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 at – 5.0 to -8.0 MPa
Creosote Bush: Larrea tridentata
What does water stress do to plant cells and
plants?
1. Loss of turgor
Plasmolyzed vacuole
Ψp = 0
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
Stressed orange trees
What does water stress do to plant cells and
plants?
3. Impact on Metabolism
•Photosynthesis ↓ Why?
•Respiration ↑ Why?
•Solutes ↑ Impact?
•Change in plant
hormone balance:
ABA (abscisic acid) ↑
Impact?
White Oak stomates (glue impression)
Adaptations to Avoid Water Stress
Problems of water
shortage and maintenance
of turgor are universal
among terrestrial plants.
Why?
Strategies: Avoidance
and Tolerance
California desert after a rain
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.)
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 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
Avoidance mechanisms: osmoregulation
Lower leaf water potential by
synthesizing solutes (amino
acids, sugars, ions, etc.)
What will this do?
Leaf anthocyanins in young leaves of
Corymbia gummifera
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
Subsidiary cells
Subsidiary cells
Avoidance Mechanisms: C4 and CAM Plants
CAM plant - pineapple
C4 grass - sugarcane
Tolerance mechanisms
Creosote Bush
Craterostigma
plantagineum
Polypodium polypodioides
Selaginella lepidophylla
How do plants that tolerate extreme water stress
survive?
• What adaptations would these plants have to conserve
water?
• In what type of habitats would these plants be found?
• 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?
• What kind of growth rates would these plants show?