Stomata close primarily due to a decrease in the turgor pressure of the guard cells that surround them, causing the cells to become flaccid and the pore to narrow or close entirely. This process is triggered by various environmental and hormonal signals.
Here's a breakdown of the process:
1. Loss of Turgor Pressure in Guard Cells
The closing of stomata hinges on the reduction of turgor pressure (internal water pressure) within the guard cells. Turgor pressure is what keeps the guard cells swollen and the stoma open.
2. Mechanisms Leading to Turgor Loss
Several factors can cause the guard cells to lose turgor:
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Water Stress: When a plant experiences water stress (drought), the hormone abscisic acid (ABA) is produced. ABA triggers a cascade of events within the guard cells, ultimately leading to the efflux (exit) of ions, particularly potassium (K+), chloride (Cl-), and malate, from the guard cells.
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Ion Efflux: The exit of these ions reduces the solute concentration inside the guard cells.
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Osmosis: Due to the lower solute concentration in the guard cells, water moves out of the guard cells and into the surrounding epidermal cells via osmosis (the movement of water across a semi-permeable membrane from an area of high water concentration to an area of low water concentration).
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Conversion of Sugar to Starch: As indicated by the reference, in the absence of light, photosynthesis stops. The sugar (glucose) present in guard cells is converted into starch. Starch is insoluble and osmotically inactive (meaning it doesn't contribute to solute concentration). This lowers the osmotic potential within the guard cells, promoting water loss.
3. Guard Cells Become Flaccid
As water moves out of the guard cells, they lose turgor and become flaccid (limp). The shape of the guard cells changes, causing them to sag inward.
4. Stomatal Closure
The inward sagging of the guard cells reduces the size of the stomatal pore, eventually leading to complete closure. This closure prevents further water loss from the plant through transpiration (evaporation of water from plant leaves).
Summary Table: Stomatal Closing Process
| Step | Description | Triggering Factors |
|---|---|---|
| 1. Turgor Loss | Guard cells lose internal water pressure. | Water stress, darkness, high CO2 levels, ABA signaling |
| 2. Ion Efflux | Potassium (K+), Chloride (Cl-), and Malate ions exit the guard cells. | ABA signaling, calcium influx |
| 3. Osmosis | Water moves out of the guard cells due to lower solute concentration inside and higher solute outside. | Lower solute concentration in guard cells compared to surrounding cells |
| 4. Starch Conversion | Sugar converts to starch when no light is available | Lack of light for photosynthesis |
| 5. Guard Cell Flaccidity | Guard cells become limp and sag inwards. | Reduced turgor pressure |
| 6. Stomatal Closure | The stomatal pore closes, reducing transpiration. | Shape change of flaccid guard cells |
In summary, stomata close due to a reduction in the turgor pressure of the guard cells caused by a combination of hormonal signals, ion efflux, osmosis, and the conversion of sugars to starch, leading to water loss and flaccidity.
