Stress Physiology

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Stress Physiology Chapter 25
Abiotic stress Water availability (drought,
flooding) Temperature
(hot, cold) Salinity
O2 concentration
Nutrient limitation (N, P, micro
nutrients) Pollution
(air, soil) Radiation
(high, low) Wind Biotic
Herbivory Disease
(fungi, bacteria, virus)
etc
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Economic importance The yield of field-grown
crops in the U.S. is only 22 of the genetic
potential yield (Boyer 1982). Ecological
importance Stress factors limit the distribution
of plant species
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Stress - a disadvantageous influence on the plant
exerted by an external factor. Disadvantageous
reduced growth reproduction (sometimes also
reduced process rates, e.g. photosynthesis)
Growth after 1 month
High T Low T
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Stress tolerance - the ability to maintain
functioning when exposed to a wide range of
conditions. Usually a relative term based on
comparisons among species or genotypes of their
responses to different levels of some factor
(temp., moisture, etc.).
Growth after 1 month
High T Low T
RED has a greater stress tolerance than BLUE
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Acclimation - an increase in stress tolerance of
an individual organism following exposure to
stress.
Growth after 1 month
Adequate Water moisture limitation
RED no previous exposure to drought no stress
tolerance BLUE previous exposure to drought
increased stress tolerance
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Adaptation - a genetically-determined increase in
stress tolerance as a result of selection over
generations.
Growth after 1 month
High T Low T
RED has a greater stress tolerance than BLUE
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Stress Stress tolerance Acclimation Adaptation O
lder literature Stress avoidance for example
early seed-set to avoid drought
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• Water stress drought tolerance
• Heat stress and heat shock
• Chilling and freezing
• Salinity
• O2 deficiency
• Much research is directed towards discovering the
  mechanisms of stress tolerance, acclimation etc.

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• Water stress drought tolerance
• Heat stress and heat shock
• Chilling and freezing
• Salinity
• O2 deficiency
• Much research is directed towards discovering the
  mechanisms of stress tolerance, acclimation etc.

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Precipitation and productivity of global
ecosystems
Fig. 3.2
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Water Stress
Fig. 3.1
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Rice (Oryza sativa L.) is the staple food for
more than two-third of the world's population
(Dowling et al, 1998). About 7.5 of total rice
production comes from irrigated lowland
production (Bouman and Tung 2001). Drought
stress is a major constraint for about 50 of the
world production area of rice.
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The timing of water stress is very important.
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Drought stress and consequences for natural
vegetation
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Dealing with water stress

• Three general ecological strategies
• Postponement of desiccation
• Ability to prevent desiccation despite reduced
  water availability.
• 2. Tolerance of desiccation
• Ability to maintain function while dehydrated
• 3. Drought escape
• Complete life cycle before the onset of drought.

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• Effects of water stress that reduce growth
• Reduction in cell and leaf expansion
• Reduction in photosynthesis, due first to
• decreased stomatal conductance, then to
  inhibition of chloroplast metabolism.
• 3. Altered allocation - greater investment in
  non-
• photosynthetic tissues such as roots
  mycorrhizae

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Fig. 3.12
Responses to deal with stress
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Leaf expansion is very sensitive to water deficit.
Fig. 25.4
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Why is leaf expansion so sensitive to drought?
YW YS YP
Leaf expansion is slowed by water stress because
turgor pressure declines.
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Acclimation to drought stress
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Additional strategies for adapting leaf area to
drought Loss of leaves Wilting Morphology -
Vertical leaves Reduction of radiation load
results in less evaporative demand
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A very important drought response stomatal
closure Advantage less loss of
water Disadvantage less transport of
CO2. Mechanism 1- loss of water from stomatal
cells, turgor drops, stoma closes 2- cell
actively decrease solute concentration YW YS
YP
Solute potential rises (less negative), turgor
drops, stoma closes Long-distance action via
hormones Abscisic acid (ABA) Split-root
experiment
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Effects of drought on photosynthesis are
generally minor 1- early effect mostly via
stomatal closure 2- late effect metabolic
breakdown
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Phloem translocation seems to be less sensitive
to water stress than photosynthesis.
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• Water uptake from the soil happens when soil
  potential is higher than plant water potential
• Osmotic adjustment helps plants cope with water
  stress.
• YW YS YP
• A decrease in YS helps maintain turgor, YP, even
  as total
• water potential decreases.
• Osmotic adjustment is a net increase in solute
  content
• per cell.
• Many solutes contribute to osmotic adjustment.
• K, sugars, organic acids, amino acids
• Osmotic adjustment may occur over a period days.

Costs of osmotic adjustment synthesis of organic
solutes, maintenance of solute gradients, and
opportunity costs, energy the could be used
for other functions
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• Responses to water stress
• Osmotic adjustment
• Stomatal closure
• hydropassive - guard cell dehydration
• hydroactive - guard cell metabolism ABA,
  solutes, etc.
• Leaf abscision and reduced leaf growth
• reduces surface area for water loss
• Smaller leaves lose more heat via convective heat
  loss
• Increased root growth
• with reduced leaf expansion, more C translocated
  to roots
• increases water supply
• Increased wax deposition on leaf surface
• reduces cuticular transpiration, increases
  reflection

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Also many responses at the cellular
level Proteins increase and decrease in
response to water stress One special group of
proteins LEA-proteins (late embryogenesis
abundant) Accumulate in dehydrating leaves, and
during seed ripening Function protection of
membranes (hydrophylic proteins)
prevention of random crystallization of proteins
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Table 25.3
2. Heat Stress And Thermotolerance
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Photosynthesis declines before respiration
Ion leakage is a sign of membrane damage due to
high temps. (or freezing.)
Fig. 25.10
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• What happens when plant tissues reach harmful
  temperatures?
• Membranes lose function because they become too
  fluid.
• Soluble proteins may denature, degrading function
• Membrane-bound proteins may become dysfunctional
  because of denaturation or excessive membrane
  fluidity.
• These effects can be seen in the changes in
  photosynthesis, respiration, and ion leakage of
  membranes.

Fig. 1.5
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• Adaptive or acclimation responses to high
  temperatures
• Vertical leaf orientation
• Leaf pubescence
• Altered membrane fatty acids
• more saturated fatty acids that dont melt as
  readily
• 4. Production of heat shock proteins (HSPs) in
  response
• to rapid heat stress
• molecular chaperones, increase enzymes
  resistance to denaturation help maintain proper
  protein folding
• 5. Increased synthesis of gamma-aminobutyric acid
  (GABA)