Abscisic acid (ABA) is a phytohormone that plays an important role in regulating plant responses to stress. ABA is produced in the roots in response to decreased soil moisture, where it then travels to the leaves and regulates the stomatal guard cells, causing them to shrink and stomata to close. This decreases plant evapotranspiration, i.e. the movement of water from the roots to the stem and out of the leaves.
