Plant Growth and Development

Growth is regarded as one of the most fundamental and conspicuous characteristics of a living being. It is an irreversible permanent increase in size of an organ or its parts.

• Open Form of Growth: Plants retain the capacity for unlimited growth throughout their life due to Meristems (Shoot Apical Meristem, Root Apical Meristem).
• Phases of Growth:
  1. Meristematic Phase: Cells with rich protoplasm, large nucleus. High respiration.
  2. Elongation Phase: Increased vacuolation, cell enlargement, new cell wall deposition.
  3. Maturation Phase: Cells attain maximal size. Protoplasmic modification.
• Growth Rates:
  - Arithmetic: Lt = L0 + rt. (Linear curve).
  - Geometric: W1 = W0 * e^rt. (Sigmoid curve).

Small, simple molecules. Indole compounds (IAA), adenine derivatives (Kinetin), carotenoid derivatives (ABA), terpenes (GA), gases (Ethylene).

A. Auxins (Indole-3-Acetic Acid)

Discovery: Charles Darwin (Canary grass). Isolated by F.W. Went (Oat coleoptile).

• Functions: Apical Dominance, Rooting in stem cuttings, Flowering (Pineapple), Parthenocarpy (Tomatoes).
• Synthetic Auxins: NAA, 2,4-D (Weedicide).

B. Gibberellins (GA3)

Discovery: Kurosawa (Bakanae disease in Rice - Gibberella fujikuroi).

• Functions: Increase stalk length (Grapes, Sugarcane), Bolting (Internode elongation in beet/cabbage), Delay senescence (Fruits on tree).

C. Cytokinins (Kinetin/Zeatin)

Discovery: Skoog & Miller (Herring sperm DNA). Zeatin (Corn kernels).

• Functions: Cell division, Overcome apical dominance, Delay leaf senescence (Mobility of nutrients), New leaves/chloroplasts.

D. Ethylene (Gas)

Discovery: Cousins (Ripened oranges hastened banana ripening).

• Functions: Fruit ripening, Triple response (Horizontal growth, swelling, hook formation), Absicission, Sprouting (Potato), Flowering (Mango).
• Source: Ethephon (Aqueous solution).

E. Abscisic Acid (ABA)

Stress Hormone. Antagonist to GAs.

• Functions: Inhibit seed germination, Close stomata (Water stress), Seed development & dormancy.

Photoperiodism

Response of plants to duration of light/dark.

• Long Day Plants: Requirement > Critical duration.
• Short Day Plants: Requirement < Critical duration.
• Day Neutral Plants: No correlation (Tomato, Pepper).
• Site of Perception: Leaves. (Hypothetical hormone: Florigen).

Vernalization

Promotion of flowering by low temperature treatment.

• Prevents precocious reproductive development.
• Examples: Winter varieties of Wheat/Rye/Barley (Planted in autumn), Biennials (Sugarbeet, Cabbage, Carrot).

Irreversible permanent increase in size of an organ or its individual cell.

Plants retain capacity for unlimited growth due to presence of meristems.

Cells have rich protoplasm, large conspicuous nuclei, thin cellulosic walls.

Increased vacuolation, cell enlargement, and new cell wall deposition.

Following mitotic cell division, only one daughter cell continues to divide. Linear curve. Lt = L0 + rt.

Both daughter cells continue to divide. Sigmoid curve (Lag, Log, Stationary).

Plants follow different pathways in response to environment (e.g., Heterophylly in Cotton, Coriander, Larkspur).

Indole-3-acetic acid (IAA) - Auxin.

N6-furfurylamino purine (Kinetin) - Cytokinin.

Abscisic Acid (ABA).

Gibberellic Acid (GA3).

Studied Phototropism in Canary grass. Concluded tip transmits influence.

Isolated Auxin from tips of coleoptiles of oat seedlings.

"Foolish Seedling" disease of rice caused by Gibberella fujikuroi. Discovered Gibberellins.

Identified Kinetin (Cytokinesis promoting substance) from autoclaved herring sperm DNA.

Confirmed release of volatile substance (Ethylene) from ripened oranges.

Auxin produced at apical bud inhibits growth of lateral buds. (Tea plantation pruning).

Synthetic Auxin. Used as weedicide to kill broad-leaved weeds (Dicots).

Internode elongation just prior to flowering. Promoted by Gibberellins (Beet, Cabbage).

Natural cytokinin isolated from corn-kernels and coconut milk.

Function of Cytokinins. Helps delay leaf senescence.

Caused by Ethylene. 1. Horizontal growth. 2. Swelling of axis. 3. Apical hook formation.

Source of ethylene. Aqueous solution. Hastens fruit ripening.

Abscisic Acid (ABA). Stimulates closure of stomata during water stress.

ABA acts as antagonist to Gibberellins (in seed dormancy/germination).

Require light period longer than critical duration (e.g., Spinach, Radish).

Require light period shorter than critical duration (e.g., Tobacco, Chrysanthemum).

Leaves are the site of perception for light/dark duration (Photoperiodism).

Promotion of flowering by low temperature.

Controlled by ABA, Phenolic acids, Para-ascorbic acid. Broken by GA or Nitrates.

Auxins induce parthenocarpy (seedless fruits) in tomatoes.

GA3 speeds up malting process in brewing induestry.

GAs hasten maturity period in conifers leading to early seed production.

Auxin to Cytokinin ratio regulates shoot/root differentiation. (High C/A = Shoot).

Monocarpic plants that flower in second season (Sugarbeet, Cabbage, Carrot). Vernalization helps.