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Stress Physiology Chapter 25. Abiotic stress: Water availability (drought, flooding) Temperature (hot, cold) Salinity O 2 concentration Nutrient limitation (N, P, micro nutrients)

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Stress Physiology

Chapter 25

Abiotic stress: Water availability (drought, flooding)

Temperature (hot, cold)


O2 concentration

Nutrient limitation (N, P, micro nutrients)

Pollution (air, soil)

Radiation (high, low)


Biotic: Herbivory

Disease (fungi, bacteria, virus)



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.

Disadvantageous = reduced growth & reproduction

(sometimes also reduced process rates, e.g. photosynthesis)

Growth after 1 month

High T Low T


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 thanBLUE


Acclimation - an increase in stress tolerance of an

individual organism following exposure to stress.

Growth after 1 month

Adequate Water

moisture limitation


no previous exposure to drought: no stress tolerance


previous exposure to drought: increased stress tolerance


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 thanBLUE



Stress tolerance



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.

Water Stress

Fig. 3.1


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.


The timing of water stress

is very important.


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.


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


Fig. 3.12


to deal

with stress


Why is leaf expansion so sensitive to drought?

YW = YS + YP

Leaf expansion is slowed by water stress because turgor pressure declines.


Additional strategies for adapting leaf area to drought

Loss of leaves


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.


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.

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


Table 25.3

2. Heat Stress





declines before


Ion leakage is

a sign of


damage due

to high temps.

(or freezing.)

Fig. 25.10


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

Vertical leaf orientation

Leaf pubescence

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)