Answer: Stomatal closure to reduce water loss.
- A Stomatal opening to fix more CO<sub>2</sub>
- B Stomatal closure to reduce water loss
- C Increased transpiration
- D Stomatal neutral response
Correct answer: B. Stomatal closure to reduce water loss
Explanation: ABA (drought hormone): binds to receptors on guard cells, triggers Ca<sup>2+</sup> signaling, K+ and water efflux, turgor decreases, stomata close.
Water can travel through cell walls (apoplast) or cell-to-cell through the cytoplasm (symplast) - but the Casparian strip at the endodermis blocks the apoplast route entirely, forcing everything through a selectively permeable membrane (symplast) before it can enter the xylem.
Concept context
Water and mineral absorption, the ascent of sap, transpiration, and phloem transport via the pressure flow hypothesis.