This MCQ module is based on: Plant Sexual Reproduction: The Floral Journey
Plant Sexual Reproduction: The Floral Journey
Study Notes and Summary
- Sexual Reproduction in Flowering Plants (Angiosperms):
- Flower as Reproductive Organ: The specialized structure evolved for sexual reproduction.
- Floral Anatomy:
- Sepals (Calyx): Outermost whorl, typically green, protect the developing bud.
- Petals (Corolla): Often brightly colored and/or fragrant, attract pollinators.
- Stamen (Androecium – Male Reproductive Part):
- Anther: Contains pollen sacs where pollen grains (containing male gametes) are produced.
- Filament: Stalk supporting the anther.
- Pistil/Carpel (Gynoecium – Female Reproductive Part):
- Stigma: Receptive tip, often sticky, for pollen landing.
- Style: Stalk connecting the stigma to the ovary.
- Ovary: Basal swollen part containing ovules.
- Ovule: Contains the female gamete (egg cell/ovum) and other cells important for fertilization and seed development.
- Types of Flowers:
- Unisexual Flower: Contains either stamens (male flower) or pistil (female flower) (e.g., Papaya, Watermelon, Maize).
- Bisexual Flower (Hermaphrodite): Contains both stamens and pistil (e.g., Hibiscus, Mustard, Rose).
- Pollination: The transfer of pollen grains from the anther to the stigma.
- Self-pollination: Pollen transferred within the same flower or to another flower on the same plant. Leads to less genetic variation.
- Cross-pollination: Pollen transferred between flowers of different plants of the same species. Requires external agents (pollinators) like wind, water, insects, birds, bats. Promotes genetic variation.
- Adaptations for Pollination: Flowers have evolved various features to attract specific pollinators (e.g., color, scent, nectar, specific shapes).
- Fertilisation (Angiosperms): The fusion of male and female gametes.
- Pollen grain landing on the stigma absorbs moisture and nutrients, swells, and germinates, producing a pollen tube.
- The pollen tube grows down through the style, towards the ovule in the ovary, guided by chemical signals.
- The pollen tube carries two male gametes (sperms).
- Double Fertilisation (Unique to Angiosperms): A key event where two fusions occur:
- One male gamete fuses with the egg cell to form the zygote (2n), which develops into the embryo.
- The second male gamete fuses with the central cell (containing two polar nuclei) to form the primary endosperm nucleus (3n), which develops into the endosperm (nutritive tissue for the developing embryo).
- Post-Fertilisation Changes:
- Ovary develops into the fruit (pericarp).
- Ovules develop into seeds.
- Zygote develops into the embryo within the seed.
- Other floral parts (sepals, petals, stamens, style, stigma) typically wither and fall off (though some may persist, e.g., sepals on brinjal).
- Seed Structure: Contains the embryo (radicle – embryonic root, plumule – embryonic shoot, cotyledons – embryonic leaves/food storage), protective seed coat, and sometimes endosperm.
- Germination: The process by which the dormant embryo within the seed resumes growth and develops into a seedling under favorable environmental conditions (typically adequate water, oxygen, and suitable temperature). Light can inhibit or promote germination depending on the species.
Practice MCQs
Assessment Worksheets
This assessment will be based on: Plant Sexual Reproduction: The Floral Journey
Experiment-Based Theories for Olympiad Preparation
Hypothetical Experiment: Manipulating Pollination Success and Seed Viability in a Model Plant
- Objective: To investigate the factors influencing pollination success (e.g., pollen viability, pollinator efficiency, stigma receptivity) and their downstream impact on seed development and viability in a model plant like a fast-growing Brassica rapa (Wisconsin Fast Plant).
- Materials:
- Brassica rapa plants at flowering stage.
- Fine paintbrushes (for manual pollination).
- Pollen viability stains (e.g., FDA – Fluorescein diacetate).
- Microscope and slides.
- Environmental chambers (to control temperature and humidity).
- Seed germination trays, filter paper.
- Procedure:
- Pollen Viability Test: Collect pollen from anthers. Apply pollen to slides with FDA stain and observe under a fluorescent microscope. Viable pollen will fluoresce. Quantify % viability under different environmental stresses (e.g., high heat, drought).
- Controlled Pollination Treatments:
- Group A (Control): Allow natural cross-pollination.
- Group B (Self-pollination): Manually transfer pollen from anther to stigma of the same flower, ensuring no cross-pollination.
- Group C (Manual Cross-pollination): Manually transfer pollen from one plant to another, ensuring no self-pollination.
- Group D (Delayed Pollination): Allow flowers to open, but delay manual pollination for a certain period (e.g., 24h, 48h) to test stigma receptivity over time.
- Group E (Simulated Pollinator Efficiency): Use varying numbers of “brush strokes” (simulating pollinator visits) to transfer pollen and compare.
- Post-Pollination Observation: After fruit/pod formation, collect seeds from each group.
- Seed Viability Test: Place a fixed number of seeds from each group on moist filter paper in germination trays. Incubate under optimal conditions. Count germinated seeds daily for a week.
- Expected Observations:
- Pollen viability will decrease with increasing environmental stress.
- Manual cross-pollination (Group C) might yield the highest number of seeds and highest seed viability, demonstrating hybrid vigor.
- Self-pollination (Group B) might result in lower seed set or reduced viability due to inbreeding depression.
- Delayed pollination (Group D) will show a decline in seed set, indicating a limited window of stigma receptivity.
- Increased “pollinator efficiency” (Group E) should correlate with higher seed set.
- Seed viability (germination rate) will vary across groups, reflecting the success of the fertilization process and genetic vigor.
- Theoretical Outcomes & Advanced Concepts:
- Pollen-Stigma Recognition: Discuss the molecular mechanisms of compatibility/incompatibility between pollen and stigma, which prevent self-pollination or interspecific hybridization.
- Energetics of Reproduction: Analyze the energy cost of producing viable pollen and seeds, and how environmental factors might limit this.
- Genetic Load and Inbreeding Depression: Explain how repeated self-pollination can expose deleterious recessive alleles, leading to reduced fitness (inbreeding depression).
- Outcrossing Mechanisms: Discuss various floral adaptations that promote cross-pollination (e.g., dichogamy, heterostyly, self-incompatibility).
- Seed Dormancy and Germination Triggers: Explore the physiological mechanisms of seed dormancy and how environmental cues (cold stratification, light, scarification) can break it.
- Real-Life Connections:
- Crop Breeding: Informing strategies for artificial pollination and hybrid seed production to improve crop yields and disease resistance.
- Conservation Biology: Understanding the reproductive biology of endangered plant species to design effective conservation and propagation programs.
- Horticulture: Optimizing pollination techniques for fruit and vegetable production in greenhouses or orchards.
- Impact of Climate Change on Pollinators: The experiment highlights the critical role of pollinators and how their decline (due to climate change, pesticides) can threaten plant reproduction and food security.
