1.0 Phytohormones: The Chemical Messengers
Unlike animals, plants lack a nervous system. Coordination is achieved entirely through Chemical Messengers known as Phytohormones (Plant Growth Regulators). These are organic substances produced in minute quantities in one part of the plant and translocated to another part to elicit specific physiological responses.
1.1 Auxins: The Master of Apical Dominance
Discovered by F.W. Went, Auxins (specifically Indole-3-Acetic Acid or IAA) are synthesized at the shoot and root tips.
- Cell Elongation: Auxins promote the elongation of cells in shoots.
- Apical Dominance: The presence of the apical bud inhibits the growth of lateral (axillary) buds. Removing the tip (decapitation) allows lateral buds to grow, a technique used in Hedge-making and tea plantations.
- Phototropism: Auxins migrate towards the shaded side of a stem, causing that side to grow faster and making the stem bend toward the light.
Parthenocarpy: The induction of fruit development without fertilization, resulting in seedless fruits (e.g., seedless watermelons, grapes). This is commercially achieved by spraying Auxins and Gibberellins.
1.2 Comparative Phytohormone Physiology
| Hormone | Primary Function | Advanced Mechanism |
|---|---|---|
| Gibberellins | Internodal Elongation | Breaks seed dormancy and induces Bolting (sudden stalk elongation). |
| Cytokinins | Cell Division | Promotes Cytokinesis; delays leaf senescence (aging). |
| Ethylene | Fruit Ripening | The only Gaseous hormone; promotes abscission (falling) of leaves. |
| Abscisic Acid (ABA) | Growth Inhibitor | The "Stress Hormone"; causes Stomatal Closure during drought. |
The Richmond-Lang Effect: This refers to the prevention of senescence (yellowing and aging) in detached leaves by the application of Cytokinins. This is a key focus in advanced agricultural studies to increase the shelf life of green vegetables.
Do not confuse Auxins and Gibberellins. While both promote growth, Auxins primarily cause cell elongation in the apical region, whereas Gibberellins are famous for the "Foolish Seedling" (Bakane) effect—causing extreme elongation of the internodes in genetically dwarf plants.
2.0 Plant Movements: Tropic and Nastic Responses
Plants exhibit movement in response to external stimuli. These movements are categorized based on whether the direction of the stimulus determines the direction of the movement. Unlike locomotion in animals, plant movements are usually Growth-Mediated or Turgor-Mediated.
2.1 Tropisms: Direction-Dependent Growth
Tropisms are slow, directional growth movements. If the growth is toward the stimulus, it is Positive; if away, it is Negative.
- Phototropism: Response to Light. Stems are positively phototropic (mediated by Auxin accumulation on the shaded side).
- Geotropism (Gravitropism): Response to Gravity. Roots are Positively Geotropic, while shoots are Negatively Geotropic.
- Hydrotropism: Response to Water. Roots always grow toward moisture, even overriding the force of gravity.
- Thigmotropism: Response to Touch. Seen in tendrils of climbers (e.g., Pea, Bitter gourd) that twining around a support.
- Chemotropism: Response to Chemicals. The growth of the Pollen Tube toward the ovule stimulated by sugary secretions from the stigma.
2.2 Nastic Movements: Non-Directional Responses
Unlike tropisms, the direction of nastic movement is independent of the stimulus direction. These are often rapid and reversible.
- Thigmonasty (Seismonasty): Response to touch/shock. Best seen in Mimosa pudica (Touch-me-not). It is caused by a sudden loss of turgor pressure in the Pulvinus (swollen leaf base).
- Photonasty: Response to light intensity. Flowers of some plants (e.g., Evening Primrose) open at night and close during the day.
- Nyctinasty: "Sleep movements." The drooping of leaves at night in response to darkness.
Statocytes and Statoliths: Specialized cells in the root cap containing heavy starch grains (Statoliths). Gravity causes these grains to settle at the bottom of the cell, signaling the root to grow downward—the molecular basis of Geotropism.
2.3 Movement Comparison
| Feature | Tropic Movement | Nastic Movement |
|---|---|---|
| Directionality | Direction-dependent. | Non-directional. |
| Mechanism | Growth-mediated (Permanent). | Turgor-mediated (Often reversible). |
| Speed | Slow. | Relatively Fast. |
Clinostat: A laboratory instrument used to eliminate the effect of gravity on plant growth. By slowly rotating a potted plant horizontally, gravity acts equally on all sides of the root and shoot, causing them to grow straight instead of bending.
Important distinction: While most stems are negatively geotropic, underground modified stems (like Potato tubers or Ginger rhizomes) grow horizontally. These are described as Diageotropic.