Abscisic acid (ABA) is a plant hormone that controls the response of plants to environmental stresses such as shortage of water or high temperatures by closing stomata.

Stomata are holes on the surface of leaves, green stems, and flowers that allow the diffusion of gases in and out of the plant.

Guard cells surround the stomata and control their opening and closing, controlling the rate of diffusion.

1. Outer wall for structural support, with a waxy cell wall that prevents water loss
2. Elastic inner walls for flexibility
3. The inner wall is thicker than the outer wall, which allows the cells to curve and open the pore
4. Many mitochondria release energy that drives opening and closing of the pore
5. Membrane folds that increase surface area for ion transport
6. Many chloroplasts for photosynthesis within guard cells
7. Starch grains that adjust in size to aid in osmotic balance
8. Multiple small vacuoles for efficient compartmentalisation and osmoregulation

When stomata open, plants gain carbon dioxide so the rate of photosynthesis increases. They also lose more water via transpiration.

When stomata close, the supply of carbon dioxide decreases so the rate of photosynthesis decreases. The rate of transpiration also decreases.

1. Darkness
2. High carbon dioxide concentrations
3. Low humidity
4. High temperature
5. Water stress

1. ATP-powered proton pumps stop transporting hydrogen ions (H⁺) out of the guard cells
2. Potassium ion (K⁺) diffuse out of guard cells down an electrochemical gradient
3. Malate ions are transported into chloroplasts and converted into starch
4. This creates a high water potential inside the cell and water leaves the cell by osmosis
5. Lower water volume inside the guard cells decreases the turgor pressure
6. The guard cells become flaccid and the stoma closes

1. ATP-powered proton pumps actively transport hydrogen ions (H⁺) out of the guard cells
2. Potassium ion (K⁺) channels open and K⁺ diffuses into guard cells down an electrochemical gradient
3. This creates a low water potential inside the cell and water enters the cell by osmosis
4. Increased water volume inside the guard cells increases the turgor pressure
5. The guard cells expand, especially the outer wall, causing the cells to curve apart
6. The stoma opens

1. ABA stimulates the movement of calcium ions (Ca²⁺) into the cytoplasm and vacuoles of guard cells
2. Ca²⁺ stimulates negatively charged malate ions and potassium ions (K⁺) to leave guard cells
3. Water potential of the guard cells increases and water leaves the cells by osmosis
4. Guard cells become flaccid and stoma closes, reducing loss of water vapour

Starch in guard cells is broken down to form negatively charged malate ions.

The malate ions enter vacuoles, and help to maintain electrical balance and contribute to reducing the water potential of the guard cells during stomatal opening.

Cellulose microfibrils prevent the expansion of the cell in all directions, directing cells to increase in length, not diameter.

This means that the outer wall stretches more than the inner walls when guard cells become turgid, causing the guard cells to curve apart and open the stoma.