Lecture 17 Plant Reproduction 1

Sexual vs. Asexual Reproduction

  • Plants exhibit two broad reproductive modes:
    • Asexual (clonal) reproduction – offspring genetically identical to parent.
    • Sexual reproduction – offspring genetically variable due to meiosis and fertilisation.

Asexual Reproduction: Strategies & Examples

  • Simple body plan + meristem branching allow many clonal tactics:
    • Underground rhizomes (e.g., bamboo, ginger).
    • Root‐branching systems (trembling aspen clones can occupy >10 km²).
    • Above-ground stolons/runners (wild strawberry).
    • Fragmentation: detachable pads or leaves (teddy-bear cactus; Mother-of-Millions forms plantlets on leaf margins).
    • Apomixis – “virgin‐birth” seeds produced by mitosis; common in dandelions (Taraxacum).

Asexual Reproduction: Advantages & Disadvantages

  • Advantages
    • No need for pollinators; resources saved on floral structures.
    • Avoids vulnerable seedling stage; propagules often larger & established.
    • Excellent in good, stable environments with little year‐to‐year change.
  • Disadvantages
    • Slow/limited dispersal (offspring deposited centimetres from parent).
    • Reduced genetic variation → slower adaptation → risky in unstable or changing environments.
    • Large‐scale monoculture = pathogen heaven (illustrated by bananas & potatoes).

Case Studies: Agriculture & Clonal Vulnerability

  • Bananas
    • Wild bananas small, fibrous, seedy. Edible cultivars (Gross Michel → Cavendish) are sterile triploids propagated clonally.
    • Panama disease fungus/virus wiped out Gross Michel; currently threatening Cavendish.
    • Continuous “arms race”: breeders search for new resistant clones.
  • Potatoes
    • Introduced from South America, clonally propagated in Europe.
    • Irish potato famines (mid 19^{th} century) when late-blight pathogen struck genetically uniform crops → mass starvation & diaspora.

Ground Plan of a Flower

  • A flower = modified stem with four concentric whorls of modified leaves:
    1. Sepals (outermost; protective; often green).
    2. Petals (usually colourful; attractants).
    3. Stamens (male; anther + filament → pollen).
    4. Carpels (innermost female; stigma–style–ovary containing ovules).
  • Evolutionary origin traces back to gymnosperm leaves that bore ovules or pollen; gradual folding & fusion produced enclosed angiosperm organs.

Evolution & Diversification of Floral Form

  • Primitive angiosperms (e.g., magnolia) show:
    • Many free carpels centrally.
    • Numerous stamens outside carpels.
    • Several whorls of petals & sepals.
  • Subsequent modifications:
    • Fusion of organs (e.g., united petals forming long corolla tubes in Asteraceae).
    • Reduction in organ number (loss of sepals; fewer stamens/carpels).
    • Fusion of carpels → compound ovaries with multiple chambers (tomato cross-section reveals locules).
    • Protective “inferior” ovaries sunken into receptacle tissue.
    • Specialised symmetry: radial → bilateral (orchids, “naked-man” orchid) guiding pollinators.

Inflorescences: Maximising Floral Output

  • Meristems often allocate to inflorescences (clusters) not single flowers.
    • Umbel of umbels (wild carrot).
    • Banksia cones – thousands of flowers; visible stamens protrude.
    • Asteraceae capitulum: disc flowers (fertile) + ray flowers (showy) – sunflower.

Genetic Control of Floral Organ Identity (ABC Model)

  • Three master regulatory genes: A, B, C determine whorl identity.
    • A alone → sepals.
    • A + B → petals.
    • B + C → stamens.
    • C alone → carpels.
  • Knock-out experiments in Arabidopsis thaliana
    • Remove A → carpels–petals–carpels (C expands).
    • Remove B → sepals–carpels–sepals.
    • Remove C → sepals–petals–sepals–petals.
    • Remove A+B+C → reversion to leaf-like organs.
  • Highlights small genetic toolkit → vast morphological diversity.

Environmental Triggers of Flowering

Vernalisation (Prolonged Cold)

  • Biennials (e.g., carrot) store resources year 1; winter cold cues conversion to reproductive shoot in year 2.
  • Horticultural note: premature bolting in lettuce/carrot converts sugars → bitter reproductive tissues.

Photoperiodism (Day-length Sensing)

  • Plants classify as:
    • Long-day (LD) – flower when day \ge \, critical length (e.g., iris needs \approx15 h light).
    • Short-day (SD) – flower when night \ge \, critical length (e.g., chrysanthemum prefers long nights of autumn).
  • Night‐break experiments
    • Flash of light in middle of long night → LD plants flower; SD plants do not.
    • Demonstrates night length, not day length, is measured.

Phytochrome Hourglass Model

  • Phytochrome photoreceptor toggles between:
    • P{\text{r}} (inactive, absorbs red) ↔ P{\text{fr}} (active, absorbs far-red).
  • In daylight (rich in red light) P_{\text{fr}} dominates ("happy").
  • Darkness converts P{\text{fr}}\rightarrow P{\text{r}} over time ("hourglass running").
  • Flowering signal initiated when P_{\text{fr}} falls below threshold → plant infers long night.
  • Sequence red flash → far-red flash resets hourglass, confirming light quality matters.

Climate-Change Implications

  • Study of 28 northeastern US species using herbarium data (1840→present):
    • Mean flowering time advanced \approx14 days.
  • Risk: pollinator emergence/migration may not shift synchronously → mismatched interactions & reduced reproductive success.

Ethical, Practical & Philosophical Notes

  • Monoculture reliance on clonal crops poses ethical responsibility to manage genetic diversity, avoid famine-scale disasters, and preserve cultural foods.
  • Herbarium collections act as temporal archives, informing ecology & climate science centuries later.
  • Plant sensory biology (light, temperature) illustrates non-neural "decision making," prompting philosophical reflections on intelligence in organisms without brains.