Stomata are more than just holes in the leaf epidermis; they are hydromechanical valves that can open and close. Each stoma consists of two guard cells surrounding a central pore. The guard cells can shrink or swell, changing the size of the pore between the guard cells (Fig. 29.5b). How does this valve system work?

Cellulose consists of long molecules that provide strength to plant cell walls. In guard cells, the cellulose molecules are oriented radially—that is, wrapped around the cell. This makes it easier for swelling guard cells to expand in length rather than in girth. Because the guard cells are firmly connected at their ends, this increase in length causes them to bow apart, opening the stomatal pore. Conversely, a decrease in guard cell volume causes the pore to close.

Guard cells control their volume by altering the concentration of solutes, such as potassium ions (K+) and chloride ions (Cl–), in their cytoplasm. ATP drives the uptake of solutes across the plasma membrane against their concentration gradient. An increase in solute concentration causes water to flow into the cell by osmosis, whereas a decrease in solute concentration causes water to flow out of the cell. Recall from Chapter 5 that osmosis is the diffusion of water molecules across a selectively permeable membrane, such as the cell’s plasma membrane, from a region of higher water concentration to a region of lower water concentration. An increase in the concentration of solutes in guard cells is accompanied by a corresponding decrease in the concentration of water molecules. Thus, as guard cells add solutes, water diffuses into the cells, causing them to swell.

Stomata close by the same process, only in reverse. Solutes leave the guard cells, and as the solute concentration decreases, water diffuses out of the cells, causing them to shrink.

To function effectively, stomata must open and close in response to both CO₂ and water loss. Stomata are thus key sites for the processing of physiological information. Light stimulates stomata to open, while high levels of CO₂ inside the leaf (a signal that CO₂ is being supplied faster than photosynthesis can take it up) cause stomata to close. Guard cell volume also changes in response to signaling molecules. For example, abscisic acid, a hormone produced during drought, causes stomata to close. Thus, stomata open when the conditions for photosynthesis are favorable, and close when a water shortage endangers the hydration of the leaf.

Our own experience leads us to think of evaporation (for example, of sweat) as a means of preventing overheating. Is there any evidence that plants control the temperature of their leaves by regulating transpiration? The answer, perhaps surprisingly, is no. Although transpiration does cool leaves, plants transpire whether their leaves are too hot or too cold, or at their optimal temperature. When leaves are above their optimal temperature for photosynthesis, cooling through transpiration is beneficial. But when temperatures are below the optimum, transpiration results in leaf temperatures that are even less favorable. In addition, evaporative cooling is most beneficial in hot environments, yet often in these environments little water is available in the soil. For this reason, plants in arid regions tend to rely on mechanisms other than evaporative cooling, such as reflective hairs and waxes and small leaf size, to prevent their leaves from overheating.