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Plant Physiology: Photoperiodism & More
1. Photoperiodism and Flowering
Definition:
Physiological response of plants to day/night length.
Plant Classification:
Short-Day Plants (SDPs):
Flower when day length < critical period (e.g., Chrysanthemums).
Long-Day Plants (LDPs):
Flower when day length > critical period (e.g., Spinach).
Day-Neutral Plants (DNPs):
Flower irrespective of day length (e.g., Tomatoes).
Molecular Mechanism of Flowering
Perception:
Occurs in leaves.
Circadian Clock & CONSTANS (CO):
Measures day length, controls CO gene expression.
Florigen (FT Protein):
Mobile flowering signal, travels from leaves to meristem.
Activation:
FT protein activates floral identity genes (e.g.,
APETALA1
,
LEAFY
) at the meristem.
2. Phytochrome and Light Perception
Phytochrome:
Photoreceptor proteins detecting red and far-red light.
Forms:
Pr:
Inactive form, absorbs red light.
Pfr:
Active form, absorbs far-red light.
Pfr/Pr ratio:
Provides information about the light environment.
3. Night Break and Day Break
Night Break:
Brief light exposure during dark period can manipulate flowering (inhibits SDPs, promotes LDPs).
Day Break (Dawn Signal):
Transition from dark to light, resets circadian clock.
4. Seed Dormancy
Definition:
Seed unable to germinate even under favorable conditions.
Hormonal Control:
Balance between
Abscisic Acid (ABA)
(induces/maintains dormancy) and
Gibberellins (GA)
(promote germination).
ABA/GA Ratio:
High ratio promotes dormancy, low ratio promotes germination.
5. Vernalisation
Definition:
Induction of flowering by prolonged exposure to cold temperatures.
Mechanism:
Epigenetic silencing of the
FLOWERING LOCUS C (FLC)
gene (a floral repressor) through histone modifications.
Memory of Winter:
Silenced FLC state maintained after cold, ensuring flowering post-winter.
6. Plant Hormones (Phytohormones)
Auxins:
Cell elongation, apical dominance, root formation.
Gibberellins:
Stem elongation, seed germination, flowering.
Cytokinins:
Cell division, shoot formation, delay senescence.
Abscisic Acid (ABA):
Growth inhibitor, seed dormancy, stomatal closure (stress hormone).
Ethylene:
Gaseous, fruit ripening, abscission, senescence.
Brassinosteroids:
Cell elongation/division, stress responses.
Strigolactones:
Inhibit shoot branching, promote mycorrhizal fungi.
7. Allied Topics
Circadian Rhythms:
Internal biological clock for daily cycles.
Shade Avoidance Syndrome (SAS):
Responses to avoid shading (e.g., stem elongation).
Thermoperiodism:
Plant response to daily temperature fluctuations.
Plant Movements:
Tropisms:
Directional growth (Phototropism, Gravitropism, Thigmotropism).
Nastic Movements:
Non-directional (Nyctinasty, Thigmonasty).