Plant Growth and Hormones

Ethylene is considered as the growth hormone which regulates promoter and inhibitor as it is used to make the fruit ripen, breaks bud dormancy, initiates flowering. It acts as inhibitor as it retards the mitotic division in root and shoot meristem and also for axillary buds.

Abscisic acid (ABA) is known to promote abscission, senescence, and dormancy. It generally inhibits growth processes, but its role in flowering is complex and often considered less direct or inhibitory compared to its other primary functions.

The plant hormone ethylene is growth inhibitor and is released in plant organs that are to undergo ripening or senescence. Ethylene triggers climacteric ripening of the fruit and induces insensitivity of senescent organs to auxins due to which the nutrients are channelized to other growing parts of the plant resulting in withering away of the plant part.

Gibberellic acid increases the yield of sugarcane crop. Cytokinins inhibit apical dominance and act antagonistically to auxins which promote apical dominance. Rhizomes, corms, tubers, seeds (e.g., peanut) and other storage organs can be made to sprout early by exposing them to ethylene. Abscisic acid is a plant growth inhibitor, which inhibits seed germination.

Presence of cytokinin in an area causes preferential movement of nutrients towards it. When applied to lateral buds, they help in their growth despite the presence of apical bud. They thus act antagonistically to auxin which promotes apical dominance. Therefore cytokinin can overcome apical dominance, caused by auxins.

The most important function of cytokinins is the promotion of cell division. The varying amounts of cytokinins, along with sufficient auxins is required for the growth of callus in tissue culture experiments. Cytokinins delay the senescence of leaves and other organs by mobilisation of nutrients. It is known as Richmond-Lang effect.

Presence of cytokinin in an area causes preferential movement of nutrients towards it. When applied to lateral buds, they help in their growth despite the presence of apical bud. They thus act antagonistically to auxin which promotes apical dominance. Therefore, cytokinin can overcome apical dominance, caused by auxins.

ABA induces seed dormancy in unfavorable environmental condition, its function is antagonistic to gibberillin which promotes seed germination. It is regarded as the stress hormone as it brings about closure of stomata during water stress to prevent water loss from transpiration.

Cytokinins (Letham, 1963) are plant growth hormones which are basic in nature, either aminopurine or phenyl urea derivatives, That promote cytokinesis (= cell division) either alone or in conjunction with auxin. Coconut milk is a rich source of cytokinin.

ABA Is known as dormin as it induces dormancy in buds, underground stems and seeds in a variety of plants. e.g., ABA induces winter bud dormancy in duckweed plant.

Ethylene inhibits longitudinal growth but stimulates transverse or horizontal growth and swelling of axis. It decreases the sensitivity to gravity. Roots become Apo-geotropic while stems turn positively geotropic. Leaves and flowers undergo drooping. The phenomenon is called epinasty. Seedlings develop tight epicotyl hook. It hastens the senescence of leaves and flowers.Abscission of various parts (leaves, flowers, fruits) is stimulated by ethylene which induces the formation of hydrolases. It breaks the dormancy of buds, seeds and storage organs. Rhizomes, corms, tubers, seeds (e.g., Peanut) and other storage organs can be made to sprout early by exposing them to ethylene. Initiates germination in peanut seeds. 

So, the correct answer is 'Ethylene'

ABA (Absisic acid) is a stress hormone which is synthesised by the plant during drought or other stress conditions. It causes rapid movement of $K^+$ ions out of the guard cells, closes stomatal pores and thus reduces the transpiration rate.

A- GA or Gibberellic acid helps in promoting the growth and the elongation of cells.