14.10 Homeostasis in Plants
Definition:
- Homeostasis in plants involves maintaining a stable internal environment to support essential processes like water balance, gas exchange, and photosynthesis.
1. Importance of Stomata in Plant Homeostasis
- Stomata: Small pores mainly found on the underside of leaves.
- Function: Regulate gas exchange (CO₂ in, O₂ out) and control water vapor loss (transpiration).
- Guard Cells: Specialized cells that surround each stoma and control its opening and closing based on environmental conditions.
2. Structure of Guard Cells
- Kidney Shape: Allows guard cells to bend outward when they swell, opening the stoma.
- Cell Wall Adaptations:
- Thicker Inner Walls: Help guard cells expand in a way that opens the pore.
- Thinner Outer Walls: Enable bending and movement.
- Cellulose Microfibrils: Arranged in bands, directing expansion lengthwise to control stomatal opening.
3. Mechanism of Stomatal Opening and Closing
Stomatal Opening (in light conditions)
- Proton Pump Activation:
- Light activates proton (H⁺) pumps in the guard cell membrane, moving H⁺ ions out of the cell.
- This creates a negative charge inside the cell.
- Potassium Ion (K⁺) Influx:
- The negative charge inside opens K⁺ channels, allowing K⁺ ions to flow into guard cells, lowering water potential.
- Water Movement by Osmosis:
- Water enters guard cells by osmosis due to the lower water potential, increasing turgor pressure.
- Cell Expansion:
- Guard cells swell outward (due to their structural adaptations), opening the stoma for gas exchange.
Stomatal Closing (in darkness or water stress)
- Ion Movement Reversal:
- Proton pumps stop, K⁺ ions exit guard cells, raising water potential.
- Water Loss:
- Water exits guard cells by osmosis, decreasing turgor pressure.
- Cell Flaccidity:
- Guard cells become flaccid, closing the stoma to conserve water.
Diurnal Rhythm
- Day: Stomata typically open to allow CO₂ for photosynthesis.
- Night: Stomata close to conserve water, as photosynthesis cannot occur in the dark.
4. Role of Abscisic Acid (ABA) in Stomatal Regulation
ABA: A plant hormone produced in response to stress, especially during drought or high temperatures.
- Function: Signals guard cells to close stomata to reduce water loss.
- Mechanism:
- ABA Binding: ABA binds to receptors on guard cells.
- Calcium Ion (Ca²⁺) Influx: Acts as a second messenger, prompting K⁺ and Cl⁻ ions to exit the cell.
- Increased Water Potential: Water exits guard cells by osmosis, causing flaccidity and stomatal closure.
5. Key Concepts in Stomatal Regulation
- Proton Pump: Moves H⁺ ions out of guard cells to initiate stomatal opening.
- Electrochemical Gradient: Negative charge inside the cell attracts K⁺ ions.
- Water Potential: Lowered by ion influx, allowing water to enter by osmosis.
- Turgor Pressure: Increased by water intake, causing guard cells to expand and open the stoma.
6. Environmental Adaptations of Stomatal Behaviour
- Light and CO₂: Stomata open when light is present and CO₂ is low, optimizing photosynthesis.
- Water Stress: Stomata close in response to high temperatures or drought to conserve water.
- Diurnal Patterns: Daytime opening for CO₂ intake; nighttime closure to reduce water loss.
7. Key Terms
- Guard Cell: Surrounds each stoma and controls its opening and closing.
- Electrochemical Gradient: Created by proton pumps, aiding K⁺ entry into guard cells.
- Abscisic Acid (ABA): Hormone that promotes stomatal closure during water stress.
Questions for Review
Answer: Ion influx lowers water potential, causing water to enter by osmosis, which increases turgor pressure and opens the stoma.
Explain how stomata adapt to environmental changes.
Answer: Stomata open in response to light and low CO₂ to support photosynthesis. They close under water stress or at night to conserve moisture.
Describe how ABA functions during water stress.
Answer: ABA binds to guard cell receptors, inhibits H⁺ pumps, triggers ion exit, raises water potential, and causes water loss, leading to stomatal closure.