Plant hormones act by affecting the activities ofa. genes.b. membranes.c. enzymes.d. genes, membranes, and enzymes.

During winter or periods of drought, which one of the following plant hormones inhibits growth and seed germination?a. ethyleneb. abscisic acidc. gibberellind. 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 … auxinb. gibberellin … abscisic acidc. cytokinin … abscisic acidd. 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 on 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.

Buds and sprouts often form on tree stumps. Which hormone would stimulate their formation?a. auxinb. cytokininc. abscisic acidd. ethylene

A plant mutant that shows normal gravitropic bending but does not store starch in its plastids would require a reevaluation of the role of in gravitropism.a. auxinb. calciumc. statolithsd. differential growth

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.