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Chapter 37 - Plant Nutrition

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Chapter 37 Plant Nutrition
Lecture Outline

Outline: A Nutritional Network

  • Every organism is an open system linked to its environment by a continuous exchange of energy and materials.
    • In ecosystems, plants and other photosynthetic autotrophs perform the crucial step of transforming inorganic compounds into organic ones.
    • Plants need sunlight as the energy source for photosynthesis.
    • They also need inorganic raw materials such as water, CO2, and inorganic ions to synthesize organic molecules.
    • Plants obtain CO2 from the air. Most vascular plants obtain water and minerals from the soil through their roots.
    • The branching root and shoot systems of vascular plants allow them to draw from soil and air reservoirs of inorganic nutrients.
      • Roots, through fungal mycorrhizae and root hairs, absorb water and minerals from the soil.
      • CO2 diffuses into leaves from the surrounding air through stomata.

Concept 37.1 Plants require certain chemical elements to complete their life cycle

  • Early ideas about plant nutrition were not entirely correct and included:
    • Aristotle’s hypothesis that soil provided the substance for plant growth.
    • van Helmont’s conclusion from his experiments that plants grow mainly from water.
    • Hale’s postulate that plants are nourished mostly by air.
  • In fact, soil, water, and air all contribute to plant growth.
  • Plants extract mineral nutrients from the soil. Mineral nutrients are essential chemical elements absorbed from soil in the form of inorganic ions.
    • For example, many plants acquire nitrogen in the form of nitrate ions (NO3?).
    • However, as van Helmont’s data suggested, mineral nutrients from the soil contribute little to the overall mass of a plant.
  • About 80–90% of a plant is water. Because water contributes most of the hydrogen ions and some of the oxygen atoms that are incorporated into organic atoms, one can consider water a nutrient.
    • However, only a small fraction of the water entering a plant contributes to organic molecules.
    • More than 90% of the water absorbed by a field of corn is lost by transpiration.
    • Most of the water retained by a plant functions as a solvent, provides most of the mass for cell elongation, and helps maintain the form of soft tissues by keeping cells turgid.
  • By weight, the bulk of the organic material of a plant is derived not from water or soil minerals, but from the CO2 assimilated from the atmosphere.
  • The dry weight of an organism can be determined by drying it to remove all water. About 95% of the dry weight of a plant consists of organic molecules. The remaining 5% consists of inorganic molecules.
    • Most of the organic material is carbohydrate, including cellulose in cell walls.
      • Carbon, hydrogen, and oxygen are the most abundant elements in the dry weight of a plant.
      • Because some organic molecules contain nitrogen, sulfur, and phosphorus, these elements are also relatively abundant in plants.
  • More than 50 chemical elements have been identified among the inorganic substances present in plants.
    • However, not all of these 50 are essential elements, required for the plant to complete its life cycle and reproduce.
  • Roots are able to absorb minerals somewhat selectively, enabling the plant to accumulate essential elements that may be present in low concentrations in the soil.
    • However, the minerals in a plant also reflect the composition of the soil in which the plant is growing.
    • Some elements are taken up by plant roots even though they do not have any function in the plant.

    Plants require nine macronutrients and at least eight micronutrients.

  • Plants can be grown in hydroponic culture to determine which mineral elements are actually essential nutrients.
    • Plants are grown in solutions of various minerals in known concentrations.
    • If the absence of a particular mineral, such as potassium, causes a plant to become abnormal in appearance when compared to controls grown in a complete mineral medium, then that element is essential.
    • Such studies have identified 17 elements that are essential nutrients in all plants and a few other elements that are essential to certain groups of plants.
  • Elements required by plants in relatively large quantities are macronutrients.
    • There are nine macronutrients in all, including the six major ingredients in organic compounds: carbon, oxygen, hydrogen, nitrogen, sulfur, and phosphorus.
    • The other three macronutrients are potassium, calcium, and magnesium.
  • Elements that plants need in very small amounts are micronutrients.
    • The eight micronutrients are iron, chlorine, copper, zinc, manganese, molybdenum, boron, and nickel.
    • Most of these function as cofactors, nonprotein helpers in enzymatic reactions.
    • For example, iron is a metallic component in cytochromes, proteins that function in the electron transfer chains of chloroplasts and mitochondria.
    • While the requirement for these micronutrients is modest (e.g., only one atom of molybdenum for every 60 million hydrogen atoms in dry plant material), a deficiency of a micronutrient can weaken or kill a plant.

    The symptoms of a mineral deficiency depend on the function and mobility of the element.

  • The symptoms of a mineral deficiency depend in part on the function of that nutrient in the plant.
    • For example, a deficiency in magnesium, an ingredient of chlorophyll, causes yellowing of the leaves, or chlorosis.
  • The relationship between a mineral deficiency and its symptoms can be less direct.
    • For example, chlorosis can also be caused by iron deficiency because iron is a required cofactor in chlorophyll synthesis.
  • Mineral deficiency symptoms also depend on the mobility of the nutrient within the plant.
    • If a nutrient can move freely from one part of a plant to another, then symptoms of the deficiency will appear first in older organs.
      • Young, growing tissues have more “drawing power” than old tissues for nutrients in short supply.
      • For example, a shortage of magnesium will initially lead to chlorosis in older leaves.
    • If a nutrient is relatively immobile, then a deficiency will affect young parts of the plant first.
      • Older tissue may have adequate supplies, which they can retain during periods of shortage.
      • For example, iron does not move freely within a plant. Chlorosis due to iron deficiency appears first in young leaves.
  • The symptoms of a mineral deficiency are often distinctive enough for a plant physiologist or farmer to make a preliminary diagnosis of the problem.
    • This can be confirmed by analyzing the mineral content of the plant and the soil.
    • Deficiencies of nitrogen, potassium, and phosphorus are the most common problems.
    • Shortages of micronutrients are less common and tend to be geographically localized due to differences in soil composition.
      • The amount of micronutrient needed to correct a deficiency is usually quite small. Care must be taken, because a nutrient overdose can be toxic to plants.
  • One way to ensure optimal mineral nutrition is to grow plants hydroponically on nutrient solutions that can be precisely regulated.
    • This technique is practiced commercially, but the requirements for labor and equipment make it relatively expensive compared with growing crops in soil.
  • Mineral deficiencies are not limited to terrestrial ecosystems or to plants.
  • Photosynthetic protists and bacteria can also suffer from mineral deficiencies.
    • For example, populations of planktonic algae in the southern oceans are limited by iron deficiency.
      • In a trial in relatively unproductive seas between Tasmania and Antarctica, researchers demonstrated that dispersing small amounts of iron produced large algal blooms that pulled carbon dioxide out of the air.
      • Seeding the oceans with iron may help slow the increase in carbon dioxide levels in the atmosphere, but it may cause unanticipated environmental effects.

Concept 37.2 Soil quality is a major determinant of plant distribution and growth

    Soil texture and composition are key environmental factors in terrestrial ecosystems.

  • The texture and chemical composition of soil are major factors determining what kinds of plants can grow well in a particular location.
    • Texture is the general structure of soil, including the relative amounts of various sizes of soil particles.
    • Composition is the soil’s organic and inorganic components.
  • Plants that grow naturally in a certain type of soil are adapted to its texture and composition and are able to absorb water and extract essential nutrients from that soil.
  • Plants, in turn, affect the soil.
  • The soil-plant interface is a critical component of the chemical cycles that sustain terrestrial ecosystems.
  • Soil has its origin in the weathering of solid rock.
    • Water that seeps into crevices and freezes in winter fractures rock. Acids dissolved in soil water also help break down rock chemically.
    • Organisms, including lichens, fungi, bacteria, mosses, and the roots of vascular plants, accelerate the breakdown by the secretion of acids and the expansion of roots in fissures.
  • This activity eventually results in topsoil, a mixture of particles from rock; living organisms; and humus, a residue of partially decayed organic material.
  • Topsoil and other distinct soil layers, called horizons, are often visible in a vertical profile through soil.
  • Topsoil, or the A horizon, is richest in organic material and is thus the most important horizon for plant growth.
  • The texture of topsoil depends on the size of its particles, which are classified from coarse sand to microscopic clay particles.
    • The most fertile soils are loams, made up of roughly equal amounts of sand, silt (particles of intermediate size), and clay.
    • Loamy soils have enough fine particles to provide a large surface area for retaining minerals and water, which adhere to the particles.
    • Loams also have enough course particles to provide air spaces that supply oxygen to the root for cellular respiration.
    • Inadequate drainage can dramatically impact survival of many plants.
    • Plants can suffocate if air spaces are replaced by water.
    • Roots can also be attacked by molds that flourish in soaked soil.
  • Topsoil is home to an astonishing number and variety of organisms.
    • A teaspoon of soil has about 5 billion bacteria that cohabit with various fungi, algae and other protists, insects, earthworms, nematodes, and the roots of plants.
    • The activities of these organisms affect the physical and chemical properties of soil.
    • For example, earthworms aerate soil by burrowing and add mucus that holds fine particles together.
    • Bacterial metabolism alters the mineral composition of soil.
    • Plant roots extract water and minerals. They also affect soil pH by releasing organic acids and reinforce the soil against erosion.
  • Humus is the decomposing organic material formed by the action of bacteria and fungi on dead organisms, feces, fallen leaves, and other organic refuse.
    • Humus prevents clay from packing together and builds a crumbly soil that retains water but is still porous enough for the adequate aeration of roots.
    • Humus is also a reservoir of mineral nutrients that are returned to the soil by decomposition.
  • After a heavy rainfall, water drains away from the larger spaces of the soil, but smaller spaces retain water because of water’s attraction for the electrically charged surfaces of soil particles.
    • Some water adheres so tightly to hydrophilic particles that plants cannot extract it, while water that is bound less tightly to the particles can be taken up by roots.
  • Many minerals, especially those with a positive charge, such as potassium (K+), calcium (Ca2+), and magnesium (Mg2+), adhere by electrical attraction to the negatively charged surfaces of clay particles.
    • Clay in soil prevents the leaching of mineral nutrients during heavy rain or irrigation because of its large surface area for binding minerals.
    • Minerals that are negatively charged, such as nitrate (NO3?), phosphate (H2PO4?), and sulfate (SO42?), are less tightly bound to soil particles and tend to leach away more quickly.
  • Positively charged mineral ions are made available to the plant when hydrogen ions in the soil displace the mineral ions from the clay particles.
    • This process, called cation exchange, is stimulated by the roots, which secrete H+ and compounds that form acids in the soil solution.

    Soil conservation is one step toward sustainable agriculture.

  • It can take centuries for soil to become fertile through the breakdown of soil and the accumulation of organic material.
  • However, human mismanagement can destroy soil fertility within just a few years.
  • Soil mismanagement has been a recurring problem in human history.
  • For example, the Dust Bowl was an ecological and human disaster that occurred in the southwestern Great Plains of the United States in the 1930s.
    • Before the arrival of farmers, the region was covered with hardy grasses that held the soil in place in spite of long recurrent droughts and torrential rains.
    • In the 30 years before World War I, homesteaders planted wheat and raised cattle, which left the soil exposed to wind erosion.
  • Several years of drought resulted in the loss of centimeters of topsoil that were blown away by the winds.
    • Millions of hectares of farmland became useless, and hundreds of thousands of people were forced to abandon their homes and land.
  • To understand soil conservation, we must begin with the premise that agriculture is not natural and can only be sustained by human intervention.
    • In natural ecosystems, mineral nutrients are recycled by the decomposition of dead organic material.
    • In contrast, when we harvest a crop, we remove essential elements.
      • In general, agriculture depletes minerals in the soil.
      • To grow 1,000 kg of wheat, the soil gives up 20 kg of nitrogen, 4 kg of phosphorus, and 4.5 kg of potassium.
    • The fertility of the soil diminishes unless minerals are replaced by fertilizers.
    • Most crops require far more water than the natural vegetation for that area, making irrigation necessary.
  • The goals of soil conservation include prudent fertilization, thoughtful irrigation, and prevention of erosion.
  • Complementing soil conservation is soil reclamation, the return of agricultural productivity to damaged soil.
  • A third of the world’s farmland suffers from low productivity due to poor soil conditions.
  • Farmers have been using fertilizers to improve crop yields since prehistory.
    • Historically, these have included animal manure and fish carcasses.
    • In developed nations today, most farmers use commercial fertilizers containing minerals that are either mined or prepared by industrial processes.
    • These are usually enriched in nitrogen, phosphorus, and potassium, the macronutrients most often deficient in farm and garden soils.
    • Fertilizers are labeled with their N-P-K ratio. A fertilizer marked “10-12-8” is 10% nitrogen (as ammonium or nitrate), 12% phosphorus (as phosphoric acid), and 8% potassium (as the mineral potash).
  • Manure, fishmeal, and compost are “organic” fertilizers because they are of biological origin and contain material in the process of decomposing.
    • The organic material must be decomposed to inorganic nutrients before it can be absorbed by roots.
    • However, the minerals that a plant extracts from the soil are in the same form whether they came from organic fertilizer or from a chemical factory.
    • Compost releases nutrients gradually, while minerals in commercial fertilizers are available immediately.
    • Excess minerals are often leached from fertilized soil by rainwater or irrigation and may pollute groundwater, streams, and lakes.
  • Genetically engineered “smart plants” have been produced. These plants produce a blue pigment in their leaves to warn the farmer of impending nutrient deficiency.
  • To fertilize judiciously, a farmer must maintain an appropriate soil pH. pH affects cation exchange and influences the chemical form of all minerals.
    • Even if an essential element is abundant in the soil, plants may starve for that element if it is bound too tightly to clay or is in a chemical form that the plant cannot absorb.
    • Adjustments to soil pH of soil may make one mineral more available but another mineral less available.
    • The pH of the soil must be matched to the specific mineral needs of the crop.
    • Sulfate lowers pH, while liming (addition of calcium carbonate or calcium hydroxide) increases pH.
  • A major problem with acidic soils, particularly in tropical areas, is that aluminum dissolves in the soil at low pH and becomes toxic to roots.
    • Some plants cope with high aluminum levels in the soil by secreting organic ions that bind the aluminum and render it harmless.
  • Water is the most common factor limiting plant growth.
    • Irrigation can transform a desert into a garden, but farming in arid regions is a huge drain on water resources.
    • Irrigation in an arid region can gradually make the soil so salty that it becomes completely infertile. Salts in the irrigation water accumulate in the soil as the water evaporates.
    • Eventually, the water potential of the soil solution becomes lower than that of root cells, which lose water to the soil instead of absorbing it.
  • Valuable topsoil is lost to wind and water erosion each year.
    • This can be reduced by planting rows of trees between fields as a windbreak and terracing a hillside to prevent topsoil from washing away.
    • Some crops such as alfalfa and wheat provide good ground cover and protect soil better than corn and other crops that are usually planted in widely spaced rows.
  • Soil is a renewable resource in which farmers can grow food for generations to come.
    • The goal is sustainable agriculture, a commitment embracing a variety of farming methods that are conservation-minded, environmentally safe, and profitable.
  • Some areas have become unfit for agriculture or wildlife as the result of human activities that contaminate the soil or groundwater with toxic heavy metals or organic pollutants.
    • In place of costly and disruptive remediation technologies such as removal and storage of contaminated soils, phytoremediation takes advantage of the remarkable abilities of some plant species to extract heavy metals and other pollutants from the soil.
    • These pollutants are concentrated in plant tissues that can be harvested.
    • For example, alpine pennycress (Thlaspi caerulescens) can accumulate zinc in its shoots at concentrations that are 300 times the level most plants can tolerate.
    • Phytoremediation is part of a more general technology of bioremediation, which includes the use of prokaryotes and protists to detoxify polluted sites.

Concept 37.3 Nitrogen is often the mineral that has the greatest effect on plant growth

    The metabolism of soil bacteria makes nitrogen available to plants.

  • Of all mineral nutrients, nitrogen has the greatest effect on plant growth and crop yields.
  • It is ironic that plants sometimes suffer nitrogen deficiencies, for the atmosphere is nearly 80% nitrogen as N2.
    • Plants cannot use nitrogen in the form of N2.
    • It must first be converted to ammonium (NH4+) or nitrate (NO3?).
    • The main source of ammonium and nitrate is the decomposition of humus by microbes, including ammonifying bacteria.
  • Nitrogen is lost from this local cycle when soil microbes called denitrifying bacteria convert NO3? to N2, which diffuses into the atmosphere.
  • Other bacteria, nitrogen-fixing bacteria, restock nitrogenous minerals in the soil by converting N2 to NH3 (ammonia) by the metabolic process of nitrogen fixation.
  • All life on Earth depends on nitrogen fixation, a process performed only by certain bacterial species.
    • In soil, these include several species of free-living bacteria and several others that live in symbiotic relationships with plants.
    • The reduction of N2 to NH3 is a complicated, multistep process, catalyzed by one enzyme complex, nitrogenase, and simplified as:
      •  
        •  
            N2 + 8e? + 8H+ + 16ATP -> 2NH3 + H2 + 16ADP + 16Pi
    • Nitrogen fixation is a very costly process, costing the bacterium 8 ATP for every ammonia molecule synthesized.
    • Nitrogen-fixing bacteria are most abundant in soils rich in organic materials, which provide fuels for cellular respiration to support this expensive metabolic process.
  • In the soil solution, ammonia picks up another hydrogen ion to form ammonium (NH4+), which plants can absorb.
  • Nitrifying bacteria in the soil oxidize ammonium to nitrate (NO3?), the required form of nitrogen for most plants.
    • After nitrate is absorbed by roots, plant enzymes reduce nitrate back to ammonium, which other enzymes then incorporate into amino acids and other organic compounds.
    • Most plant species export nitrogen from roots to shoots via the xylem, in the form of nitrate or organic compounds that have been synthesized in the roots.

    Improving the protein yield of crops is a major goal of agricultural research.

  • The ability of plants to incorporate fixed nitrogen into proteins and other organic substances has a major impact on human welfare.
    • Protein deficiency is the most common form of malnutrition.
    • Either by choice or economic necessity, the majority of the world’s people have a predominately vegetarian diet.
    • Unfortunately, plants are a poor source of protein and may be deficient in one or more of the amino acids that humans need from their diet.
  • Plant breeding has resulted in new varieties of corn, wheat, and rice that are enriched in protein.
    • However, many of these “super” varieties have an extraordinary demand for nitrogen, which is usually supplied by commercial fertilizer produced by energy-costly industrial production.
      • Generally, the countries that most need high-protein crops are the ones least able to afford to pay for the fossil fuels to power the factories that make fertilizers.
  • Agricultural scientists are pursuing a variety of strategies to overcome this protein deficiency.
    • For example, the use of new nitrogenase-based catalysts to fix nitrogen may make commercial production of nitrogen fertilizers cheaper.
    • Alternatively, improvements in the productivity of symbiotic nitrogen fixation may increase protein yields of crops.

Concept 37.4 Plant nutritional adaptations often involve relationships with other organisms

  • The roots of plants belong to subterranean communities that interact with a diversity of other organisms.
    • Among these are certain species of bacteria and fungi that have coevolved with specific plants, forming symbiotic relationships with roots that enhance the nutrition of both partners.
    • The two most important examples of mutualistic interactions are nitrogen fixation (symbiosis of plant roots and bacteria) and the formation of mycorrhizae (symbiosis of plant roots and fungi).

    Symbiotic nitrogen fixation results from intricate interactions between roots and bacteria.

  • Some plant species form symbiotic relationships with nitrogen-fixing bacteria.
    • This provides their roots with a built-in source of fixed nitrogen for assimilation into organic compounds.
    • Much of the research on this symbiosis has focused on the agriculturally important members of the legume family, including peas, beans, soybeans, peanuts, alfalfa, and clover.
  • A legume’s roots have swellings called nodules, composed of plant cells that contain nitrogen-fixing bacteria of the genus Rhizobium.
    • Inside the nodule, Rhizobium bacteria assume a form called bacteriods, which are contained within vesicles formed by the root cell.
    • Legume-Rhizobium symbioses produce more usable nitrogen for plants than all industrial fertilizers, at no cost to farmers. Subsequent crops can also benefit from the usable nitrogen left in the soil by a legume crop.
  • Nitrogen fixation requires an anaerobic environment.
    • Lignified external layers of the nodule limit gas exchange.
    • Nodules produce leghemoglobin, an iron-containing protein that binds reversibly to oxygen. Leghemoglobin provides oxygen for Rhizobium’s intense respiration, while protecting nitrogenase from free oxygen.
  • The development of root nodules begins after bacteria enter the root through an infection thread.
    1. 1. Chemical signals from the root attract the Rhizobium bacteria, and chemical signals from the bacteria lead to the production of an infection thread.
    2. 2. The bacteria penetrate the root cortex within the infection thread.
    3. 3. Growth in cortex and pericycle cells which are “infected” with bacteria in vesicles continues until the two masses of dividing cells fuse, forming the nodule.
    4. 4. As the nodule continues to grow, vascular tissue connects the nodule to the xylem and phloem of the stele, providing nutrients to the nodule and carrying nitrogenous compounds to the rest of the plant.
  • The symbiotic relationship between a legume and nitrogen-fixing bacteria is mutualistic, with both partners benefiting.
    • The bacteria supply the legume with fixed nitrogen.
      • Most of the ammonium produced by symbiotic nitrogen fixation is used by the nodules to make amino acids, which are then transported to the shoot and leaves via the xylem.
  • The plant provides the bacteria with carbohydrates and other organic compounds and protects the nitrogenase from free oxygen.
  • The common agricultural practice of crop rotation exploits symbiotic nitrogen fixation.
    • One year, a nonlegume crop such as corn is planted. The following year, alfalfa or another legume is planted to restore the concentration of fixed soil nitrogen.
    • Often, the legume crop is not harvested but is plowed under to decompose as “green manure.”
    • To ensure the formation of nodules, the legume seeds may be soaked in a culture of the correct Rhizobium bacteria or dusted with bacterial spores before sowing.
  • Species from many other plant families also benefit from symbiotic nitrogen fixation.
    • For example, alder trees and certain tropical grasses host nitrogen-fixing bacteria of the actinomycetes group.
    • Rice benefits indirectly from symbiotic nitrogen fixation because it is often cultivated in paddies with the water fern Azolla, which has symbiotic nitrogen-fixing cyanobacteria.
      • This increases the fertility of the rice paddy through the activity of the cyanobacteria.
      • The growing rice eventually shades and kills the Azolla.
      • The decomposition of water fern adds more nitrogenous compounds to the paddy.

    The molecular biology of root nodule formation is increasingly well understood.

  • The specific recognition between legume and bacteria and the development of the nodule is the result of a chemical dialogue between the bacteria and the root.
    • Each partner responds to the chemical signals of the other by expressing certain genes whose products contribute to nodule formation.
    • The plant initiates the communication when its roots secrete molecules called flavonoids, which enter Rhizobium cells living in the vicinity of the roots.
    • Each particular legume species secretes a type of flavonoid that only a certain Rhizobium species can detect and absorb.
      1. 1. A specific flavonoid signal travels from the root to the plant’s Rhizobium partner.
      2. 2. The flavonoid activates a gene-regulating protein in the bacterium, which switches on a cluster of bacterial genes called nod (for nodulation genes).
      3. 3. The nod genes produce enzymes that catalyze production of species-specific molecules called Nod factors.
      4. 4. Nod factors signal the root to initiate the infection process, enabling Rhizobium to enter the root and begin forming the root nodule.
      5. 5. The plant’s responses require activation of early nodulin genes by a signal transduction pathway involving Ca2+ as second messengers.
    • It may be possible in the future to induce Rhizobium uptake and nodule formation in crop plants that do not normally form such nitrogen-fixing symbioses.
    • In the short term, research is focused on improving the efficiency of nitrogen fixation and protein production.

    Mycorrhizae are symbiotic associations of roots and fungi that enhance plant nutrition.

  • Mycorrhizae (“fungus roots”) are modified roots, consisting of mutualistic associations of fungi and roots.
    • The fungus benefits from a hospitable environment and a steady supply of sugar donated by the host plant.
  • The fungus provides several potential benefits to the host plant.
    • First, the fungi increase the surface area for water uptake and selectively absorb phosphate and other minerals in the soil and supply them to the plant.
    • The fungi also secrete growth factors that stimulate roots to grow and branch.
    • The fungi produce antibiotics that may help protect the plant from pathogenic bacteria and fungi in the soil.
  • Almost all plant species produce mycorrhizae.
    • This plant-fungus symbiosis may have been one of the evolutionary adaptations that made it possible for plants to colonize land in the first place.
      • Fossilized roots from some of the earliest land plants include mycorrhizae.
    • Mycorrhizal fungi are more efficient at absorbing minerals than roots, which may have helped nourish pioneering plants, especially in the nutrient-poor soils present when terrestrial ecosystems were young.
    • Today, the first plants to become established on nutrient-poor soils are usually well endowed with mycorrhizae.
  • Mycorrhizae take two major forms: ectomycorrhizae and endomycorrhizae.
    • In ectomycorrhizae, the mycelium forms a dense sheath over the surface of the root.
    • Some hyphae grow into the cortex in extracellular spaces between root cells. Hyphae do not penetrate root cells but form a network in the extracellular spaces to facilitate nutrient exchange.
    • The mycelium of ectomycorrhizae extends from the mantle surrounding the root into the soil, greatly increasing the surface area for water and mineral absorption.
    • Compared with “uninfected” roots, ectomycorrhizae are generally thicker, shorter, more branched, and lack root hairs.
    • Ten percent of plant families have species that form ectomycorrhizae. Ectomycorrhizae are especially common in woody plants, including trees of the pine, spruce, oak, walnut, birch, willow, and eucalyptus families.
  • Endomycorrhizae have fine fungal hyphae that extend from the root into the soil.
    • Hyphae also extend inward by digesting small patches of the root cell walls, forming tubes by invagination of the root cell’s membrane.
    • Some fungal hyphae within these invaginations may form dense knotlike structures called arbuscles that are important sites of nutrient transfer.
    • Roots with endomycorrhizae look like “normal” roots with root hairs, but the microscopic symbiotic connections are very important.
    • Endomycorrhizae are found in more than 85% of plant species, including important crop plants such as corn, wheat, and legumes.
  • Roots can be transformed into mycorrhizae only if they are exposed to the appropriate fungal species.
    • In most natural systems, these fungi are present in the soil, and seedlings develop mycorrhizae.
    • However, seeds planted in foreign soil may develop into plants that show signs of malnutrition because of the absence of the plant’s mycorrhizal partners.
    • Researchers observe similar results in experiments in which soil fungi are poisoned.
    • Farmers and foresters are already applying the lessons learned from this research by inoculating plants with the spores from the appropriate fungal partner to ensure development of mycorrhizae.

    Epiphytes nourish themselves but grow on other plants.

  • An epiphyte is an autotrophic plant that nourishes itself but grows on the surface of another plant, usually on the branches or trunks of trees.
  • Epiphytes absorb water and minerals from rain, mostly through their leaves.
    • Examples of epiphytes are staghorn ferns, some mosses, Spanish moss, and many species of bromeliads and orchids.

    Parasitic plants extract nutrients from other plants.

  • A variety of plants parasitize other plants to extract nutrients to supplement or even replace the production of organic molecules by photosynthesis by the parasitic plant.
  • Many species have roots that function as haustoria, nutrient-absorbing roots that enter the host plant.
  • Mistletoe supplements its photosynthesis by using projections called haustoria to siphon xylem sap from the vascular tissue of the host tree.
  • Both dodder and Indian pipe are parasitic plants that do not perform photosynthesis at all.
    • The haustoria (modified roots) of dodder tap into the host’s vascular tissue for water and nutrients.
    • Indian pipe obtains its nutrition indirectly via its association with fungal hyphae of the host tree’s mycorrhizae.

    Carnivorous plants supplement their mineral nutrition by digesting animals.

  • Carnivorous plants are photosynthetic but obtain some nitrogen and minerals by killing and digesting insects and other small animals.
  • Such plants live in acid bogs and other habitats where soil conditions are poor in nitrogen and other minerals.
  • Various types of insect traps have evolved by the modification of leaves.
  • The traps are usually equipped with glands that secrete digestive juices.
  • Examples are the Venus flytrap, pitcher plant, and sundew.

    Lecture Outline for Campbell/Reece Biology, 7th Edition, © Pearson Education, Inc. 37-1

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