4.5.1.2.2: Stomatal Opening and Closure

Learning Objectives

• Relate the pattern of cell wall thickening in guard cells to their function.
• Explain the mechanism by which blue light triggers stomatal opening.
• Explain the mechanism by which water stress, signaled by abscisic acid, triggers stomatal closure.

Regulation of transpiration is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells (Figure \(\PageIndex{1}\)). Stomata must open to allow the gas exchange of carbon dioxide and oxygen for efficient photosynthesis (see Photorespiration), and light thus typically triggers stomatal opening. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between gas exchange and water loss. Water stress, high temperatures, and high carbon dioxide concentration causes stomata to close.

Figure \(\PageIndex{1}\): Italian chicory leaf epidermis showing stomata. The epidermal cells are shaped like puzzle pieces. The stomata (singular = stoma) are pores in the epidermis. Each is bordered by two guard cells, which are filled with oval, green chloroplasts. Image by Umberto Salvagnin (CC-BY).

Stomatal Opening

Guard cell walls are radially thickened such that the thickenings are concentrated around the stoma (plural: stomata; Figure \(\PageIndex{2}\)). When turgor pressure increases in guard cells, the cells swell. However, the thickened inner walls near the stoma cannot expand, so they curve to accommodate the expanding outer walls. The curving of the guard cells opens the stoma.

Figure \(\PageIndex{2}\): The guard cells that surround the stoma have radial cell wall thickenings (represented by solid black lines). The portion of the guard cell adjacent to the stoma (ventral side) has a fully thickened cell wall. The outer portion (dorsal side) has alternating bands of thick and thin cell wall. Image modified from Vojtech.dostal (public domain).

How does light cause stomata to open? Phototropins detect blue light, causing a proton pumps to export protons (H⁺). ATP, generated by the light reactions of photosynthesis, drives the pump. The cytosol usually more negative than the extracellular solution, and this difference in charge (membrane potential) increases as protons leave the cell. This increase in membrane potential is called hyperpolarization, and it causes potassium (K⁺) to move down its electrochemical gradient into the cytosol. Protons also move down their electrochemical gradient back into the cytosol, bringing chloride (Cl^-) with them through symport channels. Meanwhile, starch is broken down, producing sucrose and malate. Nitrate (NO₃^-) also enters the cell. The solute potential resulting high concentrations of potassium, chloride, sucrose, malate, and nitrate in the cytosol drives the osmosis of water into the the guard cells. This increases turgor pressure, and the guard cells expand and bend, opening the stoma (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Phototropins respond to blue light and signal to the proton pump to export protons. This active transport is fueled by the ATP produced in the light-dependent reactions of photosynthesis. This causes the cell to become hyperpolarized, stimulating an influx of potassium ions. At the same time, chloride is symported into the guard cell with protons as they reenter the cell. Nitrate (NO₃^- ) also enters the cell. Starch breaks down, producing sucrose and malate. These, along with the influx of ions, increases the solute concentration inside of the guard cells, driving water into the cells. This increases turgor pressure and causes the guard cells to expand. Due to their radial cell wall thickenings, the guard cells curve when they expand, opening the stoma (plural: stomata). Image by Jen Valenzuela (CC-BY-NC).

Table \(\PageIndex{1}\) illustrates how osmotic pressure (which results in turgor pressure) increases with light availability during the day. When the osmotic pressure of the guard cells became greater than that of the surrounding cells, the stomata opened. In the evening, when the osmotic pressure of the guard cells dropped to nearly that of the surrounding cells, the stomata closed.

Table \(\PageIndex{1}\): Osmotic pressure measured at different times of day in typical guard cells. The osmotic pressure within the other cells of the lower epidermis remained constant at ~1 MPa.

Time	Osmotic Pressure (MPa)
7 A.M.	1.46
11 A.M.	3.14
5 P.M.	1.88
12 Midnight	1.32

Stomatal Closure

When water is low, roots synthesize abscisic acid (ABA), which is transported through the xylem to the leaves. There, abscisic acid causes calcium channels to open. Calcium (Ca²⁺) opens anion channels, and malate, chloride, and nitrate exit the cell. The membrane potential decreases (the difference in charge across the membrane becomes less pronounced) as anions leave the cell. Potassium exits the cell in response to this decrease in membrane potential (called depolarization). The loss of these solutes in the cytosol results in water leaving the cell and a decrease in turgor pressure. The guard cells regain their original shape, and the stoma closes (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): Stomatal closure is triggered by abscisic acid (ABA), which causes calcium (Ca²⁺) ions to enter the cell. These open anion channels. At this point, the cytoplasm is not as negatively charged as it was before. The change in charge opens potassium (K⁺) channels, and potassium leaves the cell as well. Water leaves the cells, causing them to loose turgor pressure. The stoma then closes. Image modified from June Kwak (public domain).

Attributions

Curated and authored by Melissa Ha using the following sources:

• 16.2D Gas Exchange from Biology by John. W. Kimball (licensed CC-BY)
• 30.2 Stems and 30.5 Transport of Water and Solutes in Plants from Biology 2e by OpenStax (licensed CC-BY). Access for free at openstax.org.