14.2.1—Stomata response to environmental changes

Stomata regulate the exchange of gases between a leaf and the air. Opening allows carbon dioxide to diffuse in for photosynthesis, but it also provides a route for water vapour to leave by transpiration.

Environmental response:

• Increasing light intensity or low carbon dioxide concentration inside the leaf tends to favour opening because photosynthetic carbon supply is useful.
• Darkness or high internal carbon dioxide concentration reduces the benefit of opening.
• Low humidity, high temperature, limited water supply or high transpiration increase the cost of opening and can favour closure.

The response is a balance, not a single-trigger rule. Open stomata improve carbon-dioxide entry but increase water loss; closed stomata conserve water but restrict carbon-dioxide diffusion, so photosynthesis may decrease.

Light alone does not determine aperture, and a stoma is not “good” or “bad” in isolation. The outcome depends on carbon-dioxide demand and the leaf’s water-loss conditions. Guard-cell structure and turgor mechanics, daily rhythms, and ABA signalling are separate learning steps.