Transpiration is the loss of water from plants as water vapor, predominantly through pores in their leaves called stomata. Water absorbed by roots moves through plant tissues before evaporating and entering the atmosphere. This process connects plant water transport with the water cycle and is a component of evapotranspiration, which combines transpiration with evaporation from soil, water bodies, and other surfaces. Its rate depends on both environmental conditions and biological regulation. (usgs.gov)
Water movement through the plant
In vascular plants, roots absorb water that passes into the xylem, the conducting tissue supplying stems and leaves. Within leaves, water evaporates from moist cell walls into intercellular air spaces. The resulting vapor travels outward by diffusion, usually through stomata. Liquid-water transport and vapor movement are therefore successive parts of the same pathway, rather than water evaporating continuously throughout the stem. (openstax.org)
The cohesion–tension theory explains how leaf evaporation drives the ascent of xylem sap. Evaporation increases tension, or negative pressure, in the liquid water remaining within the plant. Cohesion between water molecules allows this tension to be transmitted through the xylem, pulling water upward from roots. Adhesion to conducting walls helps maintain the water columns. The pathway follows a water-potential gradient from soil through the plant toward the generally much drier atmosphere. Thus, the bulk movement of xylem water is largely a passive physical process, although living cells regulate water uptake and loss. (openstax.org)
Transpiration differs from guttation, in which plants release liquid droplets, often at leaf margins. The defining distinction is the form of water leaving the plant: vapor in transpiration and liquid in guttation. (open.lib.umn.edu)
Stomatal regulation and carbon uptake
Stomata provide a shared pathway for water vapor leaving a leaf and carbon dioxide entering it for photosynthesis. Their aperture therefore links carbon acquisition with water loss. Opening the pores facilitates gas exchange; closing them restricts transpiration but also limits carbon dioxide entry. Stomatal conductance describes how readily gases pass through this pathway and is distinct from the actual transpiration rate. (licor.com)
Each pore is bordered by guard cells. These cells respond to environmental and internal signals, including light, carbon dioxide concentration, and water availability. Experiments in Arabidopsis have identified interacting blue-light and carbon dioxide signaling pathways that regulate stomatal opening. During water deficiency, abscisic acid, a plant hormone, participates in signaling that promotes stomatal closure and reduces water loss. (nature.com)
Transpiration often declines at night, but it does not invariably stop. Incomplete stomatal closure can permit nocturnal water loss. Controlled experiments with bean and cotton also demonstrate that internal daily rhythms can influence nighttime conductance, so transpiration cannot always be predicted solely from immediate atmospheric conditions. (nature.com)
Environmental controls
Transpiration depends on available energy, the vapor-pressure difference between leaves and surrounding air, and the resistance encountered by escaping vapor. Solar radiation supplies energy for evaporation, while air temperature, humidity, and wind influence atmospheric evaporative demand. Soil moisture and the soil’s ability to supply water to roots also constrain the process. Consequently, the same plant can transpire at substantially different rates under different conditions. (fao.org)
The vapor pressure deficit between a leaf’s internal air spaces and the surrounding atmosphere is an important measure of the drying gradient. At a given conductance, a larger difference supports greater vapor loss. The thin layer of air adjacent to a leaf creates additional resistance: vapor must pass through both the stomatal pathway and this boundary layer before reaching well-mixed air. These separate resistances are included in leaf gas-exchange calculations. (licor.com)
Plant size, leaf area, growth stage, and canopy development affect total water loss. Transpiration and soil evaporation can occur simultaneously, but their relative contributions change as vegetation develops. Sparse young crops expose more soil to evaporation; a developed canopy shades the soil and commonly makes transpiration the larger component of field evapotranspiration. Waterlogging and salinity can also restrict root water uptake. (fao.org)
Hydrological and agricultural significance
Transpiration returns water stored in soil to the atmosphere and can contribute to soil drying during periods without rainfall. Where roots reach a shallow water table, plants may also withdraw groundwater directly. Along streams and wetlands, this withdrawal can lower groundwater levels and alter exchanges between groundwater and surface water, sometimes producing daily fluctuations associated with plant activity. (usgs.gov)
In agriculture, transpiration is an important part of crop water consumption, but it is not interchangeable with total irrigation demand. Crop-water calculations distinguish atmospheric demand, crop characteristics, soil evaporation, and water stress. Dual crop-coefficient methods separate a basal crop coefficient associated with transpiration from a coefficient representing soil evaporation, allowing the two components to be estimated separately. (fao.org)
Measurement
A potometer can measure water uptake by a cut shoot through the displacement of an air bubble in a capillary tube. Uptake is related to transpiration but is not a direct measurement of vapor loss. Gravimetric arrangements instead record changes in mass, with other sources of water loss controlled or accounted for. (saps.org.uk)
Leaf gas-exchange instruments determine transpiration from airflow, leaf area, and the difference in water-vapor concentration between air entering and leaving a chamber. Rates are commonly expressed as millimoles of water per square meter of leaf area per second. Such measurements can be combined with carbon dioxide exchange and conductance estimates to examine the relationship between photosynthetic activity and water loss. (licor.com)