During photosynthesis, plants incorporate atmospheric CO2 into carbohydrates using the energy of sunlight. CO2 enters the leaf through pore-like openings located predominately in the lower epidermis of the leaf called stomata. The sizes of the stomata are directly controlled through regulating the turgor pressure of the surrounding cells, known as guard cells. By inflating and deflating the guard cells, the plant can optimize the balance between CO2 uptake and water loss in real time as environmental conditions change.
Stomata open when the pair of guard cells is fully turgid and pressing firmly against each other. Although this seems counterintuitive, the pressurized condition results in open stomata because of the unique anatomy of these specialized cells. In particular, the interior walls of the guard cells are heavily reinforced with cellulose, making them bow open when stressed by the influx of water. But what causes this influx of water? As usual, water flows in response to the water potential gradient.
In sunny conditions, blue light stimulates a H+-ATPase in the guard cell plasma membrane, which hydrolyzes ATP and pumps H+ out of the cell. The outward flow of H+ results in the inside of the cell becoming more and more negatively charged. This hyperpolarization leads to the activation of an inward-rectifying K+ channel, resulting in the influx of K+ ions. This influx of K+ activates a H+/Cl- symporter, which imports Cl- ions. The net uptake of both K+ and Cl- significantly reduces the water potential, resulting in the influx of water from the apoplast.
Stomata close in response to a number of different signals. One such signal is water stress, usually signaled through the presence of the drought hormone, abscisic acid (ABA). The perception of ABA in the guard cell leads to the release of intracellular Ca++, which triggers the opening of a plasma membrane anion channel. This allows the efflux of Cl- ions, leading to the depolarization of the membrane. Upon membrane depolarization, an outward-rectifying K+ channel opens and allows the escape of K+. The loss of both K+ and Cl- increases the water potential of the guard cell, leading to water efflux.
One important question is, where are the large supplies of ions and water coming from that lead to stomatal opening, and where do these go when stomata close? Guard cells do not exist in isolation, but are found situated in the epidermis next to companion cells. As the concentration of ions is increasing in guard cells, those ions are being supplied by companion cells. Likewise, the ions that flow out of guard cells undergoing stomatal closing are taken up by the companion cells.