CBSE · Class 10 · Science · Control and CoordinationDiscuss the chemical coordination in plants in detail. Name the major plant hormones (phytohormones) and explain their respective physiological functions and effects on plant growth and development.

Chemical Coordination in Plants (Phytohormones)

\nUnlike animals, plants lack a nervous system for rapid transmission of electrical impulses. Instead, coordination and regulation of growth, development, and environmental responses in plants are mediated entirely by chemical substances known as plant hormones or phytohormones. These chemical messengers are synthesized in specific parts of the plant and transported to regions where they exert profound physiological effects. Phytohormones are broadly classified into two categories: plant growth promoters and plant growth inhibitors.

1. Auxins

• Site of Synthesis: Synthesized at the growing tips of roots and shoots (meristematic regions).
• Functions:
  • Promote cell elongation in stems, causing the plant to grow taller.
  • Play a crucial role in phototropism (bending towards light) and gravitropism (responding to gravity). When light falls on one side of a shoot, auxins diffuse towards the shady side, stimulating rapid cell elongation on that side, which causes the shoot to bend towards the light source.
  • Induce apical dominance, where the terminal bud suppresses the growth of lateral buds.

2. Gibberellins

• Site of Synthesis: Synthesized in young leaves, roots, and developing seeds.
• Functions:
  • Stimulate stem elongation, internodal growth, and leaf expansion.
  • Break seed dormancy and stimulate germination by inducing the synthesis of hydrolytic enzymes.
  • Promote bolting (sudden internodal elongation) in rosette plants prior to flowering and enhance fruit development.

3. Cytokinins

• Site of Synthesis: Synthesized mainly in regions of rapid cell division, such as roots, developing fruits, and seeds.
• Functions:
  • Promote rapid cell division (cytokinesis) along with auxins.
  • Delay senescence (aging) of leaves, keeping them green and healthy for longer periods.
  • Break seed dormancy and promote the growth of lateral buds, overcoming apical dominance.

4. Abscisic Acid (ABA)

• Site of Synthesis: Synthesized in leaves, roots, and green fruits (acts primarily as a growth inhibitor).
• Functions:
  • Inhibits plant growth and counteracts the growth-promoting effects of auxins, gibberellins, and cytokinins.
  • Induces seed dormancy, ensuring that seeds do not germinate under unfavorable environmental conditions.
  • Promotes stomatal closure during water stress (drought conditions) to prevent excessive water loss through transpiration.
  • Triggers abscission, leading to the shedding of leaves, flowers, and fruits.

5. Ethylene

• Site of Synthesis: Synthesized in ripening fruits, aging leaves, and flowers.
• Functions:
  • Acts as a gaseous plant hormone that promotes fruit ripening.
  • Stimulates senescence and abscission of leaves and flowers.
  • Enhances transverse cell expansion while inhibiting longitudinal growth in certain seedlings. \nIn summary, the balanced interaction, synergy, and antagonism among these five major phytohormones allow plants to respond harmoniously to internal developmental cues and external environmental stimuli.