A. Parasitic Plants B. Carnivorous Plants

CHAPTER 33PLANT NUTRITIONOUTLINEI.Plants require at least seventeen essential nutrientsA.The Chemical Composition of PlantsB.Essential NutrientsII.The symptoms of mineral deficiency depend on the function and mobility of the element III.Soil characteristics are key environmental factors in terrestrial ecosystemsA.Texture and Composition of SoilsB.The Availability of Soil Water and MineralsIV.Soil conservation is one step toward sustainable agricultureV.The metabolism of soil bacteria makes nitrogen available to plantsA.Nitrogen FixationB.Symbiotic Nitrogen FixationVI.Improving the protein yield of crops is a major goal of agricultural researchVII.Predation and symbiosis are evolutionary adaptations that enhance plant nutritionA.Parasitic PlantsB.Carnivorous PlantsC.MycorrhizaeOBJECTIVESAfter reading this chapter and attending lecture, the student should be able to:1.Describe the chemical composition of plants including:a.Percent of wet weight as water.b.Percent of dry weight as organic substances.c.Percent of dry weight as inorganic minerals.2.Explain how hydroponic culture is used to determine which minerals are essential nutrients.Plant Nutrition 6093.Distinguish between macronutrient and micronutrient.4.Recall the nine macronutrients required by plants and describe their importance in normal plantstructure and metabolism.5.List seven micronutrients required by plants and explain why plants need only minute quantities ofthese elements.6.Explain how a nutrient's role and mobility determine the symptoms of a mineral deficiency.7.Explain how soil is formed.8.Explain what determines the texture of topsoil and list the type of soil particles from coarsest tosmallest.9.Describe the composition of loams and explain why they are the most fertile soils.10.Explain how humus contributes to the texture and composition of soil.11.Explain why plants cannot extract all of the water in soil.12.Explain how the presence of clay in soil helps prevent the leaching of mineral cations.13.Define cation exchange, explain why it is necessary for plant nutrition, and describe how plants canstimulate the process.14.Explain why soil management is necessary in agricultural systems but not in natural ecosystems suchas forests and grasslands.15.List the three mineral elements that are most commonly deficient in farm soils.16.Describe the environmental consequence of overusing commercial fertilizers.17.Explain how soil pH determines the effectiveness of fertilizers and a plant's ability to absorb specificmineral nutrients.18.Describe problems resulting from farm irrigation in arid regions and list several current approachesto solving these problems.19.Describe precautions that can reduce wind and water erosion.20.Define nitrogen fixation and write the overall equation representing the conversion of gaseousnitrogen to ammonia.21.Distinguish between nitrogen-fixing bacteria and nitrifying bacteria.22.Recall the forms of nitrogen that plants can absorb and describe how they are used by plants.23.Beginning with free-living rhizobial bacteria, describe the development of a root nodule.24.Explain why the symbiosis between a legume and its nitrogen-fixing bacteria is considered to bemutualistic.25.Recall two functions of leghemoglobin and explain why its synthesis is evidence for coevolution.26.Describe the basis for crop rotation.27.Describe agricultural research methods used to improve the quality and quantity of proteins in plantcrops.28.Describe modifications for nutrition that have evolved among plants including parasitic plants,carnivorous plants and mycorrhizae.KEY TERMSmacronutrients humus nitrogen fixation leghemoglobin micronutrients cation exchange nitrifying bacteria haustoriahorizons chemical fertilizers nodules epiphytestopsoil drip irrigation Rhizobium mycorrhizaeloams610Plant NutritionLECTURE NOTESPlants and other photosynthetic autotrophs play a critical role in the energy flow and chemical cycling of ecosystems by transforming:•Light energy into chemical bond energy.•Inorganic compounds into organic compounds.To accomplish these tasks, plants require:•Sunlight as the energy source for photosynthesis.•Inorganic, raw materials, such as:⇒carbon dioxide⇒water⇒variety of inorganic, mineral ions in the soilTo acquire the resources of light and inorganic nutrients, plants have highly ramified root and shoot systems with large surface areas for exchange with their environment – soil and air.I.Plants require at least seventeen essential nutrientsA.The Chemical Composition of PlantsEarly ideas about plant nutrition:•In the fourth century B.C., Aristotle thought that soil was the substance for plant growth and that leaves only provided shade for fruit.•In the 1600's, Jean-Baptiste von Helmont concluded plants grow mainly from the water it absorbs. To discover if plants grew by absorbing soil, he:⇒planted a seed in a measured amount of soil;⇒weighed the grown plant;⇒measured how much soil was left. The soil did not decrease proportionately.•In the 1700's, Stephen Hales postulated plants were nourished mostly by air.Plants grow mainly by accumulating water in their cells’ central vacuoles.•Water is a nutrient since it supplies most of the hydrogen and some oxygen incorporated into organic compounds by photosynthesis.•80–85% of an herbaceous plant is water.•More than 90% of the water absorbed is lost by transpiration.•Retained water functions as a solvent, allows cell elongation, and keeps cells turgid.By weight, the bulk of a plant's organic material is from assimilated CO2. The compositionof the dry weight of plants is:•95% organic substances, mostly carbohydrates.Plant Nutrition 611•5% minerals, to some extent determined by soil composition.B.Essential NutrientsAn essential nutrient is one that is required for a plant to grow from a seed and complete itslife cycle. (See Campbell, Table 33.1)•Determined by hydroponic culture, in which roots are bathed in an aerated aqueous solution of known mineral content. If a mineral is omitted and plant growth isabnormal, it is essential.Macronutrients = Elements required by plants in large amounts. (See Campbell, Table 33.1)Micronutrients = Elements required by plants in small amounts.•These function primarily as cofactors of enzymatic reactions.•Optimal concentrations vary for different plant species.II.The symptoms of a mineral deficiency depend on the function and mobility of the elementSymptoms of mineral deficiencies depend on:1.The role of the nutrient in the plant.2.Its mobility within the plant.•Deficiencies of nutrients mobile in the plant appear in older organs first since some are preferentially shunted to growing parts.•Deficiencies of immobile nutrients affect young parts of plant first because older tissues may have adequate reserves.Deficiencies of N, K, and P are the most common.•Shortages of micronutrients are less common and often localized.•Overdoses of some micronutrients can be toxic.III.Soil characteristics are key environmental factors in terrestrial ecosystems Plants growing in an area are adapted to the texture and chemical composition of the soil.A.Texture and Composition of SoilsSoil is produced by the weathering of solid rock. Living organisms may accelerate theprocess once they become established.Horizons = Distinct soil layers.612Plant NutritionTopsoil = Mixture of decomposed rock of varying texture, living organisms, and humus.The texture of a topsoil depends on the particle size.•Coarse sand has diameter of 0.2 – 2 mm.•Sand is 20 – 200 mm.•Silt is 2 – 20 mm.•Clay is less than 2 mm.Loams = Mixture of sand, silt and clay; are the most fertile soils.•Fine particles retain water and minerals.•Coarse particles provide air spaces with oxygen for cellular respiration.Soil contains many bacteria, fungi, algae, protists, insects, earthworms, nematodes and plantroots. These affect the soil composition. For example, earthworms aerate soil; bacteria altersoil mineral composition.Humus = Decomposing organic material.•Prevents clay from packing together.•Builds a crumbly soil that retains water but is still porous for good root aeration.•Acts as a reservoir of mineral nutrients.Soil composition determines which plants may grow in it, and plant growth, in turn, affectssoil characteristics.B.The Availability of Soil Water and MineralsSome water is bound so tightly to hydrophilic soil that it cannot be extracted by plants.Water bound less tightly is generally available to the plant as a soil solution containingminerals. This solution is absorbed into the root hairs and passes via the apoplast to theendodermis. (See Campbell, Figure 33.5)Positively charged minerals (K+, Ca+, Mg+) adhere by electrical attraction to negativelycharged clay particles.•Clay provides much surface area for binding.•Prevents leaching of mineral nutrients.Cation exchange = H ions in soil displace positively charged mineral ions from clay, making them available to plants.•Stimulated by roots which release acids to add H+ to the soil solution.Negatively charged minerals (NO3–, H2PO4–, SO4–) are not tightly bound to soil particles.•Tend to leach away more quickly.Water and minerals are depleted from soil surrounding roots, but roots grow to bring theplant into contact with a new supply of nutrients.Plant Nutrition 613IV.Soil conservation is one step toward sustainable agricultureGood soil management is necessary to maintain soil fertility which may have taken centuries to develop through decomposition and accumulation of organic matter.•Agriculture is unnatural and depletes the mineral content of the soil, making soil less fertile.Crops also use more water than natural vegetation.•Three important aspects of soil management are:1.Fertilizers2.Irrigation3.Erosion prevention1.FertilizersFertilizers may be mined, chemically produced or organic.•Usually enriched in N, P, K.•Marked with 3 numbers corresponding to % nitrogen (as ammonium or nitrate), % phosphorus (as phosphoric acid), and % potassium (as potash).Organic fertilizers are manure, fishmeal, and compost.•Release minerals more gradually than chemical fertilizers, thus, soil retains minerals longer. Excess minerals from chemical fertilizers may be leached from soil and maypollute streams and lakes.To properly use fertilizers, one must consider the soil pH.•Acidity affects cation exchange and the chemical form of the minerals.• A change in soil pH may make one essential element more available while causing another to adhere so tightly to soil particles that it is unavailable.2.IrrigationAvailability of water limits plant growth in many environments.Problems of irrigation arid land:•Huge drain of water resources.•Can gradually make soil salty and infertile.Solutions to these problems:•Use of drip irrigation. Perforated pipes slowly drip water close to plant roots, which reduces water evaporation and drainage.•Development of plant varieties that require less water or can tolerate more salinity.3.ErosionWind and water erode away much of the topsoil each year. Measures to prevent theselosses include:•Rows of trees to divide fields act as windbreaks.•Terracing hillsides helps prevent water erosion.•Planting alfalfa and wheat provide good ground cover and protection.614Plant NutritionProper management makes soil a renewable resource which will remain fertile for manygenerations.Plant Nutrition 615 V.The metabolism of soil bacteria makes nitrogen available to plantsPlants require nitrogen to produce proteins, nucleic acids and other organic molecules.•Plants can not use nitrogen in gaseous form (N2).•To be assimilated by plants, nitrogen must be in the form of ammonium (NH4+) or nitrate (NO3–).There is a complex cycling of nitrogen in ecosystems:•Over the short term, the main source of nitrogenous minerals is the decomposition of humus by microbes.⇒For example, ammonifying bacteria.⇒Nitrogen in organic compounds is repackaged into inorganic compounds that can be absorbed as minerals by roots.•Nitrogen is lost from this local cycle when soil denitrifying bacteria convert NO3– to N2, which diffuses form the soil to the atmosphere.•Nitrogen fixing bacteria restock nitrogenous minerals in the soil by converting N2 to NH3 (ammonia), a metabolic process called nitrogen fixation.A.Nitrogen FixationNitrogen fixation = The process of converting atmospheric nitrogen (gaseous state) to nitrogenous compounds that can be directly used by plants (nitrate or ammonia). (SeeCampbell, Figure 33.8)The process is catalyzed by the enzyme nitrogenase:nitrogenaseN2 + 8e– + 8H+ + 16ATP 2NH3 + H2 + 16ADP + 1 P i •Some soil bacteria possess nitrogenase.•Very energy consuming process, costing the bacteria at least 12 ATPs for each ammonia molecule synthesized.•In the soil, ammonia is converted to the ammonium ion which plants can absorb:NH3 + H+NH4+•Plants acquire most of their nitrogen in the form of nitrate (NO3–), which is produced in soil by nitrifying bacteria that oxidize ammonium.•Nitrogen absorbed by the plant is incorporated into organic compounds.•Most plant species export nitrogen from the roots to the shoots (through xylem) in the form of organic compounds and amino acids.616Plant NutritionB.Symbiotic Nitrogen FixationLegumes (e.g. peas, beans, soybeans, peanuts, alfalfa, clover) have a built-in source of fixednitrogen because they possess root nodules.Nodules = Root swellings composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium. Each species of legume is associated with a particular species ofRhizobium.Nodules form as follows: (See Campbell, Figure 33.9)•Roots secrete chemicals that attract nearby bacteria.•Attracted bacteria emit chemicals which stimulate root hairs to elongate and curl to prepare for bacterial infection.•Bacteria enter the root through an "infection thread" that carries them to the root cortex.•Bacteria become enclosed in vesicles and assume a form called bacteroids.•Bacteroids produce a chemical that induces the host's cells to divide and form a nodule.•Nodules continue to grow and a connection with the xylem and phloem develops.This association is mutualistic; the bacteria supplies fixed nitrogen, and the plant suppliescarbohydrates and other organic compounds.Leghemoglobin = An iron-containing protein that binds oxygen.•The plant and the bacteria each make a part of the molecule.•Releases oxygen for the intense respiration needed to produce ATP for nitrogen fixation.•Keeps the free oxygen concentration low in root nodules so that the oxygen cannot inhibit the function of nitrogenase.Most of the ammonium produced is used by the nodules to make amino acids for export tothe shoots and leaves.The basis for crop rotation is that, under favorable conditions, root nodules fix more nitrogenthan the legume uses. The excess is secreted as ammonium into the soil.•One year a nonlegume crop is planted, and the next year a legume is planted to restore the fixed nitrogen content of the soil.•Legumes may be plowed under to further increase the fixed nitrogen content of the soil.Some non-legumes host nitrogen-fixing symbionts.•Alders and tropical grasses may host nitrogen-fixing actinomycetes.•Rice farms culture a fern (Azolla), containing symbiotic nitrogen-fixing cyanobacteria, with the rice.Plant Nutrition 617 VI.Improving the protein yield of crops is a major goal of agricultural researchA majority of the world's population depends mainly on plants for protein. Some food plants havea low protein content while others may be deficient in certain amino acids.Ways in which to increase the protein content of plants are:1.Plant breeding to create new varieties enriched in protein.•Unfortunately these "super" varieties require large quantities of nitrogen in the form of commercial fertilizer too expensive for many countries to afford.2.Improving the productivity of symbiotic nitrogen fixation.•Mutant strains of Rhizobium have been isolated that continue to produce nitrogenase even after fixed nitrogen accumulates, thus releasing the excess into the soil.•Rhizobium varieties may be selected that fix N2 at a lower cost in photosynthetic energy, yielding a higher total food content in the plant.3.Genetic engineering.•May create varieties of Rhizobium that can infect nonlegumes.•May transfer the genes required for nitrogen-fixation directly into plant genomes, using bacterial plasmids as vectors. (See Campbell, Chapter 19)VII.Predation and symbiosis are evolutionary adaptations that enhance plant nutrition Some plant adaptations enhance nutrition through interactions with other organisms.A.Parasitic PlantsSome parasitic plants:•Are photosynthetic, and only supplement nutrition by using haustoria (not homologous to those of parasitic fungi) to obtain xylem sap from its host plant (e.g.mistletoe).•Have lost photosynthesis entirely, drawing all nutrients from the host plant (e.g.dodder).Epiphytes are plants that:•Grow on the surface of other plants, anchored by roots, but are not parasitic.•Nourish themselves from the water and minerals absorbed from rain.•Examples include Spanish moss and staghorn ferns.B.Carnivorous PlantsCarnivorous plants:•Live in habitats with poor (usually nitrogen deficient) soil conditions.•Are photosynthetic, but obtain some nitrogen and minerals by killing and digesting insects.618Plant Nutrition•Most insect traps evolved by modifications of leaves and usually are equipped with glands that secrete digestive juices.Plant Nutrition 619C.MycorrhizaeMycorrhizae = Symbiotic associations (mutualistic) between plant roots and fungi. The fungus either forms a sheath around the root or penetrates root tissue.•Helps the plant absorb water.•Absorbs minerals and may secrete acid that increases mineral solubility and converts minerals to forms easily used by the plant.•May help protect the plant against certain soil pathogens.•The plant nourishes the fungus with photosynthetic products.Almost all plants are capable of forming mycorrhizae if exposed to the proper species offungi. Plants grow more vigorously when mycorrhizae are present.Mycorrhizae may have permitted early plants to colonize land.•Fossils indicate the earliest land plants possessed mycorrhizae.•This mutualistic association may have allowed the early plants to obtain enough nutrients to survive colonization.REFERENCESCampbell, N. Biology. 4th ed. Menlo Park, California: Benjamin/Cummings, 1996.Raven, P.H., R.F. Evert and S.E. Eichhorn. Biology of Plants. 5th ed. New York: Worth Publishers, Inc., 1992.Reganold, J.P., R.I. Papendick and J.F. Parr. "Sustainable Agriculture." Scientific American, June 1990.。