Plants release oxygen as their primary gas output, produced during photosynthesis when they use sunlight to convert carbon dioxide and water into energy. But oxygen isn’t the only gas leaving a plant’s surface. Plants also release carbon dioxide, water vapor, ethylene, and a range of volatile organic compounds depending on the time of day, the plant’s life stage, and environmental conditions.
Oxygen: The Main Event
Oxygen is the gas most people associate with plants, and for good reason. During photosynthesis, plants split water molecules inside their leaves using light energy. This reaction frees oxygen atoms, which pair up and exit through tiny pores called stomata on the leaf surface. The process simultaneously pulls carbon dioxide out of the air and uses it to build sugars the plant needs for growth.
A single mature tree absorbs more than 48 pounds of carbon dioxide per year and releases oxygen in return, enough to supply breathable air for up to four people in a day, according to the U.S. Department of Agriculture and the Arbor Day Foundation. Forests, grasslands, and agricultural crops collectively sustain much of the oxygen in Earth’s atmosphere (with ocean-dwelling algae and phytoplankton handling the rest).
Oxygen production depends heavily on light. On a bright, warm day, a healthy leaf photosynthesizes rapidly and pumps out far more oxygen than it consumes. In dim conditions or deep shade, that output drops significantly.
Carbon Dioxide: The Nighttime Release
Plants breathe too. Like animals, they run a process called cellular respiration, breaking down sugars to fuel their own metabolism and releasing carbon dioxide as a byproduct. This happens around the clock, day and night, in every living cell of the plant.
During the day, photosynthesis overwhelms respiration. A plant absorbs far more CO₂ than it produces, so the net effect is oxygen flowing out. At night, with no sunlight to drive photosynthesis, respiration continues alone, and the plant becomes a net emitter of carbon dioxide. The amount is small compared to what was absorbed during daylight hours, so over a full 24-hour cycle, a healthy, growing plant still removes more CO₂ from the air than it adds.
Water Vapor and Transpiration
Plants release enormous quantities of water vapor, a process called transpiration. The same stomata that let carbon dioxide in and oxygen out also allow water to evaporate from the moist interior of the leaf. A large oak tree can transpire over 40,000 gallons of water in a single growing season.
Transpiration isn’t wasteful. The evaporation creates a pulling force that draws water and dissolved nutrients up from the roots through the stem and into the leaves. It also cools the plant, much like sweating cools your skin. Guard cells flanking each stoma control the pore size, balancing the need to let CO₂ in for photosynthesis against the risk of losing too much water and drying out.
Ethylene: The Ripening Gas
Ethylene is a small, gaseous hormone that plants produce naturally. It plays a central role in fruit ripening, leaf aging, and flower wilting. When a banana sitting on your counter speeds up the ripening of nearby fruit, ethylene is responsible. The gas drifts from one fruit to the next, triggering a cascade of biochemical changes that soften tissue, convert starches to sugars, and shift color.
Beyond ripening, ethylene helps regulate leaf drop in autumn and coordinates responses to physical damage. A plant that gets bent by wind or chewed by an insect ramps up ethylene production, which can trigger defensive changes throughout the plant.
Volatile Organic Compounds
Plants release a surprising cocktail of airborne chemicals called volatile organic compounds, or VOCs. If you’ve ever noticed the sharp, green smell of freshly mowed grass or the piney scent of a conifer forest, you’ve encountered plant VOCs firsthand.
These emissions serve several purposes:
- Defense signaling. When a plant is attacked by insects, it can release compounds like green leaf volatiles and terpenoids that serve as a chemical alarm. Neighboring plants pick up these signals and preemptively boost their own defenses. Some of these same compounds attract predators that feed on the attacking insects, essentially calling for backup.
- Heat and light protection. Isoprene, one of the most abundantly emitted plant VOCs, helps stabilize leaf cell membranes during heat stress.
- Pathogen resistance. Certain volatiles, including methyl salicylate, trigger a broad immune-like response in both the emitting plant and its neighbors, improving resistance to fungal and bacterial infections.
Research on lima beans found that specific terpenoids released during spider mite infestations activated defense genes in undamaged leaves nearby. In Arabidopsis (a small plant widely used in research), green leaf volatiles enhanced resistance to fungal pathogens like gray mold. The chemical vocabulary plants use is remarkably specific: different blends of VOCs carry different messages about what type of threat the plant is facing.
What About Methane?
A few years ago, headlines suggested that living plants were a significant source of methane, a potent greenhouse gas. The reality turned out to be more nuanced. Plants do not have a biochemical pathway to manufacture methane. Controlled experiments found that methane emissions from various plant species were statistically indistinguishable from zero under normal growing conditions.
What researchers did find is that plants can transport dissolved methane from the soil up through their roots and release it through their leaves via transpiration. Under extreme UV stress, plant material can also break down in ways that release trace amounts of methane. But these are physical and chemical processes, not biological ones. Plants are not a major contributor to global methane levels.
How Temperature Shifts the Balance
The ratio of gases a plant emits changes with temperature. In cool conditions (around 19°C daytime highs), plants in field studies showed higher rates of carbon fixation, meaning they pulled in more CO₂ and produced more oxygen per unit of leaf area. When temperatures climbed to around 31°C, the efficiency of the key enzyme driving photosynthesis dropped, sometimes by half.
At the same time, warmer temperatures speed up cellular respiration, so the plant burns through more sugar and releases more CO₂. The combined effect is that very hot conditions can narrow the gap between oxygen produced and carbon dioxide released, reducing a plant’s net contribution of oxygen to the air. This is one reason tropical forests, despite their lush growth, are sensitive to rising global temperatures. Their carbon-absorbing advantage shrinks as nights warm and respiration accelerates.
CAM Plants: A Different Schedule
Not all plants follow the standard daytime-photosynthesis, nighttime-respiration pattern. Succulents, cacti, pineapples, and other plants adapted to arid environments use a strategy called CAM photosynthesis. These plants open their stomata at night, when the air is cooler and less water is lost to evaporation, and absorb CO₂ in the dark. They store it as an organic acid, then use it for photosynthesis during the day with their stomata closed.
The result is that CAM plants release oxygen primarily during the day (just like other plants), but their CO₂ intake happens mostly at night. Over a full day-night cycle, the total CO₂ absorbed still equals the total oxygen released, but the timing is flipped compared to a typical houseplant or shade tree. This is why some people recommend keeping succulents in bedrooms: they absorb CO₂ at night rather than emitting it.