Final answer:
The driving force behind plant transpiration is the evaporation of water at the leaf-atmosphere interface. This process creates a negative pressure that draws water up from the roots through the xylem, regulated by atmospheric conditions such as relative humidity and controlled internally by the stomata.
Step-by-step explanation:
In plants, the driving force behind transpiration is primarily the evaporation of water at the leaf-atmosphere interface. Transpiration creates negative pressure, or tension, within the leaves, drawing water up from the roots through the xylem. This tension can reach up to −2 MPa on the leaf surface, though this varies with vapor pressure deficit, meaning it is influenced by the relative humidity levels. Essentially, a lower relative humidity results in higher transpiration rates as the atmospheric demand for moisture increases. Additionally, the control of transpiration to prevent excessive water loss is managed by the stomata. The stomata will close when dehydration is a risk, halting transpiration.
At night, when stomata are closed and transpiration halts, water is retained in the stem and leaves thanks to the adhesion of water to the cell walls of the xylem and the cohesion between the water molecules themselves. This phenomenon is described by the cohesion-tension theory of sap ascent. During the day, photosynthesis and transpiration decrease osmotic pressure in leaf cells, prompting water movement from the roots to the leaves, defying gravity due to osmotic pressure changes at the root hair cells.