Stomata control water loss primarily by opening and closing their pores. This action regulates the rate of transpiration, the process by which water evaporates from the plant's leaves.
The Mechanism: Opening and Closing
- Guard Cells: Stomata are bordered by specialized cells called guard cells. These cells regulate the opening and closing of the stomatal pore.
- Turgor Pressure: The turgor pressure (water pressure) within the guard cells is the key to their function.
- Opening: When water is plentiful, the guard cells become turgid (swollen with water). This increased turgor pressure causes them to bow outwards, creating an opening – the stoma.
- Closing: When water is scarce or environmental conditions demand it, the guard cells lose turgor pressure, becoming flaccid. They then collapse against each other, closing the stoma and reducing water loss.
Factors Influencing Stomatal Movement
Several factors influence the opening and closing of stomata:
- Light: Generally, stomata open in the presence of light to allow for photosynthesis.
- Carbon Dioxide Concentration: High concentrations of carbon dioxide inside the leaf often cause stomata to close.
- Water Availability: As mentioned above, water availability directly affects turgor pressure in the guard cells. Drought conditions trigger stomatal closure.
- Hormones: The plant hormone abscisic acid (ABA) plays a critical role in stomatal closure during water stress.
- Temperature: High temperatures can also trigger stomatal closure to conserve water.
Stomatal Distribution and Adaptation
The number and location of stomata vary depending on the plant species and its environment.
- Xerophytes: Plants adapted to dry environments (xerophytes) often have fewer stomata, sunken stomata (located in pits to reduce water loss), and/or stomata concentrated on the lower leaf surface (which is cooler and shaded) to minimize water loss. For example, many desert plants have these adaptations.
- Mesophytes: Plants that grow in moderately moist environments (mesophytes) typically have more stomata distributed on both leaf surfaces.
- Hydrophytes: Plants adapted to aquatic environments (hydrophytes) often have stomata only on their upper leaf surfaces.
Summary
Stomata are critical regulators of gas exchange and water balance in plants. They minimize water loss by opening and closing based on environmental cues and internal signals, allowing plants to thrive in various habitats. This delicate control is essential for plant survival and productivity.
