34.14:
Regulation of Transpiration by Stomata
Plants require ample carbon dioxide from the atmosphere to conduct photosynthesis. The surfaces of plant leaves contain openings that facilitate gas exchange. These openings are called stomata.
Sunlight triggers the opening of stomata, allowing carbon dioxide to enter the leaf when it is needed for photosynthesis. Oxygen is a byproduct of photosynthesis and escapes into the atmosphere through the stomata.
A pair of guard cells regulates each stomatal opening. These specialized cells swell when they take in water from neighboring cells via osmosis, creating an opening that enables gas exchange. When water leaves the guard cells, they shrink and the stoma closes.
The concentration of ions influences the amount of water in the guard cells. Sunlight stimulates guard cells to take up potassium ions. The increase in the potassium concentration drives water into the cells, thereby opening the stoma.
When potassium leaves the guard cells, water follows via osmosis. The now flaccid guard cells close the stoma.
While open stomata facilitate gas exchange, they also allow water to escape from leaves through evaporation. The evaporative water loss—or transpiration—allows the long-distance movement of water through the plant.
Transpiration is typically greatest on warm and sunny days. However, if a plant cannot acquire sufficient water, its stomata will rapidly close to prevent wilting.
Interestingly, even if kept in the dark, plants will open and close stomata on a regular, 24-hour cycle, due to an internal clock.
The opening and closing of stomata are tightly regulated, allowing plants to respond to specific environmental conditions. In their function as gate-keepers, stomata efficiently balance gas exchange and transpiration.
34.14:
Regulation of Transpiration by Stomata
During photosynthesis, plants acquire the necessary carbon dioxide and release the produced oxygen back into the atmosphere. Openings in the epidermis of plant leaves is the site of this exchange of gasses. A single opening is called a stoma—derived from the Greek word for “mouth.” Stomata open and close in response to a variety of environmental cues.
Each stoma is flanked by two specialized guard cells that create an opening when these cells take up water. The transport of ions regulates the amount of water in guard cells. When trigger, pumps translocate hydrogen ions out of the guard cell. This hyperpolarization of the membrane causes voltage-gated potassium channels to open and allow solutes, such as potassium ions and sucrose, to enter the guard cells. The increased concentration of solutes drives water into the guard cells, which accumulates in the vacuole. As a result, the guard cells bow and deform into a kidney shape, creating the stoma opening. When solutes leave guard cells, water follows, resulting in guard cell shrinkage, and closure of the opening.
A variety of environmental and internal signals triggers stomata opening. For example, blue light activates light-sensitive receptors on the cell surface that initiate a molecular cascade leading to stomata opening. In addition, when the concentration of carbon dioxide falls within the leaf tissue, stomata opening is induced so cells can access this critical reactant of photosynthesis.
Loss of water vapor is critical for the establishment of transpirational pull: water evaporates on the surface of mesophyll cells and escapes into the atmosphere through open stomata. The water loss creates a transpirational pull that pulls additional water from the soil into the roots and all the way into the leaves.
When sufficient water is not available, as in conditions of drought, stomata close. The hormone abscisic acid (ABA) is important in this process, binding to receptors on guard cell membranes and increasing intracellular solute concentration. ABA is also important in circadian control of stomatal opening, causing more stomata to be open in daylight, and closed in the dark.
Suggested Reading
Daszkowska-Golec, Agata, and Iwona Szarejko. “Open or Close the Gate – Stomata Action Under the Control of Phytohormones in Drought Stress Conditions.” Frontiers in Plant Science 4 (2013). [Source]
Inoue, Shin-ichiro, and Toshinori Kinoshita. “Blue Light Regulation of Stomatal Opening and the Plasma Membrane H+-ATPase1[OPEN].” Plant Physiology 174, no. 2 (June 2017): 531–38. [Source].