Stomata

Let’s do a breathing exercise- take a deep breath in and a deep breath out. Then, do it a few more times. Well done. You’ve breathed out some carbon dioxide and in some oxygen. A plant’s stomata do a similar job, except they take in carbon dioxide for the plant and expel oxygen. Stomata are pores on the leaf surface that allow for gas exchange and help to control water loss.

The definition of stomata in biology

In particular, a plant takes in carbon dioxide (CO₂) through its stomata and expels oxygen (O₂), a byproduct of photosynthesis. Stomatal openings are found in the epidermis of the plant or, in other words, the plant's dermal tissue.

Stomata are often found on the surfaces of leaves and some stems. Leaves, being the main site of photosynthesis, must have access to carbon dioxide. Stomata allow for this intake, thus making them an important addition to the leaf surface.

How did stomata evolve?

Stomata are an important stage in the evolution of plants.

Early land plants evolving from aquatic species had to face the biggest challenge: how to not dry out in a terrestrial environment. As a result, plants evolved waxy cuticles that helped reduce the amount of water that could be lost as water vapor through the plant. However, these cuticles also prevented gases from diffusing across the plants' membranes for photosynthesis. What was the solution? Stomata, of course!

Stomata allowed plants to control gas exchange between their membranes and the air, despite having cuticles to prevent drying out. Because water vapor can also pass through stomata, they are not always open. Stomata open and close based on environmental cues, which helps to prevent excess water loss.

Stomata and transpiration

As a result of the direct opening of the stomata, a process called transpiration occurs. Transpiration is the evaporation of water through the stomata. Transpiration creates a water pressure difference in plants, helping to drive water up the xylem tissue of vascular plants.

Stomata are found in the epidermis of leaves and sometimes stems. Surrounding the stomatal pores are modified epidermal cells known as guard cells.

Guard cells have cell walls that are not uniform but can expand when water enters them. They have cellulose (the component fortifying plant cell walls) microfibrils that help the cells expand and contract depending on their turgidity. Guard cells also contain chlorophyll and chloroplasts, which make them capable of photosynthesis. The presence of chloroplasts also helps guard cells detect changes in light, which can influence whether they are open or closed.

Surrounding the guard cells are subsidiary cells, which vary in function but may offer mechanical or storage support to the guard cells². The number of subsidiary cells surrounding guard cells, their sizes, and their shapes vary from plant to plant.

Stomata: where to find them?

Most stomata are found on the dermal tissue of a leaf. This means they exist in the outer layers of a plant and its tissues. Stomata occur both on the undersides of leaves and the top of them as well.

Depending on the species or type of plant, you may observe stomata on both the abaxial and adaxial surfaces, or on one or the other.

Stomata function: how do stomata open and close?

The basic function of stomata is to allow for gas exchange between the air and the plant, allowing in carbon dioxide and releasing oxygen.

Stomata allow the exchange of gases for photosynthesis and control the loss of water, as we have discussed. What factors, then, might influence whether stomates remain open or closed?

All of these may be internal or external signals that a stoma should open to continue the exchange of gases or close to limit water loss.

A stoma may open due to:

• Increase in the amount of light

• Increase in humidity in the atmosphere

• Low levels of CO₂ in the mesophyll tissue surrounding the stomatal pore

A stoma may close due to:

• Decrease in the amount of light

• Decrease in humidity in the atmosphere

• High levels of CO₂ in the mesophyll tissue

Turgor pressure, guard cells, and stomata

When environmental cues are present, the guard cells of stomata undergo a change in turgor pressure to either open or close. When the stomata are closed, the guard cells are flaccid. However, the stomatal opening is caused by the movement of water into guard cells, causing them to become turgid, and curve outwards, allowing a direct path to the mesophyll tissue below.

Stomata in plants: the adaptations for preventing water loss

As we have discussed, the presence of stomata is important for gas exchange. However, we also learned that stomata offer an easy passage for water out of a plant through transpiration. Plants control the amount of water they lose through stomata through different mechanisms or adaptations. Controlling the amount of water lost through transpiration means controlling the stomata. One way a plant manages its stomata is by opening and closing them at strategic times.

Plants also control the number of stomata. They may do this by shedding extra leaves, or if a plant faces long drought periods it may even decrease the stomatal numbers on new leaves. Some plants have their stomata in crevices called stomatal crypts, which are indentations on the surface of leaves. The stomata are at the bottom of these crypts.

Opening and closing stomata

Most plants open their stomata during the day when the sunlight is present so that CO₂ gas that enters the plant can be used for photosynthesis. The plant, however, must respond to extreme dryness or heat in the atmosphere that can cause water stress.

Abscisic acid

Plants respond to sudden water stress brought on by high temperatures or increased drought by closing their stomata.

If the water potential is low (negative) in the mesophyll tissues of leaves, the plant will activate an abscisic acid response. This means abscisic acid will signal the plant to close the guard cells, preventing further water loss through transpiration.

Crassulacean acid metabolism (CAM) plants

Most plants open their stomata during the day when the sunlight is ample for photosynthesis to take place. However, if a plant lives in an arid climate like the desert, then opening stomata during the day is a recipe for excess water loss. As a result, some plants that live in hot, dry environments have developed a Crassulacean Acid Metabolism (CAM) which lets them open stomata during the cool night and keep them closed during the heat of the day.

At night, the stomata open, and CAM plants concentrate carbon dioxide in the mesophyll tissue, converting it into a preliminary carbon compound used in the Calvin cycle of photosynthesis. Then, during the day, the plant has carbon to carry out photosynthesis without opening stomata.