CBSE · Class 10 · Science · Control and CoordinationWhat are plant hormones (phytohormones)? List the five major types of plant hormones, classify them into growth promoters and growth inhibitors, and explain at least two major physiological functions of each.

Introduction to Plant Hormones\nPlant hormones, also known as phytohormones, are naturally occurring organic chemical substances produced in very low concentrations in plants. They regulate cellular activities, growth, development, and responses to environmental stimuli. Unlike animals, plants lack a nervous system; hence, coordination is solely achieved through chemical messengers.

Classification and Functions of Plant Hormones\nPlant hormones are broadly classified into two categories:

1. Growth Promoters (Stimulate cell division, enlargement, and flowering):

   • Auxins:
     • Promotes cell elongation in stems and shoots.
     • Responsible for phototropism (bending towards light) by accumulating on the shaded side.
   • Gibberellins:
     • Stimulates stem elongation and internodal growth (useful in increasing fruit size like grapes).
     • Breaks seed dormancy and initiates germination.
   • Cytokinins:
     • Promotes rapid cell division (cytokinesis), particularly in root tips, fruits, and shoot buds.
     • Delays aging (senescence) in leaves and helps in nutrient mobilization.

2. Growth Inhibitors (Involve in dormancy, abscission, and stress response):

   • Abscisic Acid (ABA):
     • Acts as a general plant growth inhibitor.
     • Promotes wilting and closure of stomata during water stress (drought conditions) and induces seed dormancy.
   • Ethylene:
     • A gaseous hormone that stimulates fruit ripening.
     • Promotes senescence and abscission (shedding) of leaves, flowers, and fruits.

Conclusion\nThus, plant hormones work either synergistically or antagonistically to maintain balanced plant growth and coordinate responses to external environmental challenges.