Stomatal physiology — What happens to stomatal aperture when guard cells swell (become turgid)?

Introduction / Context:
Stomata regulate gas exchange and water loss. Guard cell turgor changes are central to stomatal opening and closing, integrating environmental and hormonal signals such as light, CO2, humidity, and abscisic acid (ABA).

Given Data / Assumptions:

• Guard cells form a pore whose width depends on their turgor pressure.
• K+, Cl−, and malate^2− fluxes alter osmotic status of guard cells.
• Cell wall architecture of guard cells promotes bending when turgor rises.

Concept / Approach:
Blue light and low internal CO2 promote ion uptake into guard cells. Water follows osmotically, increasing turgor pressure. Due to differential wall thickening and cellulose microfibril orientation, turgid guard cells bow outward, widening the pore and opening the stomata.

Step-by-Step Solution:

Verification / Alternative check:
Direct microscopy shows aperture increases with guard cell turgor; ABA induces ion efflux, water loss, and closing, confirming the mechanism is reversible and ion-driven.

Why Other Options Are Wrong:

• Closing occurs when turgor drops, not when it rises.
• Photosynthesis does not stop upon opening; CO2 entry facilitates it.
• Transpiration does not cease; it typically increases when stomata open.
• Plasmolysis is loss of turgor, opposite of swelling.

Common Pitfalls:
Assuming swelling always closes pores. Due to guard cell wall anatomy, swelling opens the pore; shrinking closes it.

Final Answer:
Stomata open