Answer: Ca 2+ as second messenger activating K+ efflux channels and anion channels, causing turgor loss.
- A K+ entering guard cells through inward-rectifying channels to raise turgor
- B Ca<sup>2+</sup> as second messenger activating K+ efflux channels and anion channels, causing turgor loss
- C Selective inhibition of plasma membrane H+ ATPase with no change in ion channel activity
- D Water diffusing directly into the guard cell wall matrix independent of osmosis
Correct answer: B. Ca<sup>2+</sup> as second messenger activating K+ efflux channels and anion channels, causing turgor loss
Explanation: ABA binds receptors (PYR/PYL), inactivating PP2C phosphatases. This activates SnRK2 kinases, increasing cytosolic Ca<sup>2+</sup>. Ca<sup>2+</sup> activates anion channels (Cl- efflux) and inhibits K+ influx, reducing turgor.
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.