Stomata: How Plants Breathe, Grow, and Respond to Stress

Stomata are among the most important structures in plant physiology. These tiny pores on the leaf surface govern gas exchange, water loss, and the plant’s ability to respond to environmental stress. In hydroponics—where climate, irrigation, and nutrient delivery are fully controlled—understanding stomata helps growers optimise growth, diagnose stress more accurately, and even conduct simple plant-science experiments at home.

This blog summarises stomatal function, how plants regulate them, and how growers can observe them using a quick and inexpensive technique.

What Are Stomata?

Stomata are microscopic pores located primarily on the underside of leaves. Each stomatal pore is bordered by two guard cells, which change shape by altering internal water pressure (turgor). This opens or closes the pore.

Stomata are not the structure—only the opening.
The guard cells are the machinery that controls them.

Why Plants Open and Close Their Stomata

Plants open stomata to exchange gases:

• CO₂ diffuses in for photosynthesis
• O₂ diffuses out as a by-product
• Water vapour escapes during the process (transpiration)

Because the internal leaf environment is moist—just like human lungs—gas diffusion is efficient, but water loss is unavoidable.

When Stomata Close

Stomata close under conditions such as:

• drought
• high heat
• waterlogging
• nutrient stress
• darkness (most species)

Closing stomata helps the plant conserve water and stabilise internal pressure.

Exceptions: CAM Plants and Potatoes

Some species have unique stomatal behaviour:

• Cacti and succulents (CAM plants) open stomata at night to reduce water loss.
• Potatoes reportedly keep stomata open 24 hours a day, likely to enhance calcium uptake from moist soil.

Where Stomata Are Located

Most plants have 90–95% of their stomata on the underside of the leaf.
The top surface is usually:

• waxy
• glossy
• designed for light absorption rather than gas exchange

The internal leaf structure supports this division of labour:

• Palisade cells near the top capture light
• Spongy mesophyll near the bottom allows air movement and gas exchange
• Stomata connect this internal airspace to the outside world

How Stomatal Regulation Works

Stomatal aperture (how open the pore is) determines:

• CO₂ uptake
• Photosynthetic rate
• Water loss
• Stress sensitivity

Scientists measure this using stomatal conductance, typically with a device called a porometer. This quantifies gas flow in:

micromoles per metre² per second (µmol m⁻² s⁻¹)

Porometers are accurate but expensive, which limits their use in small-scale hydroponics. However, growers can still observe stomatal behaviour directly using a simple botanical technique.

How Hydroponic Growers Can See Stomata at Home

A practical method used by plant physiologists involves clear nail varnish.

The Nail Varnish Impression Technique

1. Select a healthy leaf on your plant.
2. Paint a very thin layer of clear nail varnish onto the underside.
3. Allow it to dry completely.
4. Gently peel off the dried film.
5. Place the thin peel on a microscope slide.
6. View under 10× to 40× magnification.

This produces a perfect mirror image of epidermal features including:

• stomata
• trichomes
• epidermal cell shapes
• vein outlines

It allows you to examine:

• stomatal density
• whether stomata are open or closed
• how plants respond to environmental changes
• differences between species or stressed vs healthy leaves

This method is widely used in research and works with simple, inexpensive microscopes—ideal for curious hydroponic growers.

What Stomatal Behaviour Reveals About Stress

Stomata react quickly to changes in the environment, making them a reliable indicator of plant stress:

• Drought → stomata close
• Flooding → stomata close
• Heat stress → stomata may close to conserve water
• Salt or nutrient stress → stomatal conductance decreases
• High light intensity → stomata open (to support photosynthesis)

New leaves formed under long-term stress may even develop more or fewer stomata, altering future gas-exchange capacity.

Why Stomata Matter in Hydroponics

Hydroponic growers benefit from understanding stomatal behaviour because it helps explain:

• growth slowdowns
• water-use changes
• nutrient uptake variation
• stress responses
• heat tolerance
• transpiration-driven calcium movement

This is especially important for crops prone to calcium-related disorders (tomatoes, peppers, lettuce), where transpiration rate directly influences nutrient transport.

Final Thoughts

Stomata are central to plant health, water balance, and photosynthesis. By understanding how they function—and by using simple tools like the nail varnish method—hydroponic growers can gain deeper insight into their plants’ physiology and detect stress earlier.

Article by Dr Russell Sharp

If you would like to keep up to date with subjects just like this, you can listen to both our podcasts! Links can be found bellow:

Hydroponics Daily Podcast: https://podcasts.apple.com/us/podcast/hydroponics-daily/id1788172771

Cereal Killers Podcast: https://podcasts.apple.com/us/podcast/cereal-killers/id1695783663