Transpiration

Transpiration is the loss of water vapor from plant surfaces, primarily through stomata in the leaves, with minor loss via the cuticle and in some stems and fruits. The process drives the ascent of water through the plant, producing a transpiration stream that moves water and dissolved minerals from roots to shoots; it also creates a negative pressure in the xylem.

Water is taken up by roots and transported through the xylem by cohesion and adhesion; water evaporates from mesophyll cells to the air spaces and diffuses out of the leaf through stomata. Stomata are opened and closed by guard cells in response to light, CO2 concentration, humidity, temperature, and hormonal signals (notably abscisic acid). A portion of transpiration occurs across the leaf cuticle, especially under high humidity or thick cuticles; in most species, stomatal transpiration dominates.

Environmental factors strongly influence transpiration: higher light and temperature increase transpiration rate, while high air humidity reduces the gradient; wind removes water vapor, increasing rate. Soil moisture limits transpiration; severe water stress reduces stomatal aperture to conserve water, sometimes causing stomatal closure. Plants vary in stomatal density and conductance, affecting total transpiration.

Transpiration serves several functions: it helps transport minerals from roots, maintains leaf turgor, and cools leaves through evaporative cooling. It also creates the driving force for xylem water transport. However, excessive transpiration without adequate water can lead to cavitation and hydraulic failure.

Transpiration rate is commonly expressed as E, measured by gas exchange, porometry, sap flow sensors, or lysimeters. The term transpiration is related to evapotranspiration, which also includes soil evaporation.