Stress PhysiologyChapter 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 only22% 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)

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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.).

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RED has a greater stress tolerance than BLUE

Acclimation - an increase in stress tolerance of an individual organism following exposure to stress.

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RED: no previous exposure to drought: no stress toleranceBLUE: previous exposure to drought: increased stress tolerance

Adaptation - a genetically-determined increase in stress tolerance as a result of selection over generations.

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RED has a greater stress tolerance than BLUE

StressStress toleranceAcclimationAdaptation

Older literature

Stress avoidance: for example: early seed-set to avoid drought

Water stress – drought toleranceHeat stress and heat shockChilling and freezingSalinityO2 deficiency

•Much research is directed towards discovering the mechanisms of stress tolerance, acclimation etc.

Water stress – drought toleranceHeat stress and heat shockChilling and freezingSalinityO2 deficiency

•Much research is directed towards discovering the mechanisms of stress tolerance, acclimation etc.

Fig. 3.2

Precipitation and productivity of global ecosystems

Fig. 3.1

Water Stress

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 stressis very important.

Drought stress and consequences for natural vegetation

Dealing with water stress

Three general ecological strategies

1. Postponement of desiccationAbility to prevent desiccation despite reduced water

availability.

2. Tolerance of desiccationAbility to maintain function while dehydrated

3. Drought escapeComplete 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 todecreased stomatal conductance, then to

inhibition of chloroplast metabolism.

3. Altered allocation - greater investment in non-

photosynthetic tissues such as roots & mycorrhizae

Fig. 3.12

Responses to dealwith stress

Fig. 25.4

Leaf expansion is very sensitive to water deficit.

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

Why is leaf expansion so sensitive to drought?

W = S + P

Acclimation to drought stress

Additional strategies for adapting leaf area to drought

Loss of leavesWiltingMorphology - Vertical leaves

Reduction of radiation load results in less evaporative demand

A very important drought response: stomatal closure

Advantage: less loss of waterDisadvantage: less transport of CO2.

Mechanism: 1- loss of water from stomatal cells, turgor drops, stoma closes2- cell actively decrease solute concentration

WWSSPP

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 minor1- early effect: mostly via stomatal closure2- late effect: metabolic breakdown

Phloem translocation seems to be lesssensitive 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. 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”, 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 StressAnd Thermotolerance

Ion leakage isa sign of membranedamage dueto high temps.(or freezing.)

Fig. 25.10

Photosynthesisdeclines beforerespiration

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 orientation2. Leaf pubescence3. 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)