Plant Growth and Responses

Plant Growth Types of Flowering Plants: Annuals – 1 year life cycle Biennials – 2 years Perennials – continuous life cycle for many years Meristem: perpetually embryonic tissues Cells divide for plant growth Apical meristem: growth at tips of roots & buds of shoots; cause primary growth (increase length) Lateral meristem: growth thickens shoots and roots; secondary growth

Secondary Growth = increase diameter Involves lateral meristems Vascular cambium:: (wood) Cork cambium: bark

Unusual nutritional adaptations: Epiphyte: grow on another plant, absorb H2O from rain through leaves

Parasitic Plants: not photosynthetic; absorb sugar and minerals from living hosts

Carnivorous Plants: photosynthetic, but obtain some nitrogen and minerals by digesting small animals

Angiosperm ( Flowering Plants) Reproduction

Angiosperms have 3 unique Features: Flowers Double Fertilization (by 2 sperm) Fruits

REPRODUCTIVE VARIATIONS Flower REPRODUCTIVE VARIATIONS

Pollination: transfer pollen from anther to stigma Be sure to review plant-animal interactions. Flower color reflects the type of animal that pollinates. Day time pollinators are attracted to colorful flowers where flowers that are white tend to be pollinated by night time pollinators.

“Pin” and “thrum” flower types reduce self-fertilization 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 Remind students the more related offspring are, the more possibility of genetic errors.

Double Fertilization 1 sperm produces the seed 1 sperm produces the endosperm (fruit) The endosperm nourishes the seed by providing sugar or starch

Fruit Egg cell  plant embryo Ovules inside ovary  seeds Ripe ovary  fruit Fruit protects enclosed seed(s) Aids in dispersal by water, wind, or animals Again dispersal of seeds is related to the method of dispersal. Wind dispersal moved light seeds (dandelions). Water dispersal tends to move well protected seeds. Seeds with barbs will stick to animal fur and move with the animal

Seeds Mature seed  dormancy (resting) Low metabolic rate Growth & development suspended Resumes growth when environmental conditions suitable for germination Stress that pollination, fertilization and germination are three separate processes in plant development.

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

(Vegetative Reproduction) Plant Reproduction Sexual Asexual (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

Humans Modify Crops Artificial selection of plants for breeding Plant Biotechnology: Genetically modified organisms “Golden Rice”: engineered to produce beta-carotene (Vitamin 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

Plant Responses

Hormones: chemical messengers that coordinate different parts of a multi-cellular organism Important plant hormones: Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide) Cytokinins – cell division (cytokinesis) & differentiation Gibberellins – stem elongation, leaf growth, germination, flowering, fruit development Abscisic Acid – slows growth; closes stomata during H2O stress; promote dormancy Ethylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)

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 AUXIN = chemical messenger that stimulates cell elongation Cells on darker side elongate faster than cells on brighter side

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

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

Plant Movement Tropisms: growth responses  SLOW Phototropism – light (auxin) Gravitropism – gravity (auxin) Thigmotropism – touch 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

Night length is a critical factor! 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? (often see this on exam)

Flooding (O2 deprivation): Drought (H2O deficit): close stoma release abscisic acid to keep stoma closed Inhibit growth roll leaves  reduce SA & transpiration deeper roots Flooding (O2 deprivation): release ethylene  root cell death  air tubes formed to provide O2 to submerged roots

Excess Salt: Heat: Cold: cell membrane – impede salt uptake produce solutes to ↓ψ - retain H2O Heat: evap. cooling via transpiration heat shock proteins – prevent denaturation Cold: alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity) increase cytoplasmic solutes antifreeze proteins

Herbivores: Pathogens: physical (thorns) chemicals (garlic, mint) recruit predatory animals (parasitoid wasps) Pathogens: 1st line of defense = epidermis 2nd line = pathogen recognition, host-specific