5.1 Types of Responses
Plants respond to internal and external environmental stimuli (light, gravity, touch, water, and chemicals) through specialized growth or movement responses to maximize survival and resource acquisition.
1. Tropism Responses
- Definition: Directional growth responses of plant organs toward or away from an external directional stimulus.
- Characteristics: Slow growth response; permanent and irreversible; direction of response is directly determined by the direction of the stimulus.
- Positive Tropism: Growth movement towards the stimulus.
- Negative Tropism: Growth movement away from the stimulus.
- Types of Tropisms:
- Phototropism: Growth response to light (e.g., shoots are positively phototropic; roots are negatively phototropic).
- Geotropism / Gravitropism: Growth response to gravity (e.g., shoots are negatively geotropic; roots are positively geotropic).
- Hydrotropism: Growth response to water (e.g., roots are positively hydrotropic).
- Thigmotropism: Growth response to touch or mechanical contact with a solid surface (e.g., tendrils wrapping around supporting structures).
- Chemotropism: Growth response to chemical substances (e.g., pollen tube growth towards chemicals secreted by the ovule).
2. Nastic Responses
- Definition: Reversible, non-directional movements of plant parts in response to external stimuli.
- Characteristics: Rapid response; reversible (non-growth movements caused by turgor pressure changes in pulvinus cells); direction of movement is independent of the stimulus direction.
- Types of Nastic Responses:
- Seismonastic / Thigmonastic: Movement triggered by touch, vibration, or shock (e.g., rapid folding of Mimosa pudica leaflets).
- Nyctinastic: Sleep movements triggered by rhythmic day-night changes (e.g., folding of leguminous leaves like Leucaena leucocephala at night).
- Thermonastic: Movement induced by temperature changes (e.g., opening of tulip flowers in warm air).
- Photonastic: Movement induced by light intensity changes (e.g., opening of morning glory flowers in sunlight).
5.2 Phytohormones (Plant Hormones)
Phytohormones are organic chemical messengers synthesized in small amounts in specific plant tissues that translocate to target cells to regulate physiological growth processes.
1. Major Classes of Phytohormones
- Auxins (e.g., Indole-3-acetic acid / IAA):
- Synthesized primarily in shoot apical meristems and young leaves.
- Promotes cell elongation in shoots, apical dominance, fruit development, and adventitious root initiation.
- Inhibits lateral bud growth and leaf abscission.
- Gibberellins (GA):
- Promotes stem elongation (internode extension), seed germination (breaks seed dormancy by stimulating amylase synthesis), and flowering.
- Cytokinins:
- Synthesized in roots and actively dividing tissues; promotes cell division (mitosis), morphogenesis, and delays leaf senescence (aging).
- Works synergistically with auxin to regulate organogenesis.
- Abscisic Acid (ABA):
- Stress hormone; inhibits growth, promotes seed and bud dormancy, and induces rapid stomatal closure during drought stress to prevent transpiration loss.
- Ethylene:
- Gaseous hormone; accelerates fruit ripening, promotes triple response in seedlings, and stimulates leaf/flower abscission.
5.3 Application of Phytohormones in Agriculture
1. Commercial Uses
- Auxins: Used as rooting powders for vegetative stem cuttings; used in synthetic form (2,4-D) as selective herbicides to kill broadleaf weeds; induces parthenocarpy (seedless fruit production without fertilization).
- Gibberellins: Applied to grape clusters to increase fruit size and space out berries; breaks seed dormancy to uniform crop germination.
- Ethylene: Used commercially to synchronize ripening of harvested fruits (e.g., bananas, mangos, tomatoes) during transport.
- Cytokinins: Used in tissue culture media alongside auxins to induce callus differentiation into shoots and roots.
- Abscisic Acid (ABA): Applied to plants in nurseries to induce stomatal closure and reduce transpiration during transport.
5.4 Mechanism of Auxin Action in Tropisms
1. Role of Auxin in Phototropism
- Auxin (IAA) is produced at the shoot tip and diffuses downward.
- When unilateral light shines on one side of the shoot, auxin diffuses away from light toward the shaded side.
- High auxin concentration on the shaded side accelerates cell elongation compared to the illuminated side.
- Differential cell elongation causes the shoot to bend toward the light source (Positive Phototropism).
2. Role of Auxin in Geotropism
- When a seedling is placed horizontally, gravity pulls auxin to accumulate on the lower side of both shoot and root.
- In Shoots: High auxin concentration on the lower side stimulates cell elongation $\rightarrow$ shoot bends upward (Negative Geotropism).
- In Roots: High auxin concentration on the lower side inhibits cell elongation (root cells are far more sensitive to auxin) $\rightarrow$ upper side cells elongate faster, causing root to bend downward (Positive Geotropism).