Stresses involving water deficit
• Water related stresses could affect
plants if the environment contains
insufficient water to meet basic needs
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Environmental conditions that
can lead to water deficit
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drought
hypersaline conditions
low temperatures
transient loss of turgor at midday
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Factors that can affect the response of
a plant to water deficit
• duration of water deficiency
• and the intensity
• if the plant was acclimated to water stress
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Tolerance to drought and salinity
• Osmotic adjustment :
many drought-tolerant plants can regulate
their solute potentials to compensate for
transient or extended periods of water stress
by a net increase in the number of solutes
particles present in the plant cell
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Water
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Drought - lack of water
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Excess – flood situation creating an anaerobic
environment where roots are unable to respire due
to lack of available oxygen.
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What causes droughts?
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•
Droughts are caused by a
lack of precipitation in an
area resulting from scarce or
less frequent storms than
normal.
•
Most major droughts last for
months or years.
•
What is considered a
drought in a rainy location
may be enough precipitation
for another region.
Weather that causes droughts
• Droughts occur when scarce or less frequent
precipitation occurs than normal.
• When dry air cools and falls, lowering relative
humidity and the level of precipitation,
conditions that can bring droughts.
• During a drought not only is there little
precipitation, but the moisture from the soil
must also leave into the
atmosphere through
evaporation and transpiration.
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When do they occur?
• Droughts can occur any time
of the year.
• In North America, droughts
occur most often between
March and September, when
it is often dry and hot.
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Where do droughts occur?
•
Areas that have frequent and
severe droughts are shown
on the map as the darkest
shaded areas, and less
sever droughts are the lightly
shaded areas.
•
They usually are most
severe around the 30°
latitudes, where dry, cool air
is falling.
•
Asia, many African countries
(I.e. Kenya, Ethiopia,
Somalia), are most often
affected by droughts.
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• Humid areas are
prone to short term
droughts, whereas
arid regions are prone
to long term droughts.
Recent droughts
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Australia has recently been
through drought conditions.
The drought is occurring
around Sydney. Fires have
begun due to the dryness of
the land.
•
Ethiopia is also in a drought
area, and is expecting over
11 million people to be
without food next year due to
crop failures.
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Destruction by droughts
• Droughts cause major damage to crops and animals,
especially if they occur during the region’s growing
season.
• During a drought not enough water is provided to
support living things. Crops decrease strongly during
a drought.
• Droughts affect reservoirs, water supplies, water
tables, and topsoil. Water levels drop, and topsoil
becomes dry and less favourable to plants.
• Areas that are in a drought are more likely to have
wildfires than other regions.
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Drought Stress
• Drought stress includes:
– Soil drought, no rain for long time and noavailable water in the soil.
– Air drought, RH<20% in atmosphere，
transpiration>>water absorption. If longer,
soil drought occurs.
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• Symptoms in plant facing to drought：
– red color in base，small cell and leaf area，
leaf yellowish and abscission. Young
leaves or/and reproductive organs wilt to
death.
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• Mechanisms of drought stress:
– Membrane damage
– Metabolic disorder:
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Water redistribution in the organ
Photosynthesis reduction
Decrease in nuclear acids and proteins
Accumulation of proline
ABA accumulation
– Mechanical damage
– Formation of S-S
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• Strategies of increasing drought resistance
– Stress avoidance:
• absorption of water, decrease of transpiration and
increase in water transportation.
– Developed root system and higher ratio of root to shoot
– Thick leaf, smaller leaf area and thick cuticle
• Accumulation of ABA and stomatal control
– Stress tolerance
• Osmotic adjustment
• Change of photosynthetic pathway
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Plant Water Stress: What is it?
Tissue Water Potential:
Mild: Ψcell ~-0.5 MPa
Moderate: Ψcell ~-0.5 to -1.5 MPa
Severe: Ψcell ~<-1.5 MPa
Relative Water Content:
Mild: ~90%
Moderate: 80-90%
Severe: <80%
Survives Ψcell of -11.5 MPa
Photosynthesizes to -8.0 MPa
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Creosote Bush: Larrea tridentata
What does water stress do to plant cells and plants?
1
Plasmolyzed vacuole
Ψp = 0
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- Loss of turgor
What does water stress do to plant cells and plants?
2. Reduction in Leaf Expansion
Smaller leaves
Less extensive canopies
Less light reception
Less photosynthesis
Stressed orange trees
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What does water stress do to plant cells and plants?
3. Impact on Metabolism
•Photosynthesis ↓
•Respiration ↑
•Solutes ↑
•Change in plant
hormone balance:
ABA (abscisic acid) ↑
White Oak stomates
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Avoidance mechanisms: root systems
Root/Shoot ratio:
Temperate forest: ~0.25
Dry Savanna woodland ~0.3 – 0.4
Prairie & deserts ~0.6-0.9
Root growth is plastic and responds to
Local conditions (water, soil, etc.)
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mycorrhizal fungi extend root systems
Avoidance mechanisms: leaf modification
Water tanks in epiphytic
bromeliads
Blade of grasses “leads” water to base
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Leaf rolling in water stressed corn
Leaf orientation in Eucalypts
Avoidance mechanisms: leaf modifications
SLA (specific leaf area)
Leaf area / dry weight
Deserts (xeric): 0.02 - 0.12
Dry forests: 0.36 - 0.70
Mesic forests: 1.4 -1.6
Lower number means smaller, thicker
more dissected leaves
Dissected leaves in Palo Verde
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Avoidance mechanisms: stomatal regulation
Hydroactive closure – first line
of defense
Regulated by ABA
Hydropassive closure – second
line of defense
Regulated by general loss of
Turgor of guard cells
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Subsidiary cells
Subsidiary cells
Tolerance mechanisms
Creosote Bush
Craterostigma
plantagineum
Polypodium polypodioides
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Selaginella lepidophylla
How do plants that tolerate extreme water
stress survive?
•
What adaptations would these plants have to conserve water?
•
How do these plants prevent damage to their cells during
dehydration-rehydration cycles?
•
How long does it take for a resurrection plant to revive after drying?
Is the length of this time important for survival of the plant?
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What kind of growth rates would these plants show?
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