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.

Verified step by step guidance

1. 1. The first step is to understand the role of ABA (Abscisic Acid) in plants. ABA is a plant hormone that plays a crucial role in stress responses, including drought. It is produced in response to water deficiency and signals the closure of stomata (the pores on the leaf surface controlled by guard cells) to reduce water loss through transpiration.
2. 2. Option a suggests that guard cells close when roots are given sufficient water. This is not necessarily evidence for ABA signaling from roots to guard cells, as the closure of guard cells could be due to other factors such as light intensity or carbon dioxide concentration.
3. 3. Option b suggests that guard cells begin to close when roots are dry, even if leaves are not experiencing water stress. This could be evidence for ABA signaling from roots to guard cells, as it indicates that the guard cells are responding to a signal from the roots (possibly ABA) rather than to local conditions in the leaves.
4. 4. Option c states that applying ABA directly on guard cells causes them to close. This is evidence that ABA can cause guard cells to close, but it does not necessarily indicate that ABA from roots signals this response.
5. 5. Option d suggests that when roots are dry, the ABA concentration in leaf cells drops dramatically. This seems counterintuitive, as we would expect ABA levels to increase in response to drought. Therefore, this option does not provide evidence that ABA from roots signals guard cells to close.