How plants breathe
Plants don’t have lungs to inhale and exhale the air that blows around them, but they do, in their own way, ‘breathe’ in and out oxygen and carbon dioxide. Here you can discover how plants carry out gas exchange and how we can make sure they breathe easy.
- Plants absorb oxygen for respiration and carbon dioxide for photosynthesis through tiny breathing pores in their leaves
- These gases move into and out of a plant by a process called diffusion, from an area of high concentration to an area of low concentration
- Roots also need oxygen, which they absorb from air spaces in the soil, so a well-aerated soil is vital for good growth
Why plants need to breathe
Just as we need to breathe to stay alive, plants must also exchange gases with the atmosphere to function. They need two key gases:
- oxygen is used in aerobic respiration, where food molecules are broken down to release energy for growth. This process releases carbon dioxide as a waste product
- carbon dioxide is used in photosynthesis, where the sun’s energy is harnessed to make food. This process releases oxygen as a waste product
How gases move
Plants don’t have lungs to draw in and push out gases. Instead, the internal structure of their tissues, with loosely packed cells and large air spaces, allows the easy exchange and movement of gases.
The cells inside leaves are loosely packed with large air spaces, allowing the easy movement of gases in and out
Gases always move from an area of high concentration to an area of low concentration, in a process called diffusion. As carbon dioxide is used by cells for photosynthesis, its concentration falls and more diffuses in from the air spaces to replace it. This, in turn, reduces carbon dioxide concentration in the air spaces, drawing more into the leaf from the atmosphere.
The same principle applies to oxygen diffusing in for respiration and to waste gases diffusing out.
Did you know?
Gardeners can boost the harvest of heavy-cropping vegetables like tomatoes and peppers by growing them in straw bales. The rapidly decomposing material inside the bale releases lots of carbon dioxide, increasing the concentration in the air around the leaves, and so the rate it diffuses in. This leads to optimal photosynthesis and improved growth and cropping.
Growing peppers in straw bales can produce a bumper crop
How leaves breathe
Gases enter leaves through thousands of tiny pores called stomata (sing. stoma). In most plants these are found on the underside of leaves, where they’re hidden from strong sunlight and protected from dust.
In daylight, plants are both respiring and photosynthesising, so oxygen and carbon dioxide are diffusing in and out of the leaves. But overnight, without sunlight, photosynthesis stops and stomata close. With just respiration taking place, only oxygen diffuses into the leaves and only carbon dioxide diffuses out.
Did you know?
It was once thought that having houseplants in your bedroom was detrimental to health, as they used up the oxygen in the air overnight. We now know that the amount they absorb is too small to make any difference to us, and studies have since found houseplants improve wellbeing and air quality and help you sleep better – so don’t be afraid to fill your bedroom with plants.
Cacti and succulents work differently. They’ve evolved to keep their stomata closed during the day as a way to prevent moisture loss in the hot, dry environments they come from. To ensure cells have enough carbon dioxide to photosynthesise, their stomata open at night instead and the gas is stored as an acid in large sacs (called vacuoles) within their cells until it’s needed. These fluid-filled vacuoles create the thick, fleshy leaves and stems that are typical of these plants.
Unlike most plants, cacti and succulents close their stomata during the day, and instead store carbon dioxide for photosynthesis
How woody stems breathe
Leaves and soft, green stems have living cells in contact with the air, and they can absorb oxygen for respiration directly through their surface. However, the bark of woody stems is impervious to gases, so to get oxygen to the active tissue beneath, it is perforated by pores called lenticels.
Lenticels can be particularly decorative in some tree species, forming dark or corky streaks on the silvery bark of many birches (Betula) and the glossy trunks of some cherries (Prunus).
Many apple and potato varieties also have prominent lenticels on their fruits and tubers, a feature that can aid their identification.
Decorative lenticels on Himalayan cherry, Prunus rufa
White lenticels on an apple
How roots breathe
It’s not only the above-ground parts of plants that carry out gas exchange – roots are living, growing organs that perform a range of functions, so they too need to absorb oxygen. This gas is found in the air spaces between soil particles, and enters roots via the fine hairs that cover their tips.
Thousands of tiny white hairs give root tips a fuzzy appearance
Plants grow poorly in soil that doesn’t contain enough air spaces, such as:
- Compacted soil – where the soil particles have been squashed together and air squeezed out. This situation is often found in new-build gardens and in lawns that are frequently walked or played on.
- Waterlogged soil – where the air spaces are filled with water. Not only is there no oxygen available to diffuse into the roots, suffocating them and hindering growth, but soil micro-organisms in these anaerobic conditions produce plant-damaging toxins. The build-up of these causes the unpleasant eggy smell we associate with soggy, stagnant soil.
Plants, like lawn grasses, grow poorly in waterlogged soil
To help your plants’ roots breathe and grow well, open up the structure of compacted soil by digging-in plenty of organic matter, like homemade compost, before planting. Regularly aerating compacted lawns introduces air into the soil layers and allows water to flow through more easily.
Waterlogging can also occur in containers if they don’t have enough drainage holes or are left standing in trays of water. So, make sure you choose a suitable container for your plants and allow excess water to drain away if it hasn’t been absorbed within an hour after watering.
Dig-in organic matter to open up the soil structure
Aerate lawns each autumn to improve drainage
Make sure container plants aren't sat in water for too long
Your next steps
Now you know more about how plants breathe, put this into practice to help your plants thrive:
- Ensure plants growing in closed environments are well-ventilated so they have a good supply of oxygen and carbon dioxide. See our guide to ventilating greenhouses and our video guide to terrariums
- Add submerged aquatic plants (oxygenators) to your garden pond. As well as improving oxygen levels, oxygenators boost biodiversity by providing cover for pond creatures. See our guide to choosing pond plants
- Improve soils with organic matter and keep plants well-watered so their breathing pores remain open and they can photosynthesise and respire efficiently. See our guide to using organic matter in the garden
- Aerate lawns each autumn so there is plenty of oxygen available for grass roots – particularly important on heavy clay soil and where lawns are regularly used. See our guide to autumn lawn care
- Check containers are draining properly to avoid damage from waterlogging. Outdoor pots should have at least one large drainage hole and water should soak into the compost within a few minutes of watering. See our guide to container gardening
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.