Step 1: Understanding the Concept:
Guard cells around each stoma change shape when their internal water content changes. When guard cells take up water and become turgid, they bulge outward and pull open the pore between them. When they lose water and go flaccid, the pore closes. So the real question is what makes water move into the guard cells in the first place.
Step 2: Key Approach:
Guard cells cannot pull in water on their own; water follows solutes by osmosis. The main solute that guard cells pump in during the day is the potassium ion, K+. Once K+ builds up inside the guard cells, the cell's solute concentration rises, water follows by osmosis, and the cell swells.
Step 3: Detailed Explanation:
Light falling on guard cells switches on proton pumps in the plasma membrane. These pumps push H+ ions out of the cell, and this outward H+ movement drives K+ channels to open and let K+ move in, not out. So the correct direction of ion flow is an influx of K+, not an efflux. Checking the other choices: efflux of K+ is what happens when stomata close at night or under water stress, the opposite of opening. H+ ions move out, not in, so influx of H+ does not fit. Fe+ ions play no known part in guard cell osmotic regulation at all.
Step 4: Final Answer:
Only an influx of K+ ions raises guard cell turgor enough to open the stomatal pore, so option 3 is correct.