Abiotic stresses

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Lecture 22
Abiotic stresses Oxygen Deficiency Air pollutants
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Oxygen deficiency Some facts

• Roots obtain oxygen for respiration from gaseous
  space in soil.
• Gas-filled pores in well-drained,
  well-structured soil permit diffusion of O2 to
• depths of several meters.
• Upon poor drainage, flooding, excessive
  irrigation, water fills pores and
• blocks O2 diffusion.
• Dissolved oxygen diffuses very slowly in
  stagnant water only a few
• centimeters of soil near surface remain
  oxygenated.
• When temperatures are low and plants are
  dormant, oxygen depletion is
• very slow, no major harm.
• At higher temperatures (gt20ºC), oxygen
  consumption by plant roots, soil
• fauna and microorganisms can deplete oxygen in
  24 h.

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Oxygen deficiency Some facts

• Flooding-sensitive plants (Pisum sativum)
• severely damaged by 24 h of anoxia (lack of
  oxygen)
• growth and survival are depressed, crop yields
  are reduced
• Flooding-tolerant plants (Iris pseudocorus,
  Oryza sativa)
• have special adaptations
• Anaerobic microorganisms are active in
  water-saturated soils
• derive their energy from the reduction of
  nitrate (NO3_) to
• nitrite (NO2_) or to nitrous oxide (N2O) and
  molecular nitrogen (N2)
• Under more reducing conditions, anaerobes reduce
  Fe3 to Fe2
• Fe2 is more soluble ? Fe2 can rise to toxic
  concentrations
• Also reduction of sulfate (SO42_) to hydrogen
  sulfide (H2S)
• bacterial metabolites (acetic acid, butyric
  acid) are released into soil
• water along with reduced sulfur compounds are
  toxic to plants at
• high concentrations

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Roots are damaged in anoxic environments
Metabolic changes during anoxia
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Roots are damaged in anoxic environments

• In healthy cells vacuole is more acidic (pH
  5.8) than cytoplasm (pH 7.4)
• Under O2 deficiency protons leak from vacuole
  into cytoplasm, adding to
• the acidity generated in initial burst of
  lactic acid fermentation
• ? these changes in pH (cytosolic acidosis) are
  associated with
• onset of cell death
• Cytosolic acidosis irreversibly
• disrupts metabolism in cytoplasm
• Timing and degree of to which
• cytosolic acidosis is limited
• distinguishes flooding-sensitive
• from flooding-tolerant plants

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Damaged O2-deficient roots injure shoots

• Anoxic or hypoxic roots lack sufficient energy
  to support physiological processes on
• which the shoots depend
• Older leaves senesce prematurely because of
  reallocation of phloem-mobile
• elements (N, P, K) to younger leaves
• Lower permeability of roots to water leads to
  decrease in leaf potential and wilting (is
• temporary if stomata close, preventing further
  water loss by transpiration)
• Hypoxia accelerates production of ethylene
  precursor ACC
• - ACC travels via xylem sap to shoot, where
  (in contact with oxygen) it is
• converted to ethylene by ACC oxidase
• - Upper (adaxial) surface of leaf petioles of
  tomato and sunflower have
• ethylene-responsive cells that expand more
  rapidly when ethylene
• concentrations are high ? epinasty
• In pea and tomato, flooding induces stomatal
  closure apparently without
• detectable changes in leaf water potential
• - stimulation of ABA production and movement of
  ABA into leaves and

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Submerged organs can acquire O2 through
specialized mechanisms and structures
Wetland vegetation is well adapted to grow for
extended periods in water-saturated soil.
Water lily (Nymphoides peltata)
Submergence traps endogenous ethylene, which
stimulates cell elongation of petiole, extending
it to the water surface
Rice internodes respond similarly to trapped
ethylene so that leaves extend above
water surface
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Submerged organs can acquire O2 through
specialized mechanisms and structures
Pondweed (Potomageton pectinatus), an aquatic
monocot stem elongation is insensitive to
ethylene elongation is promoted by acidification
of the surrounding water caused by accumulation
of respiratory CO2
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Submerged organs can acquire O2 through
specialized mechanisms and structures
Aerenchyma cells are separated by prominent,
gas-filled spaces - develops in the stem
base and newly developing roots (rice, maize)
Control, supplied with air
Oxygen-deficient root
Xyl.
Xyl.
Endo.
Endo.
Cortex
GS.
Cortex
Epid.
Epid.
Transverse sections of roots of maize (SEM)
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Submerged organs can acquire O2 through
specialized mechanisms and structures
Mangroves - restricted to tropical and
subtropical areas - grow at intertidal
locations - develop aerial roots to cope
with hypoxic conditions
Peg roots Sonneratia alba
Prop roots Rhizophora mucronata
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Submerged organs can acquire O2 through
specialized mechanisms and structures
Mangroves - restricted to tropical and
subtropical areas - grow at intertidal
locations - developed aerial roots to cope
with hypoxic conditions
Knee roots Bruguira gymnorrhiza
Pencil roots Avicennia marina
Plank roots Xylocarpus granatum
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Acclimation to O2 deficit involves synthesis of
anaerobic stress proteins

• Mechanism of sensing hypoxic or anoxic
  conditions is not completely clear.
• One of the earliest events occurring elevation
  of intracellular Ca2 ? signal
• to increase mRNA levels of alcohol
  dehydrogenase and sucrose synthase
• in maize cells in culture.
• Presumably also transcriptional (and
  translational?) control of these
• anaerobic stress genes through a G-Box element
  that bind cis-acting
• transcription factors resulting in
  transcriptional activation.

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Air pollution
Air pollution any atmospheric condition in
which substances are present at concentrations
high enough above their normal ambient levels to
produce a measurable effect on man, animals,
vegetation, or materials.
Substances any natural or anthropogenic
(man-made) chemical compounds capable of being
airborne. They may exist in the atmosphere as
gases, liquid drops, or solid particles.
Major Classes of Air Pollutants Carbon oxides
(CO CO2) Sulfur oxides (mainly SO2) Nitrogen
oxides - NO (nitric oxide) NO2 (nitrogen
dioxide) Volatile organic compounds
(hydrocarbons) - methane, benzene, propane,
chlorofluorocarbons (CFC's) Toxic
compounds (Mercury) Photochemical oxidants
(mainly ozone) Smog (complex mixture of gases but
primarily ozone)
http//people.eku.edu/ritchisong/317notes8.html
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Composition of the Earths atmosphere
http//people.eku.edu/ritchisong/317notes8.html
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Natural air pollutants
Not all air pollutants are man made. Air
Pollution is not new nor is it exclusive to man.
In 1883, Krakatoa exploded and put tons of debris
in the air. More recently St. Helens exploded
burying near by areas in ash several feet deep.
Less spectacular are the every day emissions in
the table below.

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Damage caused by acid rain
Nitric oxide sulfur dioxide released primarily
from electric power plants motor vehicles SO2
water vapor ozone ---gt H2SO4 NO sunlight
O2 ---gt NO2 various atmospheric gases ---gt HNO3
Sandstone figure over the portal of a castle in
Westphalia, Germany, photographed in 1908 (left)
and again in 1968 (right). Acid rain produced by
air pollution generated in the heavily
industrialized Ruhr region of Germany probably
accounts for the severe damage. The castle was
built in 1702.
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Effect of air pollution on human health, animals
and plants
Effect on human health Much evidence links air
pollutants to respiratory other diseases in
humans Examples of air pollution-related
diseases Pulmonary irritation impaired lung
function Chronic bronchitis Emphysema
(abnormal increase in the size of lungs,
resulting in labored breathing and an increased
susceptibility to infection) Cancer Systemic
toxicity Lead Mercury Increased
susceptibility to disease
Effects animals plants Wild domestic
animals probably affected in the same ways as
humans Plants damaged by ozone, sulfur dioxide,
acids ozone - weakens pine needles makes
them more susceptible to insects
diseases sulfur dioxide - suppresses growth
acid - damages leaves needles
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http//people.eku.edu/ritchisong/317notes8.html
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Chemical reaction of ozone formation in the
Stratosphere
http//www.theozonehole.com/ozonecreation.htm
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Chemical reaction of ozone formation in the
Stratosphere and Troposphere
Production of ozone in the Stratosphere
(photolysis) O2 UV photon --gt O O O O2
--gt O3
Atomic oxygen (very reactive) quickly combines
with molecular oxygen to yield the almost equally
reactive O3.
Ozone loss O3 UV photon --gt O2 O O O3
--gt 2O2
http//www.globalchange.umich.edu/globalchange2/cu
rrent/lectures/ozone_deplete/ozone_deplete.html
Reactions of ozone in the Troposphere
Rao et al. 2000 Plant Mol Biol 44 345
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Spreading of H2O2 in birch leaf cells challenged
with 150 nL L-1 O3

• O3 reacts with plants in
• solid phase (e.g., with cuticular components of
  plant leaves)
• gas phase (e.g., reactions with the hydrocarbons
  emitted by plants)
• liquid phase, which includes the dissolution of
  O3 in aqueous media followed by reaction with
  lipids, proteins and other cellular components.
• O3 dissociation in the leaf extracellular spaces
  has the biggest effect on plants.

Stomata
Pellinen et al. 1999 Plant J. 20 349
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Presumptive chemical reactions of O3 dissociation
in leaf extracellular spaces
Rao et al. 2000 Plant Mol Biol 44 345
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Air Quality Index (AQI) values for ozone
Generally, an AQI of 100 for ozone corresponds
to an ozone level of 0.08 parts per million
(averaged over 8 hours).
http//www.airinfonow.com/html/ed_ozone.html
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Ozone injury on plants
Soybean
Conifers
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Ozone damage in tobacco and birch leaves
fumigated with 300 ppb of ozone for 3 h
Tobacco Leaf discs exposed
1 300 ppb ozone 2 300 ppb ozone 3 ppm
isoprene
Isoprene protects due to direct ozone quenching
Birch Whole leaves exposed
Loreto et al. 2001 Plant Physiol 126 993
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Effect of ozone on leaf structure
Loreto et al. 2001 Plant Physiol 126 993
300 ppb ozone
300 ppb ozone
300 ppb ozone 3 ppb isoprene
300 ppb ozone 3 ppb isoprene
No exposure to ozone or isoprene
No exposure to ozone or isoprene