Biology – Homeostasis in plants | e-Consult

Stomata are pores on the surface of plant leaves that facilitate gas exchange – the uptake of carbon dioxide (CO2) for photosynthesis and the release of oxygen (O2) and water vapour. The opening and closing of stomata are crucial for balancing these processes. The process is primarily controlled by the guard cells, which are specialized cells that surround each stoma.

Guard Cell Mechanism:

• Turgor Pressure: Guard cells are turgid (swollen with water) when the stomata are open and flaccid (limp) when they are closed.
• Ion Transport: The opening and closing of stomata are regulated by the movement of ions (primarily potassium ions, K+) into and out of the guard cells.
• Increased Turgor (Stomata Open): When K+ ions enter the guard cells, it increases their solute concentration. This causes water to move into the guard cells by osmosis, increasing their turgor pressure. The increased turgor pressure causes the guard cells to become more curved, opening the stoma.
• Decreased Turgor (Stomata Closed): When K+ ions leave the guard cells, the solute concentration decreases, and water moves out by osmosis. This reduces turgor pressure, causing the guard cells to become less curved, closing the stoma.

Factors Influencing Guard Cell Turgor:

• Light: Light stimulates the production of a plant hormone called abscisic acid (ABA). ABA promotes the efflux of potassium ions from the guard cells, leading to stomatal closure.
• Carbon Dioxide Concentration: High concentrations of CO2 inside the leaf tend to close the stomata. This is because the plant doesn't need to take in as much CO2.
• Water Availability: When water is scarce, the plant produces ABA, which causes stomata to close to reduce water loss.
• Temperature: High temperatures can also trigger ABA production, leading to stomatal closure.
• Humidity: Low humidity increases the rate of transpiration, prompting stomatal closure to conserve water.