Sinh học - Chapter 38: Angiosperm reproduction

Chapter 38Angiosperm ReproductionAngiosperms have 3 unique Features:FlowersFruitsDouble Fertilization (by 2 sperm)REPRODUCTIVE VARIATIONSPollination: transfer pollen from anther to stigmaSome plants are self-pollinatedCross-pollinated plants:Self-incompatibility: plant rejects own pollen or closely related plantMaximize genetic variationStigmaStigmaPin flowerAntherwithpollenThrum flower“Pin” and “thrum” flower types reduce self-fertilizationThe development of a plant embryoFruitEgg cell  plant embryoOvules inside ovary  seedsRipe ovary  fruitFruit protects enclosed seed(s)Aids in dispersal by water, wind, or animalsTypes of FruitSeedsMature seed  dormancy (resting)Low metabolic rateGrowth & development suspendedResumes growth when environmental conditions suitable for germinationGerminationSeed 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 vulnerabilityPredators, parasites, windSexualAsexual(Vegetative Reproduction)Flower  SeedsRunners, bulbs, grafts, cuttingsvegetative (grass), fragmentation, test-tube cloningGenetic diversityClonesMore complex & hazardous for seedlingsSimpler (no pollinator needed) Advantage in unstable environmentsSuited for stable environmentsPlant ReproductionAsexual reproduction in aspen treesTest-tube cloning of carrotsHumans 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 insectsBiofuels – reduce CO2 emissionsBiodiesel: vegetable oilsBioethanol: convert cellulose into ethanol Chapter 39Plant Responses to Internal and External SignalsExperiments with Light and the coleoptileConclusion: Tip of coleoptile senses light  some signal was sent from tip to elongating region of coleoptileExcised tip placedon agar blockGrowth-promotingchemical diffusesinto agar blockAgar blockwith chemicalstimulates growthOffset blockscause curvatureControl(agar blocklackingchemical)has noeffectControlCells on darker side elongate faster than cells on brighter sideAUXIN = chemical messenger that stimulates cell elongationHormones: chemical messengers that coordinate different parts of a multicellular organismImportant plant hormones:Auxin – stimulate cell elongation  phototropism & gravitropism (high concentrations = herbicide)Cytokinins – cell division (cytokinesis) & differentiation Gibberellins – stem elongation, leaf growth, germination, flowering, fruit developmentAbscisic Acid – slows growth; closes stomata during H2O stress; promote dormancyEthylene – promote fruit ripening (positive feedback!); involved in apoptosis (shed leaves, death of annuals)The effects of gibberellin on stem elongation and fruit growthEthylene Gas: Fruit RipeningCanister of ethylene gas to ripen bananas in shipping containerUntreated tomatoes vs. Ethylene treatmentPlant MovementTropisms: growth responses  SLOWPhototropism – light (auxin)Gravitropism – gravity (auxin)Thigmotropism – touchTurgor movement: allow plant to make relatively rapid & reversible responsesVenus fly trap, mimosa leaves, “sleep” movementPositive gravitropism in roots: the statolith hypothesis.Thigmotropism: rapid turgor movements by Mimosa plant  action potentialsPlant Responses to LightPlants can detect direction, intensity, & wavelenth of lightPhytochromes: light receptors, absorbs mostly red lightRegulate seed germination, shade avoidanceBiological ClocksCircadian rhythm: biological clocksPersist w/o environmental cuesFrequency = 24 hoursPhytochrome system + Biological clock = plant can determine time of year based on amount of light/darknessPhotoperiodism: 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 stressDrought (H2O deficit):close stomarelease abscisic acid to keep stoma closedInhibit growthroll leaves  reduce SA & transpirationdeeper rootsFlooding (O2 deprivation):release ethylene  root cell death  air tubes formed to provide O2 to submerged rootsExcess Salt:cell membrane – impede salt uptakeproduce solutes to ↓ψ - retain H2OHeat:evap. cooling via transpirationheat shock proteins – prevent denaturationCold:alter lipid composition of membrane (↑unsat. fatty acids, ↑fluidity)increase cytoplasmic solutesantifreeze proteinsHerbivores:physical (thorns)chemicals (garlic, mint)recruit predatory animals (parasitoid wasps)Pathogens:1st line of defense = epidermis2nd line = pathogen recognition, host-specific