37. Plant Sensation and Response

During winter or periods of drought, which one of the following plant hormones inhibits growth and seed germination?

a. Ethylene

b. Abscisic acid

c. Gibberellin

d. Auxin

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.

In autumn, the amount of _________ increases and the amount of _________ decreases in fruits and leaf stalks, causing a plant to drop fruit and leaves.

a. Ethylene … auxin

b. Gibberellin … abscisic acid

c. Cytokinin … abscisic acid

d. Auxin … ethylene

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.

Plant hormones act by affecting the activities of:

a. Genes

b. Membranes

c. Enzymes

d. Genes, membranes, and enzymes

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.

Buds and sprouts often form on tree stumps. Which hormone would stimulate their formation?

a. Auxin

b. Cytokinin

c. Abscisic acid

d. Ethylene

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.

"Match each definition with the correct term.

Bending of a shoot toward light, Growth response to touch, Cycle with a period of about 24 hours Pigment that helps control flowering Relative lengths of night and day Growth response to gravity Folding of plant leaves at night

a. Phytochrome

b. Photoperiod

c. Sleep movement

d. Circadian rhythm

e. Thigmotropism

f. Phototropism

g. Gravitropism"

Indicate the response to each condition by drawing a straight seedling or one with the triple response.

Which of the following types of viruses would you expect to require periods of latency?

a. Viruses that have large genomes and require a long time for replication

b. Viruses that require a long time for transmission to new hosts

c. Viruses that require a long time for assembly into complex structures

d. Viruses that infect cells of the immune system

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.