Sexual Reproduction in Flowering Plants – Post-Fertilisation Events & Seed/Fruit Biology

Post-Fertilisation Events

  • All developmental processes occurring after double fertilisation → collectively called post-fertilisation events.

  • Principal changes
    • Development of triploid endosperm and diploid embryo.
    • Maturation of each ovule → seed.
    • Enlargement of ovary (or other floral parts) → fruit.

  • Usual fate of accessory floral whorls
    • Sepals, petals, stamens, style, stigma: wither & abscise.
    • Persistent calyx: tomato, brinjal; enlarged calyx: Physalis.

  • Antipodal cells of embryo sac degenerate soon after fertilisation.

Endosperm Formation

Origin & Role

  • Develops from primary endosperm nucleus (PEN) that is triploid (3N) after double fertilisation.

  • Begins before embryo development; serves as nutritive tissue for embryo & seedling.

  • Grows by absorbing food from parent plant, often consuming entire nucellus.

  • Retention of nucellar remnant outside endosperm → thin perisperm (e.g. black pepper, beet).

Endospermic vs Non-endospermic Seeds

  • Endospermic (albuminous): retain endosperm at maturity (cereals, coconut, castor).
    • Endosperm utilised post-germination.

  • Non-endospermic (ex-albuminous): endosperm consumed during embryogenesis (beans, peas, groundnut).

Types of Endosperm (based on development)

  1. Nuclear

    • PEN undergoes free-nuclear divisions → many nuclei; cell-wall formation delayed.

    • Central vacuole, nuclei at periphery.

    • Later cell plates form centripetally → cellularised endosperm (e.g. Capsella).

    • Coconut water = free-nuclear endosperm; coconut meat = cellular endosperm.

  2. Cellular

    • Each nuclear division immediately followed by cytokinesis.

    • Endosperm thus cellular from start; first wall transverse, later walls irregular (e.g. Datura, Petunia, Balsam).

  3. Helobial

    • Intermediate. First division followed by transverse wall → small chalazal chamber + larger micropylar chamber.

    • Micropylar chamber: free-nuclear divisions then walls; chalazal: 1–2 divisions, acts as haustorium.

    • Common in monocots order Helobiales.

Embryo Formation (Embryogenesis)

General Features

  • Zygote (diploid) remains quiescent until sufficient endosperm accumulates.

  • First zygotic division asymmetric → smaller terminal (apical) cell at chalazal end + larger basal cell at micropylar end.

  • Basal cell → suspensor (pushes embryo into endosperm, transfers nutrients).

  • Terminal cell → pro-embryo → globular → heart-shaped → mature embryo.

  • Basic embryo architecture
    • Embryonal axis: epicotyl (above cotyledonary node) ending in plumule; hypocotyl (below) ending in radicle.
    • Cotyledons: 2 in dicots, 1 (scutellum) in monocots.

Dicot Embryogenesis

  1. Zygote → basal & terminal cells (transverse division).

  2. Basal cell divisions → 6–10-celled suspensor; first suspensor cell swells = haustorium; last cell differentiates as hypophysis (forms radicle & root cap).

  3. Terminal cell → two vertical + one transverse divisions → octant (8 cells) →
    • Upper tier (near suspensor) → hypocotyl.
    • Lower tier → epicotyl, cotyledons, plumule.

  4. Stage transitions: pro-embryo → globular → heart-shaped (cotyledon primordia) → mature embryo (suspensor degenerates).

Typical Dicot Embryo

  • Components
    • Two fleshy cotyledons rich in reserve.
    • Epicotyl ending in plumule (negatively geotropic).
    • Hypocotyl ending in radicle + root cap (positively geotropic).

Monocot Embryogenesis (e.g. Zea mays)

  1. Zygote → transverse asymmetric division → apical cell (ca) & basal cell (cb).

  2. Basal cell enlarges → vesicular cell aiding nutrient uptake.

  3. Apical cell → successive transverse then vertical divisions → quadrant (4 cells) → octant (8 cells, two tiers).

  4. Periclinal divisions → Dermatogen → Periblem → Plerome.

  5. These tissues give rise to single lateral cotyledon (scutellum), shoot apex, and remaining axis.

Typical Monocot Embryo

  • One cotyledon (scutellum) attached laterally to axis.

  • Axis below scutellum: radicle + root cap enclosed by coleorhiza.

  • Axis above scutellum: epicotyl with plumule + first leaf primordia enclosed by coleoptile.

Seed Formation

Definition & Structure

  • Seed = mature ovule; culmination of sexual reproduction.

  • Components
    • Seed coat (testa + tegmen) from integuments.
    • Embryo with cotyledon(s) & axis.
    • Often residual endosperm/perisperm.
    • Funicle → stalk; hilum = scar of attachment; micropyle = pore for O2 & H2O entry.
    • Possible coloured outgrowths
    – Funicle-derived = strophiole (e.g. Acacia).
    – Micropyle-derived = caruncle (e.g. Ricinus).

Seed Types

  1. By cotyledon number

    • Dicotyledonous (bean, castor).

    • Monocotyledonous (rice, maize).

  2. By endosperm presence

    • Albuminous / endospermic: endosperm retained (castor, maize, onion).

    • Ex-albuminous / non-endospermic: endosperm absorbed (gram, pea, mustard).

  • Perisperm: persistent nucellus remnant (black pepper, beet).

Typical Dicot Seed – Bean (Lablab purpureus)

  • Kidney-shaped; hard testa (dark) & thin tegmen (light).

  • Raphe: ridge along seed coat.

  • Hilum + micropyle on concave side.

  • Two fleshy cotyledons storing starch, protein, oil.

  • Embryo between cotyledons: radicle, plumule, hypocotyl, epicotyl.

Typical Monocot Seed – Maize Grain (Caryopsis)

  • Pericarp fused with seed coat.

  • Distinct endosperm (starch) surrounded by protein-rich aleurone layer.

  • Embryo region: single scutellum separated by epithelial layer.

  • Plumule within coleoptile; radicle within coleorhiza.

Significance

  • Basis of agriculture & food reserves.

  • Supports seedling until autotrophic.

  • Long-term storage (e.g. 10{,}000-year-old Lupinus arcticus; 2000-year date palm).

  • Genetic variation via sexual origin.

  • Adaptation for dispersal.

Seed Dispersal & Need

  • Movement of seeds away from parent to reduce intra-specific competition for light, space, water, nutrients.

  • Prevents overcrowding; enhances colonisation of new habitats.

Seed Dormancy

  • Temporary failure to germinate post-maturation despite viability.

  • Causes: hard testa, low moisture, scarce growth hormones, narrow micropyle limiting O_2.

  • Benefits: ensures germination under favourable conditions; allows time for dispersal.

Seed Viability

  • Duration embryo remains alive during dormancy varies widely.

  • Remarkable records: Lupinus arcticus \approx 10^{4} years; date palm \approx 2000 years.

Fruit Formation

  • Post-fertilisation, ovary undergoes cell division & differentiation → fruit.

  • Ovary wall develops into pericarp which may differentiate into
    • Epicarp (outer)
    • Mesocarp (middle)
    • Endocarp (inner)

  • In dry fruits, pericarp remains undifferentiated, papery/woody.

  • Functions: seed protection & aid in dispersal.

True vs False Fruits

  • True fruit: derived solely from ovary.

  • False (pseudocarp): other floral parts (thalamus, receptacle, calyx) contribute (e.g. apple, pear – edible part from thalamus).

Examples of Drupes

  1. Mango

    • Thin leathery epicarp.

    • Thick fleshy mesocarp (edible).

    • Hard stony endocarp encasing single laterally compressed seed.

  2. Coconut

    • Tough thin epicarp.

    • Thick fibrous mesocarp.

    • Hard woody endocarp surrounding seed.

    • Seed: thin testa + thick white endosperm (solid) lining inner wall; central cavity with liquid endosperm when tender.

Significance of Fruits

  • Food source (energy, vitamins, minerals) for animals & humans.

  • Protect developing seeds from environment.

  • May nourish germinating seedling.

  • Facilitate seed dispersal.

Parthenocarpy

  • Formation of seedless fruits without fertilisation of ovules.

  • Causes
    • Natural absence of pollination
    • Failure of fertilisation / zygotic sterility
    • Induced via hormones (IAA, \alpha-NAA, gibberellins).

  • Types
    • Genetic (hybridisation/mutation).
    • Environmental (fog, frost, heat, freezing affecting sexual organs).
    • Chemically induced.

  • Common parthenocarpic crops: banana, citrus, grapes, pineapple, some apples & pears.

  • Horticultural value: higher edible portion, consumer preference.

Apomixis

  • Asexual reproduction that mimics sexual pathway → seeds without fertilisation; no zygote formation.

  • Occurs in some Asteraceae, grasses.

  • Apomictic seeds can originate from fruit parts, pollen male nuclei, or other vegetative tissues.

Types of Apomixis

  1. Non-recurrent

    • One functional megaspore → haploid embryo sac; embryo from unfertilised egg (haploid parthenogenesis) or other gametophytic cell (haploid apogamy).

    • Embryo is haploid; useful for producing homozygous lines.

  2. Recurrent

    • Diploid embryo sac forms directly from nucellar cell (apospory) or MMC (diplospory); egg develops parthenogenetically into diploid embryo (e.g. apple, Poa).

  3. Adventive (sporophytic budding)

    • Embryo arises directly from diploid nucellus/integument cells, pushed into embryo sac (e.g. orange, mango, Opuntia, onion).

Polyembryony

  • Presence of more than one embryo inside a single seed. Discovered by Leeuwenhoek (1719) in citrus.

  • Common in conifers; also in orange, lemon, mango, groundnut, onion.

Categories

  1. Simple – multiple embryo sacs within ovule (e.g. Brassica).

  2. Mixed – >1 pollen tube enters ovule; fertilises synergid / antipodal (e.g. Ulmus).

  3. Cleavage – a single zygotic embryo splits (e.g. orchids, Nymphaea, Nicotiana).

  4. Adventive – additional embryos from nucellus/integument cells (e.g. citrus, mango, Opuntia, Balanophora).

  • True polyembryony: extra embryos from same embryo sac; false: embryos arise outside sac.

  • Inducible via chemicals like 2,4\text{-}D in freshly harvested seeds.