The role of ethylene and abscisic acid is both positive and negative.

(a) Positive Roles of Ethylene: Ripening of fruits

– When present in low concentration, it stimulates root meristem formation and promotes the growth of lateral roots.

(b) Negative Role of Ethylene:

– Ethylene is responsible for nullifying geotropism.

(c) Positive Role of Abscisic acid:

– During stress conditions, abscisic acid concentration increases in the leaves of the plant due to which stomata close preventing water loss via transpiration.

(d) Negative Role of Abscisic acid:

– It inhibits the growth of a plant and in turn, induces dormancy of buds.