Positive Role of Ethylene :
(a) Ethylene is a ripening agent. It is involved in ripening of climateric fruits.
(b) In low concentration, ethylene stimulates root initiates and growth of lateral roots and root hair.
(c) Ethylene stimulates flowering in pineapple, mango and related plants though in other cases, the hormone causes fading of flowers.
(d) Ethylene breaks dormancy of storage organs.
(e) Ethylene increases the number of female flowers and fruits in cucumber plants.
Positive Role of Abscissic acid:
(a) During desiccation and other stresses, the concentration of abscissic acid increases in the leaves of the plants. The hormone causes closure of stomata and hence prevents transpiration.
(b) Abscissic acid increases resistance of plants to cold.
(c) ABA delays flowering in long day plants. However, in some short day plants it promotes flowering.
(d) Abscissic acid helps in tuber formation in potato.
(e) Abscissic acid promote root initiation in stem cuttings of some plants.
Negative Role of Ethylene :
(a) Ethylene inhibits longitudinal growth but stimulates transverse growth so that stem looks swollen.
(b) Ethylene nullifies geotropism. Root becomes apogeotropic, while stem turns positively geotropic. Seedlings develop epicoty hook.
(c) Ethylene inhibits the growth of lateral buds and thus cause apical dominance.
(d) Ethylene accelerates abscission of leaves, flowers
(e) Ethylene is associated with the process of senescence
of leaves and flowers.
Negative Role of Abscissic acid: (a) Abscissic acid acts as growth inhibitor and induces dormancy of buds towards the approach of winter.
(b) Towards the approach of winter, the activity of cambium is inhibited due to the formation of abscissic acid.
(c) Abscissic acid promotes abscission of flowers and fruits.
(d) Abscissic acid stimulates senescence of leaves by causing destruction of chlorophyll and inhibition of protein and RNA synthesis.
(e) Abscissic acid plays an important role in the regulation of plant growth.