Precursor of ABA (abscisic acid).

ABA (abscisic acid) is an important plant hormone that regulates numerous aspects of plant growth, development, and stress responses.
Violaxanthin is the precursor of Abscisic acid. A Whereas Precursor is any substance, mostly enzymes, which on application to an inactive substance converts it to an active one. Hence, violaxanthin activates abscisic acid for its work.