Question

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, ABA concentrations in leaf cells drop dramatically.

Short answer

b. If roots are dry, guard cells begin to close - even though leaves may not be experiencing water stress.

Step-by-step solution

Statement a analysis

a. If roots are given sufficient water, guard cells close anyway. This statement does not provide evidence for ABA signaling as it states that guard cells close despite the roots having ample water, possibly due to other factors. The role of ABA in this scenario is unclear.

Statement b analysis

b. If roots are dry, guard cells begin to close - even though leaves may not be experiencing water stress. This statement suggests that ABA might play a role in signaling guard cells to close. When roots are dry, ABA could be produced to signal guard cells to close in order to conserve water, even if the leaves are not yet experiencing water stress.

Statement c analysis

c. Applying ABA on guard cells directly causes them to close. This statement indicates that ABA has a direct effect on guard cells and can cause them to close. However, it does not confirm that ABA from roots specifically is signaling guard cells to close.

Statement d analysis

d. If roots are dry, ABA concentrations in leaf cells drop dramatically. This statement contradicts the idea that ABA from roots would signal guard cells to close. If ABA concentrations drop when roots are dry, it would not act as a signal to close guard cells during water stress. Based on the analysis, the correct answer is:

Answer

b. If roots are dry, guard cells begin to close - even though leaves may not be experiencing water stress.

Key concepts

Guard Cell Function

Guard cells play a crucial role in regulating the exchange of gases and water vapor between plant leaves and the atmosphere. They are shaped like a pair of lips surrounding a stoma, which is a tiny pore on the leaf surface. When guard cells swell with water, they open the stoma, allowing carbon dioxide to enter for photosynthesis and oxygen to exit as a waste product. Conversely, during times of water scarcity, guard cells lose water and become flaccid, closing the stoma to prevent excessive water loss through transpiration.

The exquisite control of stoma opening and closing by guard cells is vital for plant survival. It helps optimize water use efficiency by balancing the need for CO₂ for photosynthesis with the prevention of dehydration. Thus, understanding how guard cells function is essential in studying plant biology and responses to environmental stress.

Plant Water Stress Response

Plants encounter water stress when water supply is limited or when the transpiration rate exceeds water uptake. To cope with water shortage, plants have developed an array of responses that help them survive. One such response involves the closing of stomata to reduce water loss. This is achieved through the signaling pathways that coordinate the guard cells' behavior.

When roots detect dry conditions, they signal to the aboveground parts of the plant. A sophisticated communication network ensures that even before the leaves experience direct water stress, mechanisms such as stoma closure are activated preemptively. This proactive response signifies the plant's ability to adapt to changing environmental conditions and is a testament to their evolutionary success in diverse habitats.

Abscisic Acid (ABA) Role

Abscisic acid (ABA) is a plant hormone known for its pivotal role in the regulation of plant water stress response. ABA is synthesized in response to drought and serves as a crucial signal in closing guard cells, hence reducing transpiration to conserve water.

The evidence is compelling that ABA produced in the roots during dry conditions is transported to the leaves where it binds to receptors on guard cells, initiating a signaling cascade leading to stoma closure. This hormone's role extends beyond managing water stress; it is involved in seed dormancy, germination, and other critical developmental processes. Thus, ABA is a central player in equipping plants with the ability to endure and adapt to environmental stresses.