Transcript water deficit - University of Jordan

Water Stress
Prof. Samih Tamimi
Plant Physiology 751
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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: 80-75%
Severe: <75%
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Water stress is due to water shortage:
• Water stress is induced when transpiration rate (T) is
higher than absorption rate (A)
• High T– low air humidity, high temperature, high
irradiance, strong wind
• Low A – low soil moisture, high concentration of salts,
low soil temperature
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Fig. 3.12
Responses
to deal
with stress
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Decreased Leaf Area
• An early response.
• As water content of the plant falls, cells
shrink and cell walls relax.
• Solutes in the cell become more
concentrated.
• cell membrane may become more
compressed due to lower surface area.
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Turgor and Cell Growth
Cell growth peaks at night when turgor higher.
• Leaf expansion is largely governed by cell
expansion.
•  leaf area transpires less - conserves soil
water.
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Abscisic Acid
• ABA is a common plant response to stress.
• In water-stressed plants  ABA is an early
response to  leaf .
• ABA stimulates stomatal closure.
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Stomatal Closure- the solute loss of guard cells can be triggered by
decreasing water status in the rest of the leaf; probably mediated by 1)
ABA (synthesized in mesophyll) when the mesophyll becomes
dehydrated the plant moves some of the ABA from CP to the
transpiration stream to the guard cells; and 2) a net increase in rate of
production
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Abscisic Acid
• ABA is a common plant response to stress.
• In water-stressed plants  ABA is an early response to
 leaf .
• ABA stimulates stomatal closure.
• Normally, ABA accumulates in mesophyll chloroplasts.
• Accumulation depends on the relative pH of the
stroma and cell cytosol and the weak acidity of ABA.
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ABA Release from Mesophyll Cells
• As leaf cells lose turgor stromal pH falls.
• ABA(H)moves passively into cytosol and apoplast.
• Transpiration stream carries ABA to stomatal guard
cells.
• ABA stimulates stomatal closure.
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ABA mediated stomatal closure mechanisms:
Water deficit → ABA → stomatal closure
ABA → Ca2+↑ → Cl- efflux/membrane potential depolarization → K+ efflux/K+
influx is blocked → ψp decrease/water loss → volume reduction → stomatal
closure
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Effects of Turgor Loss on Leaf
Metabolism
• PS rates are not directly affected by decreased
cell turgor, however:
• a fall in leaf turgor can lead to closure of
stomates and consequent limitation of CO2
concentration within the leaf tissues, leading to
decreased PS rates.
• Other metabolic functions and individual
enzymes show altered activity in response to
mild water stress e.g. protein synthesis, nitrate
reductase activity and protochlorophyll
formation.
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T&Z Figure 25.4
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Effects of Turgor Loss on Root
System Development
• When water uptake drops so does leaf expansion,
reducing consumption of the organic products of PS.
• These assimilates can be distributed to the roots to
support an expansion of growth.
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of growth
ABA promotes root growth and inhibits shoot growth
at low water potentials
Wild-type and ABA-deficient maize mutant seedlings
grown under high and low water conditions
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Phloem translocation seems to be less
sensitive to water stress than photosynthesis.
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Water stress and protein synthesis
1) Inhibition of synthesis of some proteins
2) Stimulation of synthesis of other proteins
3) Synthesis of specific stress proteins
• A) proteins taking part in signal transduction and gene
expression, e.g. transcription factors (MYC, MYB), protein
kinases (MAPK), enzymes of phospholipid metabolism
(phospholipase C, D)
• B) proteins participating in stress tolerance, e.g. membrane
proteins, proteins of water and ion channels, protection
factors (chaperones, LEA proteins), syntases of
osmoprotectants, stress proteins localized in chloroplasts,
specific inhibitors of proteolytic activity, antioxidants,
antioxidative enzymes, proteins taking part in recovery after
stress
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Plant Adaptations – Moisture
Stress
• Water stress escapees
• Water stress avoiders
• Water stress tolerators
”
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2. DROUGHTescapers: Ephemerals (“annuals”)
 Grow only when water is available
 Life span of weeks to months
 Rapid photosynthetic and growth rates
 Cooled via transpiration (can’t tolerate drought)
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Method of saving water
Structures adapted for this purpose
Slowing/reducing transpiration
rate
Waxy cuticle on leaves
Fewer stomata
Sunken stomata & curled leaves which
provide a moist micro-environment
Stomata that close in daytime & open at
night
Fine hairs on the surface of plant to trap
moisture
Small leaves/spines (Cactus) that reduce
surface area for water loss.
Leaf loss during times of dryness
Storing water for times of
storage
Fleshy, succulent leaves w/ flexible surface
Fleshy, thick stems that can store water
(Baobab tree)
Fleshy underground tubers
Increased water uptake
Shallow, widespread roots to absorb
maximum surface water
Very long deep roots to reach underground
water
Some plants have a dual root system,
which
combines
a deep tap root with shallow
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radial roots for maximum water uptake.
Osmotic stress changes gene expression
- Accumulation of solutes due to water stress
- Several genes coding for enzymes associated with osmotic adjustment are upregulated by water stress.
- LEA proteins (LATE EMBRYOGENESIS ABUNDANT) discovered by
examination of naturally desiccating embryos during seed maturation
- play role in cellular membrane protection, although function is
not well understood
- accumulate in vegetative tissue during water stress
- hydrophilic proteins, strongly binding water (protective role may
be associated with ability to retain water and to prevent
protein denaturation during desiccation)
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LEA proteins are regulated by
osmotic
stress
Function: Presumably have a role in membrane protection
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LEA proteins are regulated by
osmotic stress
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Osmotic adjustment – Tolerance to drough
• Osmotic adjustment
• a biochemical mechanism that helps
plants acclimate to dry conditions
• Many drought-tolerant plants can
regulate their solute potentials to
compensate for transient or extended
periods of water stress by making
osmotic adjustments, which results in
a net increase in the number of solute
particles present in the plant cell.
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Osmotic adjustment
ΨP = +0.5 MPa
ΨS = -2.0 MPa
ΨW = -1.5 MPa
ΨP = 0 MPa
ΨS = -1.2 MPa
ΨW = -1.2 MPa
Water
deficit
ΨP = pressure potential
(hydrostatic pressure of
solution)
ΨS = solute (osmotic) potential
ΨW = water potential
Soil ΨW =
-1.2 MPa
Osmotic adjustment
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Osmotic adjustment
• The cell actively accumulates solutes and
as a result the solute potential (s) drops,
promoting the flow of water into the cell.
• Osmotic adjustments are believed to play a
critical role in helping plants acclimate to
drought or saline conditions.
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Osmotic stress changes gene
expression
- Accumulation of solutes due to water deficit or salinity stress causes osmotic stress
- Several genes coding for enzymes associated with osmotic adjustment are upregulated by water stress
- pyrroline-5-carboxylate synthase, a key enzyme in the proline
biosynthetic pathway
- betaine aldehyde dehydrogenase, an enzyme involved in glycine betain
accumulation
- myo-Inositol 6-O-methyltransferase, a rate-limiting enzyme in the
accumulation of the cyclic sugar alcohol pinitol
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Synthesis and degration of proline
L-Glu - L-glutamate, GSA - glutamate semialdehyde, P5C - -pyrroline-5carboxylate, P5CS - P5C syntetase, P5CR - P5C reductase, ProDH - proline
dehydrogenase, L-Pro - proline
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Plant Environment: Water
• Water deficiency in plants
– Slight water stress causes stomates to close;
photosynthesis reduced
• Reduction in growth
• Smaller leaves
• Shorter internodes
• Smaller plants
– Slight water stress can effectively prevent
fast growth
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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
Induction of CAM in facultative CAM plants
•in response to water or osmotic stress
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What does water stress do to plant cells and plants?
Loss of turgor
Plasmolyzed vacuole
Ψp = 0
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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
Internal turgor ‘powers’
cell expansion
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Many plants acclimate to drought
conditions by osmotic adjustment: s
of cell sap is reduced through
accumulation of organic and inorganic
solutes in the cytoplasm and vacuole.
The solutes in the cytosol usually
have low physiological activity.
Examples: sorbitol (a sugar alcohol)
or proline (an amino acid).
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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.
1. W = S + P
A decrease in S helps maintain turgor, P, 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”,
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energy the could be used
for other functions
Main pathway of proline synthesis in higher plants
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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
Leaf pubescence
In oaks
Blade of grasses “leads” water to base
Water tanks in epiphytic
bromeliads
Leaf rolling in water stressed cornPlant Physiology 751
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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: osmoregulation
Lower leaf water potential by
synthesizing solutes (amino
acids, sugars, ions, etc.)
What will this do?
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Avoidance Mechanisms: C4 and CAM Plants
CAM plant - pineapple
C4 grass - sugarcane
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The Transpiration Ratio Measures the Relationship
between Water Loss and Carbon Gain
• Transpiration ratio = the effectiveness of plants in moderating water
loss while allowing sufficient CO2 uptake for photosynthesis
• Transpiration ratio = Amount of water lost by transpiration
Amount of CO2 fixed by photosynthesis
• Also termed Water Use Efficiency (WUE) = inverse of transpiration
ratio (e.g. plant with transpiration ratio of 500 has a WUE of 1/500 =
0.002)
• Generally, H2O efflux is more than CO2 influx; this is due to:
– Concentration gradient driving water loss is ~ 50 times larger
than driving the influx of CO2 (due to low concentration of CO2 in
air) and the relatively higher water vapor in leaf
– CO2 diffuses ~ 1.6 times more slowly through air than water does
(CO2 molecule is larger than H2O and has a smaller diffusion
coefficient)
– CO2 uptake must cross the plasma membrane, cytoplasm,
chloroplast envelope before it is assimilated in the chloroplast →
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these membranes add to
the
resistance
of CO2 diffusion pathway