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2.4 Plant Physiology: Chemical Coordination in Plants
1. Plant Growth Regulators (Phytohormones)
Chemical messengers regulating plant growth, development, and responses.
1.1. Auxins
Primary Role:
Cell elongation and differentiation.
Physiological Effects:
Cell Elongation:
Promotes stem growth.
Apical Dominance:
Inhibits lateral bud growth.
Root Initiation:
Promotes adventitious roots on cuttings.
Fruit Development:
Promotes growth, prevents premature drop.
Herbicide Action:
High concentrations act as selective herbicides (e.g., 2,4-D).
1.2. Gibberellins (GAs)
Primary Role:
Stem elongation.
Physiological Effects:
Stem Elongation:
Significant increase in length.
Seed Germination:
Break dormancy, promote enzyme synthesis.
Flowering:
Can induce flowering in some plants.
Fruit Development:
Promote fruit growth (e.g., grapes).
Bolting:
Stem elongation in rosette plants.
1.3. Cytokinins
Primary Role:
Cell division and differentiation.
Physiological Effects:
Cell Division:
Promotes cell proliferation.
Delay Senescence:
Delays aging and yellowing of leaves.
Break Apical Dominance:
Promotes lateral bud growth.
Chloroplast Development:
Promotes chloroplast formation.
1.4. Abscisic Acid (ABA)
Primary Role:
Stress hormone, growth inhibitor.
Physiological Effects:
Stomatal Closure:
Reduces water loss during stress.
Seed Dormancy:
Induces and maintains dormancy.
Bud Dormancy:
Promotes dormancy in buds.
Abscission:
Promotes shedding of leaves, fruits, flowers.
1.5. Ethylene
Primary Role:
Fruit ripening and senescence (gaseous hormone).
Physiological Effects:
Fruit Ripening:
Promotes ripening of climacteric fruits.
Senescence and Abscission:
Accelerates aging and shedding.
Triple Response:
Inhibition of stem elongation, radial swelling, horizontal growth in seedlings.
2. Tropic Movements in Plants
Directional growth responses to external stimuli, regulated by hormones (especially auxins).
2.1. Phototropism
Definition:
Growth movement in response to light.
Mechanism:
Auxins migrate to shaded side, causing differential cell elongation.
Types:
Positive:
Towards light (shoots).
Negative:
Away from light (roots, weak).
2.2. Geotropism (Gravitropism)
Definition:
Growth movement in response to gravity.
Mechanism:
Auxins accumulate on the lower side; roots are more sensitive to auxin inhibition.
Types:
Positive:
Towards gravity (roots).
Negative:
Away from gravity (shoots).
2.3. Hydrotropism
Definition:
Growth movement in response to water.
Type:
Roots typically exhibit positive hydrotropism (grow towards water).
2.4. Thigmotropism
Definition:
Growth movement in response to touch or physical contact.
Mechanism:
Differential growth causes coiling around support.
Example:
Coiling of tendrils.
2.5. Chemotropism
Definition:
Growth movement in response to a chemical stimulus.
Example:
Pollen tube growth towards the ovule (positive chemotropism).