Plant Growth and Development

Key Concepts

Characteristics of Plant Growth

Growth is an irreversible permanent increase in size of an organ or its parts or even of an individual cell. It is usually accompanied by metabolic processes (anabolic and catabolic).

• Indeterminate Growth: Plants retain the capacity for unlimited growth throughout their life due to the presence of meristems (apical, lateral, and intercalary).
• Open Form of Growth: New cells are constantly added to the plant body by meristematic activity.
• Measurability: Growth is measured by parameters such as increase in fresh weight, dry weight, length, area, volume, and cell number.

Phases of Growth

1. Meristematic Phase: Cells at root and shoot apices. They are rich in protoplasm, have large nuclei, and thin cellulosic walls with abundant plasmodesmata.
2. Elongation Phase: Cells proximal to the meristematic zone. Characterized by increased vacuolation, cell enlargement, and new cell wall deposition.
3. Maturation Phase: Cells attain maximal size in terms of wall thickening and protoplasmic modifications.

Growth Rates and Curves

• Arithmetic Growth: Only one daughter cell continues to divide while the other matures (e.g., root elongation). Expressed as $L_t = L_0 + rt$.
• Geometric Growth: Both daughter cells continue to divide. It follows a Sigmoid (S-curve) consisting of:
  • Lag phase: Initial slow growth.
  • Log/Exponential phase: Rapid growth.
  • Stationary phase: Growth slows down due to limited nutrients.
• Absolute vs. Relative Growth Rate: Absolute is total growth per unit time; Relative is growth per unit initial parameter.

Differentiation, Dedifferentiation, and Redifferentiation

• Differentiation: Cells from meristems mature to perform specific functions (e.g., forming tracheary elements).
• Dedifferentiation: Differentiated cells regain the capacity to divide under certain conditions (e.g., formation of interfascicular cambium from parenchyma).
• Redifferentiation: Cells produced by dedifferentiated tissues lose their capacity to divide again and mature for specific functions (e.g., secondary xylem/phloem).

Development and Plasticity

Development is the sum of growth and differentiation. Plants show plasticity, the ability to follow different pathways in response to environment or life phases.

• Heterophylly: Different shapes of leaves in juvenile vs. mature phases (e.g., cotton, coriander) or in air vs. water (e.g., buttercup).

Plant Growth Regulators (PGRs)

PGRs are small, simple molecules that regulate growth and development.

1. Auxins (e.g., IAA): Produced at apices. Promote apical dominance, rooting in cuttings, and prevent early fruit drop. Used as herbicides (2,4-D).
2. Gibberellins (e.g., $GA_3$): Increase length of axis (grapes, sugarcane), delay senescence, and promote bolting in rosette plants.
3. Cytokinins (e.g., Zeatin, Kinetin): Promote cell division, overcome apical dominance, and delay leaf senescence by nutrient mobilization.
4. Ethylene (Gaseous): Highly effective in fruit ripening (respiratory climactic). Promotes senescence, abscission, and breaks seed/bud dormancy.
5. Abscisic Acid (ABA): Known as the stress hormone. Inhibits growth and metabolism, promotes stomatal closure, and induces seed dormancy to withstand unfavorable conditions.

Activities

• Observe heterophylly in local plants like coriander.
• Study the effect of apical bud removal on lateral branches in a garden.
• Compare the growth of seedlings in light vs. dark.