Carbon Dioxide and Stomata

The structure of a stoma

A stoma (plural: stomata) is a tiny pore in the epidermis of a leaf. Each stoma is surrounded by two guard cells. The guard cells are the only cells of the epidermis that normally contain chloroplasts, and they control the size of the pore.

Stomata open into the air spaces of the spongy mesophyll. Gases move between these air spaces and the outside air by diffusion, so the pore is the gateway for gas exchange.

Which gases move, and in which direction

Three gases matter most at the stomata (nitrogen also passes through, but with no net effect on the plant):

• Carbon dioxide diffuses in in daylight, because photosynthesis uses it up and keeps its concentration inside the leaf low.

• Oxygen diffuses out in bright light, because photosynthesis makes more than respiration uses.

• Water vapour diffuses out (this is transpiration), because the air spaces are saturated and the outside air is drier.

In the dark, photosynthesis stops. Respiration continues, so oxygen moves in and CO₂ moves out.

How guard cells open and close a stoma

Guard cells work like two long balloons joined at the ends. Their walls are unevenly thickened: the wall next to the pore is thick and inelastic, while the outer wall is thinner and more flexible. Cellulose microfibrils are arranged in rings around the cell.

When the guard cells take in water they swell. Because the ring-like microfibrils stop them getting wider, they get longer. The thin outer wall bulges outwards and pulls the thick inner wall with it. The cells bow apart and the pore opens.

How do they take in water?

1. Potassium ions (K⁺) are actively pumped into the guard cells (using ATP).

2. The solute concentration rises, so the water potential of the guard cells falls (becomes more negative).

3. Water enters by osmosis from the neighbouring epidermal cells.

4. The guard cells become turgid and the stoma opens.

To close the stoma, K⁺ leaves, the water potential rises, water leaves by osmosis, the guard cells become flaccid and the walls straighten, closing the pore.

Carbon dioxide as a control factor

The CO₂ concentration in the air spaces of the leaf acts as a signal. This is negative feedback:

• Photosynthesis uses up CO₂, so its level inside the leaf falls. The guard cells respond by opening the stomata, letting more CO₂ in.

• If the level rises (for example in the dark, when respiration releases CO₂ and there is no photosynthesis), the stomata close, saving water.

In stomata, the guard cells act as receptor and effector together, so the response happens within the cells themselves. Compare this with breathing in humans, where a rise in CO₂ is detected by the medulla of the brain, which speeds up the diaphragm and intercostal muscles. In both cases CO₂ is the controlling factor, but humans respond to high CO₂ by getting rid of it, while plants respond to low CO₂ by taking more in.

Light also opens stomata and darkness closes them, so in most plants they are open by day and closed at night.

The compensation point

As a plant photosynthesises in a sealed container, the CO₂ level falls. It does not fall to zero. It stops at the compensation point: the CO₂ concentration (or light intensity) at which the rate of photosynthesis equals the rate of respiration. There is then no net gas exchange and the level stays steady.

Stomatal density and habitat

The number of stomata per mm² differs between species and between the two leaf surfaces. This is an adaptation to the habitat.

• Most land plants have more stomata on the lower surface. It is shaded and cooler, and out of direct sunlight, so less water is lost by evaporation.

• A floating leaf such as a water lily has stomata only on the upper surface, in contact with the air. The lower surface touches water, where stomata would be useless.

Water or carbon dioxide? Drought and abscisic acid

Open stomata let CO₂ in but also let water out. This is the plant's main conflict: conserving water versus obtaining CO₂. In drought, water conservation wins.

Roots and leaves in dry soil release the hormone abscisic acid (ABA). ABA binds to receptors on the guard cells and causes K⁺ (and other ions) to leave them. The water potential of the guard cells rises, water leaves by osmosis, they become flaccid and the stomata close. Transpiration falls, but so does CO₂ intake and therefore photosynthesis.

Xerophytes live in dry habitats (for example marram grass on Irish sand dunes). Structural adaptations that reduce the conflict include:

• Thick, waxy cuticle reducing water loss through the epidermis.

• Stomata sunken in pits or crypts, trapping a layer of humid air.

• Rolled leaves (marram grass), enclosing the stomata in a humid space.

• Hairs on the leaf surface, trapping moist air and reducing air movement.

• Reduced leaf surface area (spines or needles).

Raised CO₂ and stomatal conductance

Stomatal conductance measures how easily gases pass through the stomata (units mmol m⁻² s⁻¹). More open stomata means higher conductance.

Atmospheric CO₂ is about 0.04% (around 400 ppm). Higher CO₂ in the leaf air spaces causes stomata to partially close, so conductance falls. This reduces water loss per unit leaf area.

Irish glasshouse growers sometimes raise CO₂ to about 0.1%. At first this boosts photosynthesis because CO₂ was the limiting factor (a factor in short supply that restricts the rate). But growth cannot rise indefinitely. Once CO₂ is plentiful, another factor becomes limiting: light intensity, temperature, water or mineral supply, or the amount of enzyme (rubisco) and chlorophyll. The rate then levels off at a plateau. Very high CO₂ may also close stomata too far.

Link to climate change: as atmospheric CO₂ rises, stomata of many crops and forest trees partially close. This can reduce water use per leaf, but warmer, drier summers and a larger leaf area could offset this, so the overall effect on Irish water use needs to be discussed with evidence rather than assumed.

Summary

• A stoma is a pore surrounded by two guard cells, which contain chloroplasts and control the pore size.

• In daylight CO₂ diffuses in, and O₂ and water vapour diffuse out.

• Guard cells open a stoma when K⁺ is pumped in, water potential falls, water enters by osmosis and they become turgid. The thick inner wall makes them bow apart.

• Low CO₂ in the leaf opens stomata and high CO₂ closes them (negative feedback). At the compensation point, photosynthesis equals respiration.

• Most land plants have more stomata on the lower surface. Floating leaves have them on the upper surface.

• In drought, ABA makes K⁺ leave the guard cells so the stomata close. Xerophytes reduce water loss with cuticles, sunken stomata, rolled leaves and hairs.

Back to the start: the tomato leaf opens its stomata when CO₂ inside is low and light is available, and closes them when CO₂ is high, it is dark or the plant is short of water.