Chapter 38Chapter 38Angiosperm Reproduction

Angiosperms have 3 unique Angiosperms have 3 unique Features:eatures:

1.1. FFlowers2.2. FFruits3. Double

FFertilization (by 2 sperm)

REPRODUCTIVE VARIATIONS

Pollination: transfer pollen from anther to stigma

Some plants are self-pollinatedCross-pollinated plants:

◦Self-incompatibility: plant rejects own pollen or closely related plant

◦Maximize genetic variation

Stigma Stigma

Pin flower

Antherwith

pollen

Thrum flower

““PinPin”” and and ““thrumthrum”” flower types reduce self-fertilization flower types reduce self-fertilization

The development of a plant embryoThe development of a plant embryo

FruitFruitEgg cell Egg cell plant embryoplant embryoOvulesOvules inside ovary seedsseedsRipe ovaryovary fruitfruitFruit protects enclosed seed(s)Aids in dispersal by water, wind,

or animals

SeedsSeedsMature seed dormancy (resting)

◦ Low metabolic rate◦ Growth & development suspended◦ Resumes growth when

environmental conditions suitable for germination

GerminationGermination Seed take up water (imbibition)

trigger metabolic changes to begin growth

◦ Root develops shoot emerges leaves expand & turn green (photosynthesis)

Very hazardous for plants due to vulnerability Predators, parasites, wind

SexualAsexual

(Vegetative Reproduction)

Flower Seeds

Runners, bulbs, grafts, cuttings

vegetative (grass), fragmentation, test-tube

cloning

Genetic diversity Clones

More complex & hazardous for seedlings

Simpler (no pollinator needed)

Advantage in unstable environments

Suited for stable environments

Plant ReproductionPlant Reproduction

Asexual Asexual reproduction in reproduction in aspen treesaspen trees

Test-tube cloning Test-tube cloning of carrotsof carrots

Humans Modify CropsHumans Modify CropsArtificial selection of plants for breedingPlant Biotechnology:

◦Genetically modified organisms “Golden Rice”: engineered to produce

beta-carotene (Vit. A) Bt corn: transgenic – expresses Bt

(bacteria) gene produces protein toxic to insects

◦Biofuels – reduce CO2 emissions Biodiesel: vegetable oils Bioethanol: convert cellulose into

ethanol

Chapter 39Chapter 39Plant Responses to Internal and External Signals

Experiments with Light and the coleoptile

Excised tip placedon agar block

Growth-promotingchemical diffusesinto agar block

Agar blockwith chemicalstimulates growth

Offset blockscause curvature

Control(agar blocklackingchemical)has noeffect

Control

Cells on darker Cells on darker side elongate side elongate faster than cells faster than cells on brighter sideon brighter side

AUXINAUXIN = chemical = chemical messenger that messenger that stimulates cell stimulates cell elongationelongation

Important plant hormones:1.Auxin – stimulate cell elongation

phototropism & gravitropism (high concentrations = herbicide)

2.Cytokinins – cell division (cytokinesis) & differentiation

3.Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development

4.Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy

5.Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

The effects of The effects of gibberellin gibberellin on stem on stem elongation and fruit growthelongation and fruit growth

Ethylene Gas: Fruit RipeningEthylene Gas: Fruit Ripening

Canister of ethylene gas to ripen bananas in shipping container

Untreated tomatoes vs. Ethylene treatment

Plant MovementPlant Movement

1. Tropisms: growth responses SLOW Phototropism – light (auxin) Gravitropism – gravity (auxin) Thigmotropism – touch

2. Turgor movement: allow plant to make relatively rapid & reversible responses Venus fly trap, mimosa leaves, “sleep”

movement

Positive Positive gravitropismgravitropism in roots: the in roots: the statolith hypothesis.statolith hypothesis.

ThigmotropismThigmotropism: rapid turgor : rapid turgor movements by Mimosa plant movements by Mimosa plant action potentialsaction potentials

Plant Responses to LightPlant Responses to LightPlants can detect direction, intensity,

& wavelenth of lightPhytochromes: light receptors,

absorbs mostly red light◦Regulate seed germination, shade

avoidance

Biological ClocksBiological Clocks

Circadian rhythm: biological clocks Persist w/o environmental cues Frequency = 24 hours

Phytochrome system + Biological clock = plant can determine time of year based on amount of light/darkness

PhotoperiodismPhotoperiodism: physiological : physiological response to the relative length of response to the relative length of night & day (i.e. flowering)night & day (i.e. flowering)

Short-day plants: flower when nights are long (mums, poinsettia)

Long-day plant: flower when nights are short (spinach, iris, veggies)

Day-neutral plant: unaffected by photoperiod (tomatoes, rice, dandelions)

How does How does interrupting interrupting the dark the dark period with a period with a brief exposure brief exposure to light affect to light affect flowering?flowering?

Plant responses to Plant responses to stressstress

1. Drought (H2O deficit): close stoma release abscisic acid to keep stoma

closed Inhibit growth roll leaves reduce SA & transpiration deeper roots

2. Flooding (O2 deprivation): release ethylene root cell death

air tubes formed to provide O2 to submerged roots

3. Excess Salt: cell membrane – impede salt uptake produce solutes to ↓ψ - retain H2O

4. Heat: evap. cooling via transpiration heat shock proteins – prevent

denaturation

5. Cold: alter lipid composition of membrane

(↑unsat. fatty acids, ↑fluidity) increase cytoplasmic solutes antifreeze proteins

6. Herbivores: physical (thorns) chemicals (garlic, mint) recruit predatory animals (parasitoid

wasps)

7. Pathogens: 1st line of defense = epidermis 2nd line = pathogen recognition,

host-specific