Chapter 36, Problem 4
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 on guard cells directly causes them to close. d. If roots are dry, the ABA concentration in leaf cells drops dramatically.
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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.
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.