The term hormone was first used by William Ballice and Ernest Starling. For plants, the term photochrome was given to distinguish them from animal hormones. When plants respond to stimuli by producing a chemical called chemical hormones. Are also known as chemical messengers.
Auxin was given by Francis Darwin. The meaning oxygen is to grow as it is derived from the Greek word auxein. Auxins are powerful growth stimuli.
- Promotes the growth of stem, root, and fruits by cell elongation.
- It delays leaf senescence aging and falling of leaves.
- Promotes the growth of apical buds and lateral buds.
- This phenomenon of the suppression of the growth of lateral buds by apical buds is called apical dormancy.
A different form of gibberellins is reported from different plants named GA1, GA2, and GA3, and so on. Mainly distributed in meristematic tissues or regions such as stem apex, root apex, buds, seeds, etc.
- Promotes the growth of internodes.
- It breaks seed dormancy and initiates germination.
- It promotes fruit growth and is capable of inducing parthenocarpy.
- It delays senescence.
- It is widely used in horticulture and the food industry.
- It enhances the longitudinal growth of the internodes of dwarf plants.
- It is used to speed up the malting process in the brewing industry.
This hormone was discovered by Skoog and Miller. It is widely distributed in plants. It is produced in root tips. It is transported through the xylem. A large number of cytokinins are found in germinating seeds, embryo, developing fruits, etc.
- It stimulates plant growth against auxin and gibberellins against cell division.
- It causes the expansion of cotyledons in seeds.
- It breaks seed dormancy.
- It promotes germination.
- It promotes chlorophyll synthesis.
- It inhibits apical dominance.
It is the only hormone that is present in the gaseous state at ordinary temperature. It was discovered by R. Gane.
- It causes a reduction in stem elongation.
- It accelerates senescence.
- It helps in the ripening of fruits.
- It initiates germination in peanut seeds and germination in potato tubers.
- It promotes root growth and root hair formation.
- It induces flowers in mango trees.
Abscisic Acid (ABA)
It is a growth retaining hormone. Basically, it is found in angiosperms, gymnosperms, and in some moses. It is found in the chloroplast of leaves. Fruits and seeds contain a higher amount of ABA.
- It acts as a general growth inhibitor by slowing down the plant’s metabolism.
- It inhibits seed germination and development.
- It induces seed dormancy.
- It helps the seed to withstand desiccation.
- It accelerates senescence and abscission.
- It stimulates the closure of stomata in the epidermis.
- It increases the tolerance of plants.