Question

List five main groups of natural plant growth regulators. Write a note on the discovery, physiological functions and agriculture/horticultural applications of any one of them.

Answer 1

Hint: The chemical substances which regulate all the factors or aspects of growth and development within plants are termed as Plant growth regulators. Plant growth hormones and phytohormones are some of the other names of these regulators. Broadly, Plant growth regulators are categorized into two main groups depending on their actions including,
> Plant growth promoters
> Plant growth inhibitors.

Complete answer:
Five major groups of natural plant growth regulators are found which include
> Auxins
> Gibberellins
> Cytokinins
> Abscisic acid
> Ethylene
Discovery of Auxin
Charles Darwin and his son Francis Darwin performed experiments in 1881 on coleoptiles of canary grass. They observed that when coleoptiles are exposed to light coming from a unidirectional source then it bends towards the light. They concluded on observing that the tip of the coleoptile is responsible for sensing light and also formulated that a messenger is sent in the downward direction from the coleoptile tip inducing it to bend.
Peter Boysen-Jensen in 1913, concluded or surmised that the substance named gelatin was secreted at the tip which is water-soluble.
F.W. Went in 1928 exhibited the diffusion of the chemical messenger from the tip of the coleoptile and it results in the growth of the coleoptile towards the light. Later, He formulated that chemical substance (messenger) is a growth-promoting hormone of the plant which he referred to as auxin.
Physiological functions of Auxins include:
> Apical dominance and control of abscission
> Parthenocarpy (formation of seedless fruit)
> Differentiation of xylem and phloem
> Formation of the root in stem cuttings
Agricultural/Horticultural Applications
> In the case of orchards, flowering is promoted by the applications of auxin. For example, Litchi, pineapple etc.
> In some plants, parthenocarpy is induced by the auxins. Examples, cucumber, pepper and tomatoes etc.
> Maturation of the fruits like apples, grapes, and oranges on trees is regulated by auxins.

Note:
The students might get confused between Plant growth promoters and plant growth inhibitors.
Abscisic acid is considered to be one of the plant growth inhibitors as it facilities the dormancy of seeds and buds. It also controls the closing of the stomata and the organ size. Auxin is considered to be one of the plant growth promoters because it is responsible for promoting the elongation of the cells, shoots and root growth.