Guard cells directly regulate transpiration by controlling the opening and closing of stomata, the pores through which water vapor exits the plant.
Transpiration is the process where water moves through a plant and evaporates from aerial parts, such as leaves, stems, and flowers. This process is essential for nutrient transport and cooling. The stomata, tiny pores primarily on the undersides of leaves, are the main sites of water vapor exit. Guard cells are specialized cells that surround each stoma, and their turgor pressure (internal water pressure) determines whether the stoma is open or closed.
Mechanism of Guard Cell Action
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Stomata Opening: When guard cells are turgid (swollen with water), they bend outwards, opening the stoma. This typically occurs when the plant is well-hydrated and environmental conditions are favorable (e.g., sufficient light). Light stimulates guard cells to accumulate potassium ions (K+), chloride ions (Cl-), and malate. This increases the solute concentration inside the guard cells, causing water to enter by osmosis, leading to increased turgor pressure.
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Stomata Closing: When guard cells lose water and become flaccid, they straighten, closing the stoma. This happens when the plant is water-stressed (dehydrated) or under unfavorable environmental conditions (e.g., darkness, high temperatures). The loss of potassium, chloride, and malate from guard cells reduces the solute concentration. Water then exits by osmosis, decreasing the turgor pressure and causing the stomata to close.
Factors Affecting Guard Cell Function and Transpiration
| Factor | Effect on Stomata | Impact on Transpiration |
|---|---|---|
| Light Intensity | Open | Increased |
| Water Availability | Open (if plentiful), Closed (if scarce) | Increased (if open), Decreased (if closed) |
| Temperature | Open (moderate), Closed (high) | Increased (if open), Decreased (if closed) |
| CO2 Concentration | Closed (if high) | Decreased |
Summary
In essence, guard cells act as gatekeepers, strategically opening and closing stomata to balance the plant's need for carbon dioxide (for photosynthesis) with the need to conserve water. By responding to various environmental signals, guard cells play a critical role in regulating the rate of transpiration.
