Transpiration pull

Transpiration pull is the negative pressure (tension) generated in the xylem vessels of plants when water evaporates from leaf surfaces through stomata. This tension is the driving force behind the cohesion-tension mechanism that lifts water from roots to crowns. Transpiration pull is not a biological force per se; it is a physical consequence of the atmospheric demand for water vapor and the plant's anatomical architecture. The rate of transpiration is controlled by stomatal conductance, which responds to light, humidity, and CO₂ concentration — making transpiration pull a point of coupling between plant physiology and the global carbon cycle. The magnitude of transpiration pull is quantified by the concept of water potential, which measures the potential energy of water per unit volume relative to pure water in reference conditions.