Auxin stimulates stem elongation in low concentrations only. It also promotes the formation of lateral and adventitious roots. Auxin plays a main role in regulating the development of fruits, enhancing apical dominance, functions in phototropism and gravitropism, promotes vascular differentiation, as well as retard leaf abscission.
Cytokinins regulates cell division in shoots as well as roots. They help modify apical dominance and promotes lateral bud growth. Cytokinins promotes movement of nutrients into sink tissues, stimulates seed germination, and delay leaf senescence.
Gibberellins (GA)
Gibberellins stimulate stem elongation, pollen development, pollen tube growth, fruit growth, and seed development and germination. This hormone also regulates sex determination and helps with the transition from the plant's juvenile phase to its adult phase.
Abscisic Acid (ABA)
Abscisic acid inhibits growth, yet promotes the stomata's closure during drought stress. ABA helps with seed dormancy and inhibiting early germination. The hormone is a primary promotor of leaf senescence and desiccation tolerance.
Ethylene
Ethylene is the promotor of the ripening of fruit, leaf abscission, the triple response in seedlings, lateral expansion, horizontal growth, root and root hair formation, and flowering in pineapples. Ethylene inhibits stem elongation; it enhances rate of senescence.
Brassinosteroids (BR)
Brassinosteroids promote cell expansion and cell division in shoots, root growth at low concentrations, xylem differentiation, seed germination, and pollen tube elongation. BR inhibits traits such as root growth at high concentrations, and phloem differentiation.
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