is due to cell elongation in response to auxin.

Which of the following statements about phytochrome is not correct?

a. It is photoreversible.

b. Its function was understood long before the protein itself was isolated.

c. The Pfr form activates the responses to red light.

d. It is involved in guard-cell opening.

Which of the following statements about hormones is correct?

a. They tend to be large molecules.

b. They exert their effects only on the same cells that produce them.

c. They can exert strong effects only when they are present in high concentrations.

d. They trigger a response by binding to target-cell receptors.

What evidence suggests that ABA from roots can signal guard cells to close?

a. If roots are given sufficient water, guard cells close anyway.

b. If roots are dry, guard cells begin to close—even though leaves may not be experiencing water stress.

c. Applying ABA to guard cells directly causes them to close.

d. If roots are dry, the ABA concentration in leaf cells drops dramatically.

Why was it logical to predict that amyloplasts function as statoliths?

a. They are dense and settle to the bottom of gravity-sensing cells.

b. They are present only in gravity-sensing cells.

c. They make a direct physical connection with membrane proteins that have been shown to be gravity-receptor molecules.

d. Their density changes in response to gravity.

Phytochromes can be considered 'shade detectors,' while phototropins such as PHOT1 can be considered 'sunlight detectors.' Explain why these characterizations are valid.

To explore how hormones function, researchers have begun to transform plants with particular genes. In one experiment, a gene involved in cytokinin synthesis was introduced into tobacco plants. Which one of the following results would be expected?

a. Individuals produced more lateral branches.

b. Stems grew extremely tall and slender.

c. Roots were incapable of responding to gravity.

d. Stomata were closed most of the time.

Suppose that a mutant plant is unable to make methyl salicylate. Explain why it is not likely to survive in the wild.

In general, small seeds that have few food reserves must be exposed to red light before they will germinate. (Lettuce is an example.) In contrast, large seeds that have substantial food reserves typically do not depend on red light as a stimulus to trigger germination. State a hypothesis to explain these observations.

In many species native to tropical wet forests, seeds do not undergo a period of dormancy. Instead, they germinate immediately. Predict the role of ABA in these seeds. How would you test your prediction?

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved.

How fast can the leaflets close?

How does this occur?

And more importantly, what benefit could this unusual response provide to the plant?

Researchers studied the rate of closing and reopening of leaflets by measuring changes in the distance between opposing leaflets after being touched. Their results are shown in the accompanying graph (d/dmax is actual leaflet distance relative to maximal leaflet distance).

How long does it take for leaflets to close?

How long to reopen?

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved.

How fast can the leaflets close?

How does this occur?

And more importantly, what benefit could this unusual response provide to the plant?

Which of the following terms best describes the leaflet movement?

a. Thigmonastic movements

b. Thigmotropism

c. Thigmomorphogenesis

d. Apical dominance

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved.

How fast can the leaflets close? How does this occur?

And more importantly, what benefit could this unusual response provide to the plant?

The mechanism of leaflet closure is similar to what happens during the shrinking of guard cells and closing of stomata (see Figure 37.22). When in the open position, special cells on the upper surface of Mimosa leaflets are filled with water and are under pressure.

Explain how osmosis and flow of ions into and out of these special cells may be involved in leaflet closure and reopening.

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved.

How fast can the leaflets close?

How does this occur?

And more importantly, what benefit could this unusual response provide to the plant?

If just the leaflets located toward the end of a leaf are touched, the adjacent leaflets close in fairly rapid succession until all leaflets on a leaf close up.

Explain how electrical signaling may be involved in this response.

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved.

How fast can the leaflets close? How does this occur?

And more importantly, what benefit could this unusual response provide to the plant? Researchers have hypothesized that rapid leaf movements in Mimosa serve as a defense mechanism (e.g., closing leaflets may deter plant-eating insects).

Propose an experiment to test this hypothesis.

Leaflets of Mimosa pudica (common names: sensitive plant, touch-me-not) have a remarkable ability to close up in response to being touched or physically moved.

How fast can the leaflets close?

How does this occur?

And more importantly, what benefit could this unusual response provide to the plant?

Mimosa leaflets also close on their own at dusk and reopen at dawn (this cycle develops as a circadian rhythm).

What receptor molecule mentioned in this chapter is likely involved in regulating this process?