Bài giảng Biology - Chapter 37: Plant Nutrition

Chapter 37Plant NutritionOverview: A Nutritional NetworkEvery organismContinually exchanges energy and materials with its environmentFor a typical plantWater and minerals come from the soil, while carbon dioxide comes from the airThe branching root system and shoot system of a vascular plantEnsure extensive networking with both reservoirs of inorganic nutrientsFigure 37.1Concept 37.1: Plants require certain chemical elements to complete their life cyclePlants derive most of their organic mass from the CO2 of airBut they also depend on soil nutrients such as water and mineralsFigure 37.2CO2, the sourceof carbon forPhotosynthesis,diffuses intoleaves from theair throughstomata.Throughstomata, leavesexpel H2O andO2.H2OO2CO2Roots take inO2 and expelCO2. The plantuses O2 for cellularrespiration but is a net O2 producer.O2CO2H2ORoots absorbH2O andminerals fromthe soil.MineralsMacronutrients and MicronutrientsMore than 50 chemical elementsHave been identified among the inorganic substances in plants, but not all of these are essentialA chemical element is considered essentialIf it is required for a plant to complete a life cycleResearchers use hydroponic cultureTo determine which chemicals elements are essentialFigure 37.3 TECHNIQUE Plant roots are bathed in aerated solutions of known mineral composition. Aerating the water provides the roots with oxygen for cellular respiration. A particular mineral, such as potassium, can be omitted to test whether it is essential. RESULTS If the omitted mineral is essential, mineral deficiency symptoms occur, such as stunted growth and discolored leaves. Deficiencies of different elements may have different symptoms, which can aid in diagnosing mineral deficiencies in soil.Control: Solutioncontaining all mineralsExperimental: Solutionwithout potassium APPLICATION In hydroponic culture, plants are grown in mineral solutions without soil. One use of hydroponic culture is to identify essential elements in plants.Essential elements in plantsTable 37.1Nine of the essential elements are called macronutrientsBecause plants require them in relatively large amountsThe remaining eight essential elements are known as micronutrientsBecause plants need them in very small amountsSymptoms of Mineral DeficiencyThe symptoms of mineral deficiencyDepend partly on the nutrient’s functionDepend on the mobility of a nutrient within the plantDeficiency of a mobile nutrientUsually affects older organs more than young onesDeficiency of a less mobile nutrientUsually affects younger organs more than older onesThe most common deficienciesAre those of nitrogen, potassium, and phosphorusFigure 37.4Phosphate-deficientHealthyPotassium-deficientNitrogen-deficientConcept 37.2: Soil quality is a major determinant of plant distribution and growthAlong with climateThe major factors determining whether particular plants can grow well in a certain location are the texture and composition of the soilTextureIs the soil’s general structureCompositionRefers to the soil’s organic and inorganic chemical componentsTexture and Composition of SoilsVarious sizes of particles derived from the breakdown of rock are found in soilAlong with organic material (humus) in various stages of decompositionThe eventual result of this activity is topsoilA mixture of particles of rock and organic materialThe topsoil and other distinct soil layers, or horizonsAre often visible in vertical profile where there is a road cut or deep holeFigure 37.5The A horizon is the topsoil, a mixture ofbroken-down rock of various textures, living organisms, and decaying organic matter.The B horizon contains much less organicmatter than the A horizon and is lessweathered.The C horizon, composed mainly of partiallybroken-down rock, serves as the “parent”material for the upper layers of soil.ABCAfter a heavy rainfall, water drains away from the larger spaces of soilBut smaller spaces retain water because of its attraction to surfaces of clay and other particlesThe film of loosely bound waterIs usually available to plantsFigure 37.6a (a) Soil water. A plant cannot extract all the water in the soil because some of it is tightly held by hydrophilic soil particles. Water bound less tightly to soil particles can be absorbed by the root.Soil particle surrounded byfilm of waterRoot hairWater available to plantAir spaceAcids derived from roots contribute to a plant’s uptake of mineralsWhen H+ displaces mineral cations from clay particlesFigure 37.6b(b) Cation exchange in soil. Hydrogen ions (H+) help make nutrients available by displacing positively charged minerals (cations such as Ca2+) that were bound tightly to the surface of negatively charged soil particles. Plants contribute H+ by secreting it from root hairsand also by cellular respiration, which releases CO2 into the soil solution, where it reacts with H2O to form carbonic acid (H2CO3). Dissociation of this acid adds H+ to the soil solution.H2O + CO2H2CO3HCO3– +Root hairK+Cu2+Ca2+Mg2+K+K+H+H+Soil particle–––––––––Soil Conservation and Sustainable AgricultureIn contrast to natural ecosystemsAgriculture depletes the mineral content of the soil, taxes water reserves, and encourages erosionThe goal of soil conservation strategiesIs to minimize this damageFertilizersCommercially produced fertilizersContain minerals that are either mined or prepared by industrial processes“Organic” fertilizersAre composed of manure, fishmeal, or compostAgricultural researchersAre developing ways to maintain crop yields while reducing fertilizer useGenetically engineered “smart” plantsInform the grower when a nutrient deficiency is imminentFigure 37.7No phosphorus deficiencyBeginning phosphorusdeficiencyWell-developed phosphorusdeficiencyIrrigationIrrigation, which is a huge drain on water resources when used for farming in arid regionsCan change the chemical makeup of soilErosionTopsoil from thousands of acres of farmlandIs lost to water and wind erosion each year in the United StatesCertain precautionsCan prevent the loss of topsoilFigure 37.8The goal of soil managementIs sustainable agriculture, a commitment embracing a variety of farming methods that are conservation-mindedSoil ReclamationSome areas are unfit for agricultureBecause of contamination of soil or groundwater with toxic pollutantsA new method known as phytoremediationIs a biological, nondestructive technology that seeks to reclaim contaminated areasConcept 37.3: Nitrogen is often the mineral that has the greatest effect on plant growthPlants require nitrogen as a component ofProteins, nucleic acids, chlorophyll, and other important organic moleculesSoil Bacteria and Nitrogen AvailabilityNitrogen-fixing bacteria convert atmospheric N2To nitrogenous minerals that plants can absorb as a nitrogen source for organic synthesisFigure 37.9AtmosphereN2SoilN2N2Nitrogen-fixingbacteriaOrganicmaterial (humus)NH3 (ammonia)NH4+ (ammonium)H+ (From soil) NO3– (nitrate)NitrifyingbacteriaDenitrifyingbacteriaRootNH4+SoilAtmosphereNitrate and nitrogenousorganiccompoundsexported inxylem toshoot systemAmmonifyingbacteriaImproving the Protein Yield of CropsAgriculture research in plant breedingHas resulted in new varieties of maize, wheat, and rice that are enriched in proteinSuch researchAddresses the most widespread form of human malnutrition: protein deficiencyConcept 37.4: Plant nutritional adaptations often involve relationships with other organismsTwo types of relationships plants have with other organisms are mutualisticSymbiotic nitrogen fixationMycorrhizaeThe Role of Bacteria in Symbiotic Nitrogen FixationSymbiotic relationships with nitrogen-fixing bacteriaProvide some plant species with a built-in source of fixed nitrogenFrom an agricultural standpointThe most important and efficient symbioses between plants and nitrogen-fixing bacteria occur in the legume family (peas, beans, and other similar plants)Along a legumes possessive roots are swellings called nodulesComposed of plant cells that have been “infected” by nitrogen-fixing Rhizobium bacteriaFigure 37.10a(a) Pea plant root. The bumps onthis pea plant root are nodules containing Rhizobium bacteria.The bacteria fix nitrogen and obtain photosynthetic productssupplied by the plant.NodulesRootsInside the noduleRhizobium bacteria assume a form called bacteroids, which are contained within vesicles formed by the root cellFigure 37.10b(b) Bacteroids in a soybean root nodule. In this TEM, a cell froma root nodule of soybean is filledwith bacteroids in vesicles. The cells on the left are uninfected.5 mBacteroidswithinvesicleThe bacteria of a noduleObtain sugar from the plant and supply the plant with fixed nitrogenEach legumeIs associated with a particular strain of RhizobiumDevelopment of a soybean root noduleFigure 37.11InfectionthreadRhizobiumbacteriaDividing cellsin root cortexBacteroid2 The bacteria penetrate the cortex within the Infection thread. Cells of the cortex and pericycle begin dividing, and vesicles containing the bacteria bud into cortical cells from the branching infection thread. This process results in the formation of bacteroids.BacteroidBacteroidDevelopingroot noduleDividing cells in pericycleInfectedroot hair123Nodulevasculartissue43 Growth continues in the affected regions of the cortex and pericycle, and these two masses of dividing cells fuse, forming the nodule. Roots emit chemical signals that attract Rhizobium bacteria. The bacteria then emit signals that stimulate root hairs to elongate and to form an infection thread by an invagination of the plasma membrane.14 The nodule develops vascular tissue that supplies nutrients to the nodule and carries nitrogenous compounds into the vascular cylinder for distribution throughout the plant.The Molecular Biology of Root Nodule FormationThe development of a nitrogen-fixing root noduleDepends on chemical dialogue between Rhizobium bacteria and root cells of their specific plant hostsSymbiotic Nitrogen Fixation and AgricultureThe agriculture benefits of symbiotic nitrogen fixationUnderlie crop rotationIn this practiceA non-legume such as maize is planted one year, and the following year a legume is planted to restore the concentration of nitrogen in the soilMycorrhizae and Plant NutritionMycorrhizaeAre modified roots consisting of mutualistic associations of fungi and rootsThe fungusBenefits from a steady supply of sugar donated by the host plantIn return, the fungusIncreases the surface area of water uptake and mineral absorption and supplies water and minerals to the host plantThe Two Main Types of MycorrhizaeIn ectomycorrhizaeThe mycelium of the fungus forms a dense sheath over the surface of the rootFigure 37.12aa Ectomycorrhizae. The mantle of the fungal mycelium ensheathes the root. Fungal hyphae extend from the mantle into the soil, absorbing water and minerals, especially phosphate. Hyphae also extend into the extracellular spaces of the root cortex, providing extensive surface area for nutrient exchange between the fungus and its host plant.Mantle(fungal sheath)EpidermisCortexMantle(fungalsheath)EndodermisFungalhyphaebetweencorticalcells(colorized SEM)100 m(a)In endomycorrhizaeMicroscopic fungal hyphae extend into the rootFigure 37.12bEpidermisCortexFungalhyphaeRoothair10 m(LM, stained specimen) Cortical cellsEndodermisVesicleCasparianstripArbuscules2 Endomycorrhizae. No mantle forms around the root, but microscopic fungal hyphae extend into the root. Within the root cortex, the fungus makes extensive contact with the plant through branching of hyphae that form arbuscules, providing an enormous surface area for nutrient swapping. The hyphae penetrate the cell walls, but not the plasma membranes, of cells within the cortex.(b)Agricultural Importance of MycorrhizaeFarmers and forestersOften inoculate seeds with spores of mycorrhizal fungi to promote the formation of mycorrhizaeEpiphytes, Parasitic Plants, and Carnivorous PlantsSome plantsHave nutritional adaptations that use other organisms in nonmutualistic waysExploring unusual nutritional adaptations in plantsFigure 37.13Staghorn fern, an epiphyteEPIPHYTESPARASITIC PLANTSCARNIVOROUS PLANTSMistletoe, a photosynthetic parasiteDodder, a nonphotosynthetic parasiteHost’s phloemHaustoriaIndian pipe, a nonphotosynthetic parasiteVenus’ flytrapPitcher plantsSundewsDodder