Stomata open due to the active control of specialized cells called guard cells.
The Role of Guard Cells
A pair of guard cells surrounds each stoma, the tiny pore on the surface of leaves. These guard cells are responsible for regulating the size of the stomatal pore, controlling the exchange of gases (like carbon dioxide and oxygen) and water vapor between the plant and its environment. [Reference: A pair of guard cells surrounds each stoma, and these cells control the opening and closing of the stomatal pore between them. Guard cells regulate this opening and closing in response to a wide variety of environmental signals, such as day/night rhythms, CO2 availability, and temperature.]
The Mechanism of Opening
The opening and closing of stomata is a complex process involving changes in turgor pressure (water pressure) within the guard cells. When guard cells take in water, they become turgid, expanding and curving, thus opening the stoma. Conversely, when guard cells lose water, they become flaccid, causing the stoma to close. This turgor pressure change is influenced by several factors:
- Light: Light triggers photosynthesis, which leads to increased sugar concentrations in the guard cells, drawing in water and causing them to swell.
- Carbon Dioxide Levels: Low CO2 levels inside the leaf stimulate stomata to open to take in more CO2 for photosynthesis.
- Temperature: Temperature affects the rate of transpiration (water loss) and influences stomata opening to maintain an optimal water balance.
These factors influence the transport of ions (like potassium) into and out of the guard cells, directly affecting water movement and thus turgor pressure.
Environmental Signals and Stomatal Regulation
Stomatal opening and closing are not simply mechanical responses. They're precisely regulated responses to environmental cues:
- Day/night cycles: Stomata typically open during the day when sunlight is available for photosynthesis and close at night to conserve water.
- CO2 availability: The plant monitors internal CO2 levels. If levels are low, stomata open to allow for more CO2 uptake.
- Temperature: High temperatures increase transpiration; plants may close their stomata to prevent excessive water loss.
