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

The five main groups of natural plant growth regulators are:

1. Auxins
2. Gibberellins
3. Cytokinins
4. Abscisic acid (ABA)
5. Ethylene

I'll focus on Auxins for further elaboration:

Auxins:

Discovery: The discovery of auxins dates back to the early 20th century, primarily credited to the experiments conducted by Charles Darwin and his son Francis. However, it was the work of Dutch scientist Frits Warmolt Went that provided substantial evidence for the existence and role of auxins. Went demonstrated that the bending of coleoptiles (the protective sheath covering the emerging shoot) in response to light was due to the migration of a growth-promoting substance from the tip of the coleoptile.

Physiological Functions: Auxins play crucial roles in various aspects of plant growth and development. Some of their key functions include:

• Promotion of cell elongation, particularly in stems and coleoptiles, which aids in tropisms such as phototropism and gravitropism.
• Inhibition of lateral bud growth, thereby promoting apical dominance.
• Stimulation of root initiation in stem cuttings and adventitious root formation.
• Regulation of vascular tissue differentiation.
• Fruit development and ripening.

Agricultural/Horticultural Applications: Auxins find extensive applications in agriculture and horticulture:

• Rooting Hormones: Synthetic auxins such as indole-3-butyric acid (IBA) and naphthaleneacetic acid (NAA) are commonly used as rooting hormones to promote root formation in cuttings. This application aids in vegetative propagation, allowing for the efficient cloning of desirable plant varieties.
• Weed Control: Synthetic auxins like 2,4-Dichlorophenoxyacetic acid (2,4-D) are used as herbicides to control broadleaf weeds in crops like cereals and turf grasses. These herbicides disrupt normal plant growth processes, leading to uncontrolled growth and eventual death of the target plants.
• Fruit Development: Auxin sprays are sometimes used to promote fruit set and development, particularly in seedless fruit varieties. By influencing fruit growth and development, auxins can enhance crop yields and quality.

In conclusion, auxins are fundamental plant growth regulators with diverse physiological functions and wide-ranging applications in agriculture and horticulture. Their discovery revolutionized our understanding of plant growth processes and continues to be instrumental in modern plant science and crop management practices.