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.

Verified step by step guidance

1. Understand the role of ABA (abscisic acid) in plant physiology: ABA is a plant hormone that plays a crucial role in regulating stomatal closure, especially under water stress conditions.
2. Identify the function of guard cells: Guard cells are responsible for opening and closing stomata, which are pores on the leaf surface that regulate gas exchange and water loss.
3. Analyze the options provided: Each option suggests a different scenario related to ABA and guard cell behavior.
4. Evaluate option b: 'If roots are dry, guard cells begin to close—even though leaves may not be experiencing water stress.' This suggests that a signal from the roots, likely ABA, is causing the guard cells to close, indicating a systemic response to root water status.
5. Consider the evidence: The scenario in option b supports the idea that ABA produced in the roots can travel to the leaves and signal guard cells to close, even if the leaves themselves are not directly experiencing water stress.

Key Concepts

Abscisic Acid (ABA)

Abscisic Acid (ABA) is a plant hormone that plays a crucial role in regulating plant water balance and stress responses. It is synthesized in response to environmental stress, such as drought, and can signal guard cells to close stomata, reducing water loss. Understanding ABA's role in signaling is essential for interpreting how plants manage water stress.

Guard Cells and Stomatal Closure

Guard cells are specialized cells surrounding stomata, which are openings on the leaf surface that regulate gas exchange and water loss. When ABA levels increase, guard cells respond by closing the stomata, thereby conserving water during periods of drought. This mechanism is vital for maintaining plant homeostasis under water-limited conditions.

Root-to-Shoot Signaling

Root-to-shoot signaling involves communication between roots and aerial parts of the plant, often mediated by hormones like ABA. When roots detect dry conditions, they can produce ABA, which travels to the leaves to signal stomatal closure, even if the leaves themselves are not yet experiencing water stress. This concept is key to understanding how plants integrate environmental signals to optimize survival.