3 8.3 Stomatal Conductance and Leaves

Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat. For example, the leaves of plants growing in tropical rainforests usually have larger surface areas than those of plants growing in deserts or very cold conditions (which are likely to have a smaller surface area to minimize water loss).

The Stomata

The dermal tissue of the stem consists primarily of epidermis, a single layer of cells covering and protecting the underlying tissue. The epidermis, or dermal tissue, of a leaf contains openings known as stomata, through which the exchange of gases takes place (Figure 30.8). Two cells, known as guard cells, surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor.

Figure  30.8   Openings called stomata (singular: stoma) allow a plant to take up carbon dioxide and release oxygen and water vapor. The (a) colorized scanning-electron micrograph shows a closed stoma of a dicot. Each stoma is flanked by two guard cells that regulate its (b) opening and closing. The (c) guard cells sit within the layer of epidermal cells. (credit a: modification of work by Louisa Howard, Rippel Electron Microscope Facility, Dartmouth College; credit b: modification of work by June Kwak, University of Maryland; scale-bar data from Matt Russell)

Stomatal Conductance

Regulation of transpiration is achieved primarily through the opening and closing of stomata on the leaf surface. Guard cells work to control stomatal conductance via opening and closing in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration.

A Battle Between Needs

There is a constant tension between opening a plant’s stomata to take in carbon dioxide (CO₂) for photosynthesis and the risk of losing water through the same openings via transpiration. When stomata open, CO₂ diffuses into the leaf, enabling sugar production. This is essential for plant processes and growth. But, when this occurs, water vapor escapes into the atmosphere. To balance exchange, plants regulate stomatal opening based on environmental conditions. For example, plants might close their stomata to conserve water during drought, even if it limits photosynthesis.