Plant Response to Temperature

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Plant Response to Temperature

• Chapter 21 in Hopkins, 1999

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Temperature Thresholds
Each species responds to their own critical
temperatures. These threshold temperatures are
called Minimum Optimum Maximum
3
Growing Degree-Days or Thermal Units

• General Formula for Cotton
• (Max Temp Min Temp)/2 - 60ºF
• DD60s
• Caveat If temperature is above the maximum for a
  given species, this simplified formula does not
  correct for it. Also, it does not handle
  negative DD60s.

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Typical Temperature Thresholds for Selected
Crops

• Wheat (0-40ºC) Optimum (23ºC)
• Corn (12-40ºC) Optimum (32ºC)
• Cotton (15-40ºC) Optimum (33ºC)

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What Causes Different Temperature Optimums
Primarily, it is the ability of the plant to
photosynthesize at a selected temperature. Factor
s such as the assemblage of the photosynthetic
apparatus, membrane constituents, sequence and
conformation of growth-related proteins are
likely candidates. For a given plant, its
exposure (acclimation) to cold or warm
temperatures will affect its optimum temperature.
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Vernalization of Winter Annuals and Biennials
In some species, such as winter wheat, seedlings
must be exposed to 1ºC for several days in order
to make the plants sensitive to photoperiod. Once
sensitive, the winter wheat will flower. Some
biennials are day-neutral and need only the cold
to flower.
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What is the Physiological Cause of Vernalization
The answer is not totally clear but the following
appear true 1. The receptor for the stimulus
appears to be in the meristem. Cells may be
changed genetically. 2. A chemical messenger
(e.g., GA) appears to be involved but definitive
experiments identifying the cause are lacking.
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Bud Dormancy - How Do Cold Temperatures Prevent
Bud Growth
1. Like vernalization, the receptor is also in
the apical meristem. 2. Lower respiratory
activity. 3. Lower concentrations of GA, IAA. 4.
Role of ABA is unclear. 5. Sustained temperatures
of 1ºC to 7ºC ultimately break dormancy.
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Plant Stress
Chapter 22 in Hopkins, 1999
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Types of Stress ?
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Types of Stress ?
1. Heat 2. Cold 3. Flood 4. Drought 5.
Salinity 6. UV Radiation 7. Chemical 8.
Biotic from Hopkins, 1999, page 452
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How can plants respond or handle stress?
1. Escape (growth only during good times). 2.
Avoidance (e.g., deep tap root when upper soil is
dry). The stress is not encountered
physiologically. 3. Tolerance (the tissue
actually experiences the stress but is able to
survive, e.g., leaf angle changes). 4. Resistance
(similar to tolerance only that specific traits,
often genetic, are responsible for response). 5.
Acclimation (nonheritable) and Adaptation
(heritable).
Chapter 22 in Hopkins, 1999
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Water Stress Affects
1. Cytosolic Solute Concentration 2. Membrane
and Protein Integrity 3. Stomatal Closure 4.
Root to Shoot ratio 5. Leaf Area
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High Temperature Stress
1. Temperature stress is often associated with
drought. 2. Stomata are likely to be closed
causing leaf temperature to be high. 3. Leaf
orientation, pubescence, and rolling may be
coping mechanisms. 4. Heat-tolerant species tend
to have a higher percentage of saturated fatty
acids in their membranes than cold-tolerant
species. Saturated fatty acids exhibit less
increase in membrane fluidity associated with
heat than unsaturated fatty acids.
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High Temperature Stress
5. The thylakoid membrane and oxygen-evolving
complex is very susceptible to heat. a. Heat
disrupts electron donation to PS II. b. Movement
of proteins within the membrane is reduced under
high temperatures c. Heat damages Photosystem
II resulting in increased chlorophyll
fluorescence. d. Enzymes such as rubisco
activase are compromised by heat.
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Heat Shock Proteins
1. Heat shock proteins are low molecular weight
proteins produced when plants are exposed to heat
stress (10º C above normal). 2. Heat shock
proteins may help to prevent protein degradation,
disassembly, or aggregation. 3. One heat shock
protein, ubiquitin, may help to degrade and
remove heat-damaged proteins.
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Salt Stress
Why cant we just breed crops with marine-adapted
genes for salt-stressed land? I dont
know. Besides salt-marshes, inland deserts and
highly-irrigated agricultural land is often
saline.
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Salt Stress
Some halophytes grow in 500 mM NaCl by excluding
salt from their roots. Others excrete salt on
their leaf surface. Still others sequester salt
in the vacuole. Crop species vary in their
tolerance of salt. Cotton, wheat, and tomato are
relatively tolerant but corn and soybean are not.