The opening and closing of stomata are regulated by several environmental factors, including light, CO₂ levels, temperature, and water availability. Plants can also change the total number of stomata in response to longer-term changes in environmental conditions.

LIGHT AND CO₂ CONCENTRATION Guard cells can respond to changes in light and CO₂ concentration in a matter of minutes by changing their solute potential. The absorption of light by a pigment in the guard cell’s cell membrane activates a proton pump (see Figure 34.4), which actively transports H⁺ out of the guard cells and into the apoplast of the surrounding epidermis. The resulting electrochemical gradient drives K⁺ into the guard cells, where it accumulates (Figure 34.8B). Negatively charged chloride (Cl^–) ions and organic ions also move into and out of the guard cells along with the K⁺ ions, maintaining electrical balance. The increased concentration of K⁺ and other solutes inside the guard cells makes the solute potential of the guard cells more negative. Water then enters by osmosis, increasing the turgor pressure of the guard cells. The guard cells change their shape, becoming more turgid in response to the increase in pressure potential, so that a space—the stoma—appears between them. The stoma closes in the absence of light: the proton pump becomes less active, K⁺ ions diffuse passively out of the guard cells, water follows by osmosis, the pressure potential decreases, and the guard cells sag together and seal off the stoma. Guard cell membranes are particularly rich in aquaporins, making guard cells well adapted for the rapid water movements involved in stomatal responses.

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WATER Stomata also respond to water availability. Water stress is a common problem for plants, especially on a hot, windy day, when plants might close their stomata even when the sun is shining. The water potential of the leaf’s mesophyll cells is the cue for this response. If the mesophyll becomes dehydrated, its cells release the hormone abscisic acid, which causes the stomata to close.

STOMATA NUMBER A plant can regulate water loss not just by the opening or closing of stomata, but by changing the number of stomata. The process of controlling the number of stomata takes place over days or weeks. Trees, for example, can reduce stomata numbers by shedding leaves, or by making new leaves that have few stomata.