Ethylene and abscisic acid are plant growth hormones which have both positive as well as negative effects on a plant.

Positive Roles of Ethylene :
(a) Ethylene acts as a ripening agent involved in ripening of fruits.

(b) When present in low concentration, it stimulates root meristem formation and promotes growth of lateral roots.

(c) It is responsible for breaking dormancy of seeds and hence has a role in seed germination.

(d) It increase the number of female flowers in the cucumber plant.

Negative Role of Ethylene :
(a) Ethylene is responsible for nullifying geotropism.
(b) Ethylene inhibits the growth of lateral buds thus stimulating growth of apical buds or leading to apical dominance.
(c) Ethylene promotes abscission of leaves, flowers and fruits thus leading to senescence.

Positive Role of Abscisic acid:
(a) During stress conditions, abscisic acid concentration increases in the leaves of the plant due to which stomata closes preventing water loss via transpiration. Thus it increases the tolerance. Hence, it aids during stress conditions because of which it is called stress hormone.

(b) It increases the resistance of plants to cold climatic conditions.

(c) It leads to tuber formation in potato.
(d) It is also responsible for initiation of roots in stem cuttings of some selected plants. root initiation in stem cuttings of some plants.

Negative Role of Abscissic acid:

(a) It inhibits growth of a plant and in turn induces dormancy of buds.

(b) It leads to abscission of flowers and fruits.

(c) It promotes senescence of leaves by destroying the photosynthetic pigment called chlorophyll and inhibiting the synthesis of RNA and proteins.

Thus we see how these two hormones have both positive and negative effects on a plant.