Transcript Bio426Lecture33Apr26 - NAU jan.ucc.nau.edu web server

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
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
Stress - a disadvantageous influence on the plant
exerted by an external factor.
Growth after 1 month
Disadvantageous = reduced growth & reproduction
(sometimes also reduced process rates, e.g.
photosynthesis)
High T
Low T
Stress tolerance - the ability to maintain functioning
when exposed to a wide range of conditions.
Growth after 1 month
Usually a relative term based on comparisons among
species or genotypes of their responses to different
levels of some factor (temp., moisture, etc.).
High T
Low T
RED has a greater stress tolerance than BLUE
Growth after 1 month
Acclimation - an increase in stress tolerance of an
individual organism following exposure to stress.
Adequate
moisture
Water
limitation
RED:
no previous exposure to drought: no stress tolerance
BLUE:
previous exposure to drought: increased stress tolerance
Growth after 1 month
Adaptation - a genetically-determined increase in
stress tolerance as a result of selection over
generations.
High T
Low T
RED has a greater stress tolerance than BLUE
Stress
Stress tolerance
Acclimation
Adaptation
Older literature
Stress avoidance:
for example: early seed-set to avoid drought
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.
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.
Precipitation and productivity of global ecosystems
Fig. 3.2
Water Stress
Fig. 3.1
Rice (Oryza sativa L.) is the staple food for more than twothird 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.
The timing of water stress
is very important.
Drought
stress and
consequences
for natural
vegetation
Dealing with water stress
Three general ecological strategies
1. 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.
Effects of water stress that reduce growth
1. Reduction in cell and leaf expansion
2. Reduction in photosynthesis, due first to
decreased stomatal conductance, then to
inhibition of chloroplast metabolism.
3. Altered allocation - greater investment in nonphotosynthetic tissues such as roots &
mycorrhizae
Fig. 3.12
Responses
to deal
with stress
Leaf expansion is very sensitive to water deficit.
Fig. 25.4
Why is leaf expansion so sensitive to drought?
YW = YS + YP
Leaf expansion is slowed by water stress because
turgor pressure declines.
Acclimation to drought stress
Additional strategies for adapting leaf area to drought
Loss of leaves
Wilting
Morphology - Vertical leaves
Reduction of radiation load results in less evaporative demand
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
Effects of drought on photosynthesis are generally minor
1- early effect: mostly via stomatal closure
2- late effect: metabolic breakdown
Phloem translocation seems to be less
sensitive to water stress than photosynthesis.
Water uptake from the soil happens when soil potential is
higher than plant water potential
Osmotic adjustment helps plants cope with water stress.
1. 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
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
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
2. Heat Stress
And
Thermotolerance
Table 25.3
Photosynthesis
declines before
respiration
Fig. 25.10
Ion leakage is
a sign of
membrane
damage due
to high temps.
(or freezing.)
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
Adaptive or acclimation responses to high temperatures
1. Vertical leaf orientation
2. Leaf pubescence
3. Altered membrane fatty acids
more saturated fatty acids that don’t 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)