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.

Five main groups of natural plant growth regulators

• Auxins
• Gibberellins
• Cytokinins
• Abscisic acid (ABA)
• Ethylene

Note on gibberellins

• • Discovery

    • First recognized in 1926 by Eiichi Kurosawa studying “bakanae” or “foolish seedling” disease in rice

    • Caused by the fungus Gibberella fujikuroi; isolated active compound named gibberellic acid (GA₃)

  • Physiological functions

    • Stimulate stem and internode elongation by promoting cell division and expansion

    • Break seed dormancy; trigger synthesis of α-amylase in aleurone cells during germination

    • Promote bolting in rosette plants, delay senescence, induce flowering under favorable conditions

  • Agricultural and horticultural applications

    • Enhance grape cluster size and bunch elongation in seedless grapes (e.g., Thompson variety)

    • Increase sugarcane yield by stimulating internode elongation, boosting biomass by up to 20 t/acre

    • Accelerate barley malting by uniform germination and enzyme activation

    • Improve fruit shape in apples, induce seedlessness in citrus, and extend floral and foliage vase life