Regulation of Transpiration by Stomata

Plants need enough carbon dioxide from the atmosphere to carry out photosynthesis. The surfaces of plant leaves contain openings that help with gas exchange. These openings are called stomata.

Sunlight is one of the primary signals that promote stomatal opening. This allows carbon dioxide to enter the leaf when it is needed for photosynthesis. Oxygen is a byproduct of photosynthesis and escapes into the atmosphere through the stomata.

A pair of guard cells controls each stomatal opening. These specialized cells swell when they take in water from neighboring cells by osmosis. This creates an opening that allows gas exchange. When water leaves the guard cells, they shrink and the stoma closes.

The concentration of ions affects the amount of water in the guard cells. Sunlight stimulates guard cells to take up potassium ions. The increase in potassium concentration drives water into the cells, opening the stoma.

When potassium leaves the guard cells, water follows by osmosis. The now-flaccid guard cells close the stoma.

Open stomata help with gas exchange, but they also allow water to escape from leaves through evaporation. This evaporative water loss, or transpiration, allows water to move long distances through the plant.

Transpiration is usually greatest on warm, sunny days. However, if a plant cannot get enough water, its stomata close quickly to prevent wilting.

Interestingly, even when kept in the dark, many plants open and close their stomata in a roughly 24-hour rhythm because of an internal clock.

The opening and closing of stomata are tightly controlled, allowing plants to respond to specific environmental conditions. As gatekeepers, stomata efficiently balance gas exchange and transpiration.