4. Abscicic acid:

Abscisic acid (ABA) is the major phytohormone that controls plant’s ability to survive in harsh, changing environment. ABA promotes abscission of leaves and fruits; hence it is this action that gave rise to the name of this hormone ‘abscisic acid’. ABA is a naturally occurring compound in plants. The ABA signaling pathway is conserved across all plants, including mosses, and it is considered as an early adaptation to the terrestrial environment. ABA is found in leaves (where it is partially synthesized), stems, and green fruits. It is generally associated with negative-feedback interactions or stress-related environmental signals such as drought, freezing temperatures and environmental pollutants

Figure 9: Abscisic acid