ABSTRACT
Abscisic acid (ABA) is now well established to play a significant role as stress hormone in higher and lower plants. Usually, its biosynthesis is increased under stress conditions. After transport to target cells it improves water relations of terrestrial plants by closing their stomata and affecting their meristems. When ABA is externally applied, similar phenomena can be observed (Trewavas and Jones, 1991; Hartung et al, 1999). Attempts to produce ABA-free mutants or transgenic plants failed. Such plants would not be able to exist in a terrestrial environment with all its natural stress stimuli. It is therefore not surprising that ABA can be detected universally in all plants and that it must play an important role in roots as well. Hence, root systems are often those tissues that experience first the emergence of a stress situation.
