Transpiration

Transpiration is how water moves through a plant and then evaporates from its above-ground parts—leaves, stems, and flowers. This process is passive, meaning the plant doesn’t use energy to make it happen. Besides moving water, transpiration helps cool the plant, adjusts the osmotic pressure inside cells, and drives the mass flow of mineral nutrients.

Plants take in water and dissolved minerals through their roots via osmosis. However, only a tiny fraction—between 0.5% and 3%—of that water is actually used for growth and metabolism. The rest, 97% to 99.5%, is lost through transpiration or guttation. The water travels upward through the xylem, a network of tubes, thanks to adhesion (water sticking to the tube walls) and cohesion (water molecules sticking to each other). The cohesion-tension theory explains this: when a water molecule evaporates from a leaf, it pulls on the next molecule, creating a continuous column of water from roots to leaves. This flow is mainly driven by differences in water potential. If the air outside the leaf has a lower water potential than the air inside the leaf’s stomatal pores, water vapor moves out. That lowers the water potential in the leaf, causing liquid water to evaporate from the mesophyll cell walls. The resulting tension pulls water up through the xylem, even against gravity in tall trees. The rate of water flow from soil to roots depends on the soil’s hydraulic conductivity and the pressure gradient through the soil.

The word “transpiration” comes from Latin: *trans* (meaning “across”) and *spīrāre* (meaning “to breathe”), with a suffix that adds “the act of”—so it literally means “the act of breathing across.”

Capillary action also plays a role. This is the movement of liquid in narrow spaces without help from—or even against—external forces like gravity. It happens because of intermolecular forces between the liquid and solid surfaces. In a thin tube, surface tension (from cohesion within the liquid) and adhesive forces between the liquid and the tube wall work together to pull the liquid upward.

Plants control transpiration by adjusting the size of their stomatal openings. The rate is also influenced by the surrounding atmosphere: boundary layer conductance, humidity, temperature, wind, and sunlight. Soil temperature and moisture can affect stomatal opening too. Other factors include the plant’s size, how much water the roots absorb, soil moisture, excessive soil fertility or salt, poorly developed root systems, and infections by pathogens like *Pythium* or *Rhizoctonia*. When water loss exceeds uptake, plants close their stomata to conserve water, but this also slows nutrient uptake and reduces carbon dioxide absorption, limiting photosynthesis, metabolism, and growth.

During a growing season, a leaf transpires many times its own weight in water. An acre of corn releases about 3,000–4,000 U.S. gallons (11,000–15,000 liters) per day, and a large oak tree can transpire 40,000 U.S. gallons (150,000 liters) per year. The transpiration ratio—the mass of water transpired per mass of dry matter produced—for crops typically ranges from 200 to 1,000. Scientists measure transpiration rates using tools like potometers, lysimeters, porometers, photosynthesis systems, and thermometric sap flow sensors. Isotope studies show that transpiration is the larger part of evapotranspiration, and recent global evidence indicates that transpired water is isotopically different from groundwater and streams, meaning soil water isn’t as well mixed as often assumed.

Desert plants have special adaptations to reduce water loss: thick cuticles, smaller leaves, sunken stomata, and hairs. Many cacti perform photosynthesis in their succulent stems instead of leaves, which keeps the shoot surface area very low.