Plant Responses to Environmental Challenges

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Plant Responses to Environmental Challenges
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39 Plant Responses to Environmental Challenges

• 39.1 How Do Plants Deal with Pathogens?
• 39.2 How Do Plants Deal with Herbivores?
• 39.3 How Do Plants Deal with Climate Extremes?
• 39.4 How Do Plants Deal with Salt and Heavy
  Metals?

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39.1 How Do Plants Deal with Pathogens?

• Plants and pathogens have been evolving together
  in an evolutionary arms race.
• Pathogens evolve mechanisms to attack plants,
  plants evolve defenses against pathogens.
• Plants use both mechanical and chemical defenses.

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39.1 How Do Plants Deal with Pathogens?

• The epidermis and cork tissues protect the outer
  surfaces they often have cutin, suberin, or
  waxes.
• If pathogens pass these barriers, other
  nonspecific mechanisms are used.

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39.1 How Do Plants Deal with Pathogens?

• Plants generally do not repair tissue damaged by
  pathogens, but instead seal it off to prevent the
  rest of the plant from being infected.
• Because plants are modular, they can grow new
  modules to replace damaged ones.

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39.1 How Do Plants Deal with Pathogens?

• One of plant cells first lines of defense is to
  deposit more polysaccharides on the inside of the
  cell wall to reinforce this barrier.
• The polysaccharides block the plasmodesmata, and
  serve as a base for laying down lignin. Lignin is
  a mechanical barrier, and the precursors are
  toxic.

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Figure 39.1 Signaling between Plants and Pathogens
ANIMATION!
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39.1 How Do Plants Deal with Pathogens?

• Plants produce many kinds of defensive compounds.
• Phytoalexins are toxic to many fungi and
  bacteria. Most are phenolics or terpenes, and
  protect against herbivores as well.

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Table 39.1 Secondary Plant Metabolites Used in
Defense
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39.1 How Do Plants Deal with Pathogens?

• Enzymes from a pathogenic fungus cause plant cell
  walls to release signaling molecules,
  oligosaccharins, which trigger phytoalexin
  production.
• The action is nonspecific, so they can kill many
  species in addition to the one that triggered
  their production.

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39.1 How Do Plants Deal with Pathogens?

• Pathogenesis-related proteins (PR proteins)
• Some are enzymes that break down the cell walls
  of pathogens. Chemicals released from the walls
  can trigger other defense mechanisms.
• Other PR proteins serve as signals of attack to
  other cells.

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39.1 How Do Plants Deal with Pathogens?

• Hypersensitive response cells around site of
  infection die, preventing nutrients from reaching
  site. Some produce phytoalexins before they die.
• Dead tissue is called a necrotic lesion, contains
  and isolates what is left of the microbial
  invasion.

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Figure 39.2 The Aftermath of a Hypersensitive
Response
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39.1 How Do Plants Deal with Pathogens?

• One chemical produced during the hypersensitive
  response is salicylic acid, from which aspirin is
  derived.
• People have used willow (Salix) since ancient
  times for pain and fever.
• Now it appears that all plants have some
  salicylic acid. It often evokes a second complex
  defensive response.

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39.1 How Do Plants Deal with Pathogens?

• Systemic acquired resistance general increase in
  resistance of whole plant to a wide range of
  pathogens. Can be long-lasting.
• Accompanied by production of PR proteins.
• Salicylic acid treatment provides protection
  against tobacco mosaic virus and other viruses.

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39.1 How Do Plants Deal with Pathogens?

• Salicylic acid also acts as a hormone.
• A microbial infection in one part of a plant
  leads to export of salicylic acid to other parts
  of the plant, and PR proteins are produced to
  help stop the spread of infection.
• Infected plant parts also produce methyl
  salicylate, which is volatile and may travel in
  the air to nearby plants, where it triggers PR
  protein production.

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39.1 How Do Plants Deal with Pathogens?

• Triggering of both responses is a very specific
  mechanism gene-for-gene resistance.
• Plants have many R genes (resistance genes)
  pathogens have sets of Avr genes (avirulence
  genes).
• Dominant R alleles favor resistance dominant Avr
  alleles make pathogens less effective.

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39.1 How Do Plants Deal with Pathogens?

• If a plant has a dominant allele of one R gene
  and a pathogen has a dominant allele for the
  corresponding Avr gene, the plant will be
  resistant.
• This is true even if none of the other RAvr
  pairs have corresponding dominant alleles
  (epistasis).
• Most gene-for-gene interactions trigger the
  hypersensitive response.

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Figure 39.3 Gene-for-Gene Resistance
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39.1 How Do Plants Deal with Pathogens?

• Plant response to RNA viruses
• Plant uses its own enzymes to convert virus RNA
  to double-stranded RNAs (dsRNA), and to chop it
  into small piecessmall interfering RNAs (siRNA).
• Some of the viral RNA is transcribed, but the
  siRNAs degrade the mRNA, blocking viral
  replication.

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39.1 How Do Plants Deal with Pathogens?

• This is an example of RNA interference (RNAi), or
  posttranscriptional gene silencing.
• Immunity conferred by RNAi spreads quickly
  through the plant.
• Plant viruses fight back by evolving mechanisms
  to confound RNAi.

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39.2 How Do Plants Deal with Herbivores?

• Herbivores are predators that prey on plants.
  They can have positive and negative effects on
  plants.
• Grazing herbivores eat part of plant without
  killing it.
• Plants and their predators have evolved together
  has favored increased production in some
  grazed-upon plant species.

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39.2 How Do Plants Deal with Herbivores?

• Removing some leaves from a plant can increase
  photosynthesis in remaining leaves.
• Nitrogen doesnt have to be divided among so many
  leaves remaining leaves may increase production
  to keep up with demand from roots
• Grazing can decrease shading of younger, more
  active leaves.

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39.2 How Do Plants Deal with Herbivores?

• Grasses grow from the base of the shoot, so
  growth is not cut short by grazing.
• Scarlet gilia, grazed by elk and mule deer,
  regrows four stems instead of one, and produce
  three times as many fruits as ungrazed plants.

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Figure 39.4 Overcompensation for Being Eaten
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39.2 How Do Plants Deal with Herbivores?

• Plants may benefit from herbivory because the
  animals may spread its seeds, fruit, or pollen.
• But resisting herbivory is often advantageous for
  plants.

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Lepidopteran larvae feeding on leaves
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Raphide extrusion in Dieffenbachia
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39.2 How Do Plants Deal with Herbivores?

• Many plants have chemical defenses, which are
  secondary metabolites.
• Primary metabolites are proteins, nucleic acids,
  lipids, and carbohydrates that are produced and
  used by all living things.
• Secondary metabolites are compounds not used for
  basic metabolism. Plants can have vastly
  different secondary metabolites.

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39.2 How Do Plants Deal with Herbivores?

• There are more than 10,000 known secondary plant
  metabolites. Some are produced by a single
  species, others characteristic of whole groups.
• They have many different effects. Some act on the
  nervous system of herbivores some mimic insect
  hormones some damage digestive tracts.

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39.2 How Do Plants Deal with Herbivores?

• Humans use many secondary metabolites as
  pesticides and pharmaceuticals.
• Nicotine was one of first insecticides to be
  used. This was tested in tobacco plants in which
  the enzyme for nicotine synthesis had been
  silenced. These plants suffered much more damage
  than wild types.

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Figure 39.5 Some Plants Use Nicotine to Reduce
Insect Attacks (Part 1)
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Figure 39.5 Some Plants Use Nicotine to Reduce
Insect Attacks (Part 2)
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39.2 How Do Plants Deal with Herbivores?

• Canavanine is an amino acid similar to arginine,
  but is not found in proteins.

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39.2 How Do Plants Deal with Herbivores?

• Canavanine is a nitrogen-storing compound in
  seeds, and also an insecticide.
• When insects eat plant tissue with canavanine, it
  is incorporated into proteins where arginine
  should be. This results in defective proteins and
  developmental abnormalities and death for the
  insect.

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39.2 How Do Plants Deal with Herbivores?

• Some insect larvae can eat canavanine-containing
  plants. They have tRNA that can discriminate
  between arginine and canavanine, so proteins are
  made correctly.

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39.2 How Do Plants Deal with Herbivores?

• When some plants are attacked by herbivorous
  insects, they synthesize chemical signals to
  attract other insects to prey on the herbivores.
• Most signals are produced in the leaves, but
  roots attacked by beetle larvae were found to
  send signals to attract predaceous nematodes.

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Figure 39.6 Roots May Recruit Nematodes as
Defenders (Part 1)
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Figure 39.6 Roots May Recruit Nematodes as
Defenders (Part 2)
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Figure 39.6 Roots May Recruit Nematodes as
Defenders (Part 3)
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39.2 How Do Plants Deal with Herbivores?

• Many plant defenses involve complex signaling.
• Systemin is a polypeptide hormone formed in
  response to insect herbivores.
• It initiates production of jasmonates from
  linolenic acid. These activate genes that encode
  for a protease inhibitor.
• The inhibitor interferes with protein digestion
  in the insect and stunts growth.

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Figure 39.7 A Signaling Pathway for Synthesis of
a Defensive Secondary Metabolite
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39.2 How Do Plants Deal with Herbivores?

• Jasmonates are also formed when signals are
  released from leaves by chewing caterpillars.
• Jasmonates trigger production of volatile
  compounds that attract insect predators of the
  caterpillars.

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39.2 How Do Plants Deal with Herbivores?

• In experiments with wild and cultivated beans,
  researchers determined that the seed protein
  arcelin in the wild plants conferred resistance
  to two species of bean weevils.
• Work is now being done using recombinant DNA to
  introduce genes for arcelin into crop plants.

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39.2 How Do Plants Deal with Herbivores?

• Crop plants that produce their own pesticides
  have already been engineered.
• Tomatoes, corn, and cotton have been engineered
  to express the gene for the toxin from a
  bacterium Bacillus thuringiensis, which kills
  insects.

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39.2 How Do Plants Deal with Herbivores?

• Plants that produce toxic secondary metabolites
  protect themselves from the toxin by
• Isolating the toxin in special compartments.
• Producing toxic substances only after the cells
  have been damaged.
• Using modified enzymes or receptors that do not
  recognize the toxins (e.g., canavanine-producing
  plants).

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39.2 How Do Plants Deal with Herbivores?

• Plants store toxins in vacuoles if toxin is
  water-soluble.
• If toxin is hydrophobic, it can be stored in
  lacticifers (tubes containing rubbery latex) or
  dissolved in waxes on the cuticle.

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39.2 How Do Plants Deal with Herbivores?

• Some plants store precursors of the toxin in one
  place, and enzymes to convert it to a toxic form
  in another place. Toxin is produced only if a
  cell is damaged.
• Sorghum and some legumes produce cyanide
  (inhibits cellular respiration) in this way.

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39.2 How Do Plants Deal with Herbivores?

• Milkweeds are latex-producing plants they are
  lacticiferous).
• When damaged, a large amount of latex is
  released, keeping some insects from eating the
  leaves.
• One beetle that feeds on milkweed first cuts some
  leaf veins and causes latex flow, then moves to
  an upstream part of the leaf to feed.

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Figure 39.8 Disarming a Plants Defenses
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39.3 How Do Plants Deal with Climate Extremes?

• Desert plants may have structural adaptations for
  water conservation, or alternative strategies.
• Some desert plants carry out their entire life
  cycle in a brief period of rainfall.
• Plants adapted to dry environments are called
  xerophytes (xero Greek, dry)

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Figure 39.9 Desert Annuals Evade Drought
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39.3 How Do Plants Deal with Climate Extremes?

• Structural adaptations include thick cuticles or
  a dense covering of hairs to retard water loss.
• The stomata may be in sunken pits below the
  surface, which may have hairs as well, called
  stomatal crypts.

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Figure 39.10 Stomatal Crypts
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39.3 How Do Plants Deal with Climate Extremes?

• Many have thick, water storing leavessucculence.
• Some produce leaves only when water is available,
  (e.g., ocotillo).
• Cacti have spines instead of typical leaves
  photosynthesis occurs in the stems.

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Figure 39.11 Opportune Leaf Production
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39.3 How Do Plants Deal with Climate Extremes?

• Corn and other grasses have leaves that roll up
  in dry weather, reducing leaf surface area for
  water loss.
• Eucalyptus trees have leaves that hang vertically
  all the time, avoiding direct exposure to the
  sun.
• Xerophytic adaptations can also minimize uptake
  of CO2many grow slowly but use water more
  efficiently.

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39.3 How Do Plants Deal with Climate Extremes?

• Roots may also have adaptations.
• Mesquite trees have long taproots, reaching water
  far underground.
• Cacti have shallow root systems that intercept
  water near the surface may die back during dry
  periods.

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Figure 39.12 Mining Water with Deep Taproots
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39.3 How Do Plants Deal with Climate Extremes?

• Some xerophytes accumulate proline and other
  solutes in central vacuoles.
• Solute potential and water potential of cells
  become more negative, they can extract more water
  from the soil.
• Plants in salty environments also do this.

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39.3 How Do Plants Deal with Climate Extremes?

• Some plants live in environments with too much
  water in the soil.
• Saturated soils limit diffusion of oxygen to
  roots.
• Adaptations include shallow, slow-growing roots
  that can carry out alcoholic fermentation.

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39.3 How Do Plants Deal with Climate Extremes?

• Pneumatophores are root extensions that grow up
  out of the water have many lenticels and spongy
  tissue for gas exchange, (e.g., cypresses and
  mangroves).

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Figure 39.13 Coming Up for Air
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39.3 How Do Plants Deal with Climate Extremes?

• Aquatic plants may have large air spaces in the
  parenchyma of leaf petioles, called aerenchyma.
• Oxygen produced by photosynthesis is stored
  there, and diffuses to other plant parts for
  respiration.
• Aerenchyma also provides buoyancy.

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Figure 39.14 Aerenchyma Lets Oxygen Reach
Submerged Tissues
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39.3 How Do Plants Deal with Climate Extremes?

• Temperature extremes can also stress plants.
• High temperatures can denature proteins and
  destabilize membranes.
• Low temperatures cause membranes to loose
  fluidity and alter permeability.
• Freezing can cause ice crystals to form, damaging
  membranes.

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39.3 How Do Plants Deal with Climate Extremes?

• Transpiration can cool a plant, but increases its
  need for water.
• Plants in hot environments have many adaptations
  similar to xerophytes.
• Spines and hairs help radiate heat.
• CAM metabolism allows some metabolic processes to
  occur at night.

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39.3 How Do Plants Deal with Climate Extremes?

• Plants can respond to high temperatures by
  producing heat shock proteins.
• Some are chaperonins, which help prevent other
  proteins from denaturing.
• Chilling and freezing can also produce these
  proteins.

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39.3 How Do Plants Deal with Climate Extremes?

• Low temperatures (above freezing) can harm some
  plants, (e.g., corn, cotton, tropical
  plants)chilling injury.
• Many plants can be cold-hardened to resist
  chilling injuryrepeated exposure to cool
  temperatures.
• The relative amount of unsaturated fatty acids in
  cell membranes increasesthey solidify at lower
  temperatures, membranes retain fluidity.

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39.3 How Do Plants Deal with Climate Extremes?

• Low temperatures can induce formation of heat
  shock proteins that protect against chilling
  injury.
• Sometimes heat shock proteins can provide cross
  protection, protecting against both heat and low
  temperatures.

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39.3 How Do Plants Deal with Climate Extremes?

• Ice crystals inside cells can puncture organelles
  and membranes.
• If ice crystals grow outside the cell, they draw
  water from the cell and can dehydrate them.
• Freeze-tolerant plants have many adaptations to
  cope, including production of antifreeze proteins
  that slow formation of ice crystals.

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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Saline (salty) habitats support few plant
  species. They can be hot and dry, or cool and
  moist.
• Salinization of agricultural land is an
  increasing global problem, which can occur in
  irrigated lands, and from salt water intrusion.

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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Saline environments pose osmotic problems for
  plants, because of the negative water potential
  of the environment.
• Plants must have an even more negative water
  potential to obtain water from that environment.
• Salt ions are also toxic in large quantities.

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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Halophytes are plants adapted to saline
  conditions.
• Most accumulate sodium and chloride ions in
  vacuoles. The increased salt concentration makes
  the water potential of cells more negative.

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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Some halophytes have salt glands in their leaves
  that excrete salt (e.g., some desert plants such
  as tamarisk, and some mangroves).
• One desert shrub has glands that secret salt into
  small bladders on leaves which increases gradient
  in water potential, helping leaves get more water
  from roots. Also reduces transpirational loss of
  water.

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Figure 39.15 Excreting Salt
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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Halophytes and xerophytes have similar
  adaptations.
• Some accumulate proline in vacuoles, making water
  potential of cells more negative. It is not
  toxic, unlike sodium.
• Succulents occur in saline environments where
  uptake of water is difficult.

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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Many succulents, both halophytes and xerophytes,
  have CAMcrassulacean acid metabolism.
• CO2 can be taken up at night and stored as
  carboxyl groups, the CO2 is released for
  photosynthesis during the day. Allows stomata to
  be closed during the day, minimizing water losses.

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Figure 39.16 Stomatal Cycles
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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Heavy metals are also toxic. Some sites naturally
  have high concentrations of chromium, mercury,
  lead, and others.
• Other sites include acid rain-affected sites
  (leads to release of aluminum from soils), and
  mine sites.
• Mine tailings have high concentrations of heavy
  metals, but some plants can survive.

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Figure 39.17 Life after Strip Mining
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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Tolerant plants take up the heavy metals, in
  concentrations that would kill other plants. This
  is being used in bioremediationdecontaminating
  sites by using living organisms.
• The mechanism for tolerance is known for one
  planta buckwheat exposed to aluminum secretes
  oxalic acid which combines with the Al and does
  not inhibit root growth.

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39.4 How Do Plants Deal with Salt and Heavy
Metals?

• Tests of bent grass, which grows near mines
  showed that plants tolerated the metals that had
  been most abundant in their environment, but were
  sensitive to others.
• Tolerant populations can colonize new
  environments rapidly (e.g., bent grass around one
  copper mine is tolerant of copper, but the copper
  mining was only 100 years ago).