flowering lecture 14
Fruit Development
Ovule Development: Ovules transform into seeds after fertilization.
Ovary Wall: The wall of the ovary matures and becomes the fruit.
Function of Fruit:
Promotes seed dispersal once mature.
Fleshy Fruits: Entice animals with nutritious pulp to aid in seed transportation.
Dry Fruits: Utilize mechanical methods for dispersal, including:
Adhering to animal fur.
Being carried by wind.
Dormancy
Embryo Maturation: As the embryo grows, it loses moisture, achieving approximately 10% water content.
Hormonal Regulation: Hormone abscisic acid (ABA) plays a crucial role.
Metabolic Activity: The embryo enters a state of dormancy with significantly slowed metabolism, ranging from days to potentially years, waiting for optimal environmental conditions for growth.
Seed Germination
Gibberellin Production: Upon germination, the embryo produces gibberellin (GA), a vital hormone.
Target Tissue: The hormone diffuses to the aleurone, the outer layer of the endosperm.
Aleurone Cells: Activate to synthesize a-amylase, an enzyme crucial for breaking down starch into sugars.
Nutrient Transfer: Sugars from the digested endosperm are sent to the embryo, acting as a 'sink', which provides essential food reserves for seedling growth.
Control of Flowering
Angiosperms Growth Phases:
Initial Phase: Vegetative growth enabled by the shoot apical meristem (SAM), forming leaves, stems, and buds.
Reproductive Phase: Further development leads to flower formation, including sepals, petals, stamens, and carpels.
Flowering and Life Histories
Significance of Flowering: Essential for completing a plant's sexual lifecycle and influences lifespan.
Types of Plants:
Perennials: Flower yearly; survive multiple seasons.
Annuals: Complete their life cycle in one year.
Biennials: Require two years, often need a chilling period for flowering after the first year.
Evolutionary Implications: Shorter life cycles can accelerate evolutionary processes.
Factors Influencing the Switch from Vegetative to Reproductive Growth
Internal Cues: Occur in all plants based on maturity or "phase change".
External Cues: Include photoperiod (daylength) and vernalization (chilling).
Phase Change in Plant Development
Seed Germination: Initially, seeds cannot flower.
Growth Period: Post-germination, plants transition from a juvenile phase to an adult phase capable of flowering.
Advantages of Juvenile Phase
Resource Accumulation: Juvenile phase allows the plant to gather resources necessary for successful flowering, seed production, and fruit development.
External Cues: Photoperiod
External environmental changes like temperature and water availability impact flowering times.
Daylength Variations: Important in the timing of flowering events.
Regular (day length) vs. Irregular (temperature, water availability) patterns affect flowering progression.
Photoperiodic Plants
Short Day Plants (SDP): Flower when the day length is shorter than a critical threshold.
Long Day Plants (LDP): Flower under prolonged day conditions.
Day Neutral Plants (DNP): Flowering is unaffected by day length considerations.
Mechanism of Photoperiodic Induction of Flowering
Historical Experiments: In the 1920s, tobacco plants demonstrated that flowering can be induced by altering day length, specifically in winter conditions.
Critical Daylength
Each species has specific daylength requirements for flowering:
SDP: Requires days shorter than a certain threshold.
LDP: Requires days longer than a specific threshold.
Night Break Experiments
Flowing Trigger: The length of the night is the critical factor, rather than the length of the day.
Experiment Details: Dark treatment during the day shows no effect; light treatment during the night influences flowering.
Night Break Mechanism
A night break transforms a long night into two short nights, adjusting to photoperiod influences.
Effects seen include:
LDP flowering with specific conditions.
SDP flowering under contrasting conditions.
Example: Maryland Mammoth Tobacco
Flowering Conditions: The Maryland Mammoth tobacco only flowers when days are less than 14 hours long, a critical threshold distinct from Henbane, which flowers when days exceed 14 hours.
Commercial Uses of Night Breaks
Commercial Applications: Used to synchronize flowering with market demand, as seen with the Poinsettia, an SDP.
Other External Cues for Flowering
Vernalization: Chilling affects biennials like parsley and carrots, impacting their ability to flower.
Mechanism of Photoperiodic Induction
Organ of Perception: Leaves perceive photoperiod variations impacting flowering.
Photoperiod Influence: Covering a small part of a leaf can still maintain valid photoperiodic effects.
Florigen Production and Movement
Florigen: A factor synthesized in leaves that travels through phloem, facilitating the switch to reproductive growth in the shoot meristem.
Grafting: Florigen can also transfer between grafted plants, inducing flowering in partner plants.
Photoperiodic Response Mechanism
Action Spectrum: Phytochrome in leaves is responsible for time perception, influencing flowering responses.
Types of Light: White light and red light significantly impact plant responses, while far red light is less effective.
Plant Induction Complexity
Genetic Regulation: Different gene factors play critical roles in managing flowering across species.
Phytochrome States: These conditions regulate internal clocks affecting plant responses to day length.
CONSTANS/FT Interaction: Flow of signaling from CONSTANS protein (CO) regulates FLOWERING LOCUS T (FT), which triggers flowering in respective plants.