Chapter 38 Angiosperm Reproduction

Angiosperms have 3 unique eatures: Angiosperms have 3 unique Features: 1. F 1. Flowers 2. F 2. Fruits F 3. Double Fertilization (by 2 sperm)

REPRODUCTIVE VARIATIONS

Pollination: transfer pollen from anther to stigma

Some plants are self-pollinated Cross-pollinated plants: ◦ Self-incompatibility: plant rejects own pollen or closely related plant ◦ Maximize genetic variation Stigma Pin flower Anther with pollen Thrum flower “Pin” and “thrum” flower types reduce self-fertilization

The development of a plant embryo

Fruit Egg cell plant embryo Egg cell  plant embryo Ovules seeds Ovules inside ovary  seeds ovary fruit Ripe ovary  fruit Fruit protects enclosed seed(s) Aids in dispersal by water, wind, or animals

Types of Fruit

Seeds Mature seed  dormancy (resting) ◦ Low metabolic rate ◦ Growth & development suspended ◦ Resumes growth when environmental conditions suitable for germination

Germination 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

Sexual Asexual (Vegetative Reproduction) Flower  Seeds Runners, bulbs, grafts, cuttings vegetative (grass), fragmentation, test-tube cloning Genetic diversityClones More complex & hazardous for seedlings Simpler (no pollinator needed) Advantage in unstable environments Suited for stable environments Plant Reproduction

Asexual reproduction in aspen trees Test-tube cloning of carrots

Humans Modify Crops Artificial selection of plants for breeding Plant 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 CO 2 emissions  Biodiesel: vegetable oils  Bioethanol: convert cellulose into ethanol

Chapter 39 Plant Responses to Internal and External Signals

Experiments with Light and the coleoptile Conclusion: Tip of coleoptile senses light  some signal was sent from tip to elongating region of coleoptile

Excised tip placed on agar block Growth-promoting chemical diffuses into agar block Agar block with chemical stimulates growth Offset blocks cause curvature Control (agar block lacking chemical) has no effect Control Cells on darker side elongate faster than cells on brighter side AUXIN = chemical messenger that stimulates cell elongation

Hormones: chemical messengers that coordinate different parts of a multicellular organism 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 H 2 O stress; promote dormancy 5. Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

The effects of gibberellin on stem elongation and fruit growth

Ethylene Gas: Fruit Ripening Canister of ethylene gas to ripen bananas in shipping container Untreated tomatoes vs. Ethylene treatment

Plant 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 gravitropism in roots: the statolith hypothesis.

Thigmotropism: rapid turgor movements by Mimosa plant  action potentials

Plant Responses to Light Plants can detect direction, intensity, & wavelenth of light Phytochromes: light receptors, absorbs mostly red light ◦ Regulate seed germination, shade avoidance

Biological 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

Photoperiodism: physiological response to the relative length of 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) Night length is a critical factor!

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

Plant responses to stress

1. Drought (H 2 O deficit):  close stoma  release abscisic acid to keep stoma closed  Inhibit growth  roll leaves  reduce SA & transpiration  deeper roots 2. Flooding (O 2 deprivation):  release ethylene  root cell death  air tubes formed to provide O 2 to submerged roots

3. Excess Salt:  cell membrane – impede salt uptake  produce solutes to ↓ ψ - retain H 2 O 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