# Temperature basics and heat stress

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```					                                                                                 Basic concepts
Temperature basics                                            • Temperature is, at some level, a measure of the kinetic
energy of molecules and atoms that make up matter

and heat stress                                             • At “absolute 0” (0 Kelvin, -270 ºC, -473 ºF), random
vibratory movement of atoms and molecules come to a
complete halt

• As the temperature increases, the energy level of the
CSES 4344/5344                                   atoms and molecules become greater and they vibrate
or “wiggle” more rapidly
April 8, 2010
• The temperature reaches about 10,000 ºC, there is too
much energy for the atoms to remain together as atom
and normal matter; they disperse into a plasma – the 4th
state of matter

Temperature effects within the
Basic concepts (cont.)
“life range”
• Living matter can exist only over a very small range (0-
50 ºC)                                                        • As temperature increases, the enzymes and substrates
bump together more frequently; and the likelihood of the
collision being one will be increased
• For most crops the upper limit is 45-50 ºC
• Q10: general relationship between a reaction rate and the
• Life does not proceed much below 0 ºC because ice               temperature:
forms and/or things are vibrating too slowing to allow          Q10 = (rate at temperature x + 10 ºC)/rate at temperature x
enzyme reactions to take place at sufficient pace
• For typical non-enzymatic reactions: Q10 = 1.1 to 1.2 (rates
increase 10 to 20 % for each 10 ºC increase in
• As temperatures approach 50 ºC, the molecules are               temperature)
vibrating so rapidly that they can be torn apart or
denatured
• For enzyme-catalyzed reactions Q10 2.0

Temperature effects within the
Growing degree days (GDD)
“life range” (cont.)
• GDD is calculated by determining the average
• If Q10 = 2, the living process takes place twice as fast at       temperature for the day ([high+low]/2) and then taking
15 ºC as at 5 ºC                                                  into account the plant’s growth range

• For example, photosynthesis light reaction has a Q10            • The basal temperature for a crop is the lowest
about 1 and the dark reaction has a Q10 about 2-3,                temperature at which it would be expected to make
Calvin cycle takes place 1-2 times fast at 25 ºC as at 15         much growth. That is not automatically 0 ºC (32 ºF)
ºC
• Basal temperature: wheat = 0 ºC and corn = 10 ºC; if the
average air temp.= 12 ºC, wheat gets 12 GDD, corn gets
2 GDD
Growing degree days (GDD)
Global warming
(cont.)
• The global mean temperature
• Seasonal temperature changes and the GDD
accumulation are related with crop planting date in wheat     increased by 0.6 ºC between 1990 to 2000
development
• It is projected to increase another 1.4 to 5 ºC by 2100
• Early planting, wheat grows longer and faster (GDD
accumulate faster)
• It is estimated up to a 17% decreases in crop yield for
each 1 ºC increase in average growing season
• Wheat will switchover to reproductive growth quickly          temperature (Rao et al. 2006)
when the weather begins to warm up in the spring (mid
April)
• Crops suffer the ups and downs of temperature of their
• Late planting will reduce the number of tillers, LAI, and     environment
grain yield

The thermal environment of
Plant heat dissipation
plants and soils
• In a well-watered plant, three
• Leaf                                                          major processes can act to
–   Time of day                                              dissipate heat from leaves
–   Month of year                                             – Re-radiation (long
–   Cloudiness, wind velocity, etc.                             wavelength)
–   Position of canopy (“sun” vs. “shade” leaf)               – Convection of heat
–   Height above the soil surface                                 • High wind velocity, small
–   Canopy characteristics (shape, surface properties,              leaves
etc.)                                                     – Transpiration
• Soil                                                               • Low air humidity, low leaf
– Depth below the soil surface                                      diffusive resistance –
– Soil properties (moisture content, bulk density, etc.)            open stomata and thin
boundary layer

Temperature responses of                                   Responses of respiration and
C3 and C4 plants                                      photosynthesis to high temperature
in C3 and C4 plants
• Cool-season crops (C3) – grow best/fastest when the
weather is cool (in the spring and fall) but experience     • Pn = Pg – R (dark respiration and photorespiration in C3)
summer stress
• Warm-season crops (C4) – grow the fastest in the mid of     • The respiratory temperature-response curve for C3
the summer                                                    plants will be higher at any temperature and get
increasingly higher relative to C4’s curve because:
- C3 has photorespiration (high Q10)
- Rubisco’s affinity for O2 increases as temperatures
increases

• C4’s gross photosynthesis is higher than C3’s at any
temperature because of Rubisco’s faster, more frequent
mode of carboxylation in C4 relative to C3
Response of respiration and                                Effects of leaf temperature on rate of net
photosynthesis to high temperature                           photosynthesis in C3 and C4 crop species
in C3 and C4 plants (cont.)
Wheat
• Photosynthesis has a lower Q10 (relative to respiration),                                                            Maize
so it is not as responsive to temperature as respiration
because:

- The light reactions are non-enzymatic (Q10 ~1)
- Although the Calvin cycle (Q10 = 2) can double the
pace with a 10 ºC increase, but the Z-scheme (for
making NADPH and ATP to power the cycle) cannot

• At high temperatures, C3 plants experience a “metabolic
imbalance” – where respiratory processes take place
faster than photosynthetic process, while C4 plants have
a favorable “metabolic balance”
Basra, 2001

Three cardinal temperatures
(Topt, Tmin, Tmax)
Temperature compensation point
• Temperature at which the rate of respiration
equals the rate of gross photosynthesis (Pn = 0)
(Fitter and Hay, 2002)

• In most temperate crop species, Pg ceases at
temperatures just below 0 ºC and above 40 ºC,
with the highest rates being achieved at 20-35
ºC

• Rates of respiration tend to be low below 20 ºC,
but rise sharply up to the compensation point

Fitter and Hay, 2002

Effects of heat stress on
Effects of heat stress on                                    photosynthesis and respiration
photosynthesis and respiration                                              (cont.)
• Heat directly damage photosynthetic apparatus, with          • For wheat and most C3 crops, net photosynthesis is
PSII being the most sensitive                                  stable in the range 15 to 30 ºC, and above and below
this plateau region, the rate of net photosynthesis
declines by 5-10% per ºC
– Rubisco deactivation
– Membrane damage (increasing permeability and              • At 15 to 30 ºC, the gross rate of CO2 fixation actually
uncoupling electron transport in thylakoids)                increases with temperature
– Electron transport chain damage
– Stomatal closure                                          • The increased rates of CO2 fixation are counterbalanced
by increased rates of whole- plant dark respiration and
photorespiration, in particular
Effects of heat stress on                                      Effects of heat stress on
photosynthesis and respiration                                 photosynthesis and respiration
(cont.)                                                        (cont.)
• As temperatures rise from 30 to 40 ºC (>                       • C4 plants exhibits higher temperature optima for
temperature compensation point), the increase                    net photosynthesis than C3 plants because of
in the rate of respiration becomes greater than                  more efficient CO2 fixation and less CO2 loss via
the increases in CO2 fixation rate                               respiration at elevated temperatures

• High temperatures reduce Pn (Pn < 0) because                   • When temperatures > 40 ºC, photosynthetic
activity of both C3 and C4 declined rapidly and
the temperature optimum for respiration is higher                PSII deactivation occurs, the damage is
than that for CO2 fixation                                       generally “irreversible”

Heat stress effects on seedling                                  Effects of heat stress on crop
establishment                                                    development
• Heat stress may reduce germination rate and seedling           • Rate of development increases curvilinearly with
growth                                                           temperature, so the duration of each
developmental phase declines as temperature
• Seedling growth is particularly sensitive to heat stress as      rises
both roots and shoot apex are located in the soil
• Elevated temperatures may reduce crop yields
• For most crops, seedling growth continues as a
diminished rate until temperatures reach about 45 ºC, at         because heat stress may result in:
which point growth stops                                          – Fewer organs
– Smaller organs
• If heat stress remains long enough, cell injury and death         – Reduced total light interception
may occur

Effects of heat stress on
reproductive development
• Pollen viability is particularly sensitive to heat stress
(wheat < 30 ºC; rice < 34 ºC; common bean pollen
viability reduced by 50% at 37/27 ºC (d/n), Prasad et al.,
2002)

• Stigma and style have a higher temperature threshold
than pollens

• Ovules: heat stress may interrupt hormonal signal which
guides the tubes of germinated pollen toward ovules

• Heat stress reduces duration of grain filling, rate of grain
growth, and kernel mass at moderately high temperature
Basra, 2001
The effect of duration of very high
temperature stress on individual kernel
mass of wheat
Heat-tolerant cultivar.
Heat-sensitive cultivar.
Heat stress (40 ºC, for 6
hrs) was centered around
20 days after anthesis, to
reduce the confounding
effects of timing of heat
stress, and the response
compared with controls
maintained at 21/16 ºC)

Stone and Nicolas, 1998

Effects of heat stress on                          The effect of timing of very high
reproductive development (cont.)                    temperature stress on individual kernel
• Grains are particularly sensitive to very high
mass of wheat
temperature (33-40 ºC)                                                                      Heat-tolerant cultivar.
Heat-sensitive cultivar.
– Exposure of wheat to temperatures of 35/30                                             Heat stress (40 ºC, for 6
ºC or 35/25 ºC for 4 days during grain filling                                         hrs) was applied for 5
days, and the response
can reduce individual kernel mass by over                                              compared with controls
20% (Stone and Nicolas, 1994)                                                          maintained at 21/16 ºC)

• The time and duration of heat stress affect
kernel mass
Stone and Nicolas, 1995

Effect of heat stress on crop                           Mechanisms of heat injury
quality                                 • Starvation
• Grain quality: the complex balance of grain            – If temperature rises above the compensation point,
constituents                                             the plant’s reserves will begin to be depleted
– The deficit increases particularly rapidly in plants with
• Heat stress may impact certain proteins and              active photorespiration (C3 plants)
reduce dough properties in wheat
• Protein breakdown/denaturalization:
Chowdhury and Wardlaw,
– The earliest explanation of heat injury (Belehradek,
1978;
Sofield et al., 1978;         1935) and is the commonly accepted one to this day
tone et al., 1996.      • Cell membrane damage
– Increased permeability causes ion efflux
• Other metabolic damages (ROS, NH3, etc.)
Un-saturation level in cell membrane
Crop adaptation to heat stress                                lipids may decrease during heat stress
• As temperatures
• Alteration in membrane properties
increases, fluidity of
– Membranes have to be strong enough to divide cells
cell membranes may
from their surroundings and into compartments, and          be too high
also fluid enough to allow proteins to move
– Membrane lipids may increases degree of saturation        • Membranes
containing more
• Heat shock proteins/thermal proteins                           saturated lipids have
– Heat shock proteins – proteins that are not expressed       less fluidity and better
or not very prominent in the plants grown in moderate       tolerance to heat
temperature but which become prominent when the
plants are subjected to heat shock
stress                             Unsaturated lipids   Saturated lipids
More fluidity       Less fluidity

Heat resistance – heat avoidance                                 Heat resistance – heat tolerance
• Insulation – mature tree seedlings with a good protective    • Membrane alteration (reduced degree of unsaturation in
layer of bark are more heat resistant than immature
seedlings with thin bark                                       the lipids)
- Transgenic tobacco plants with increased lipid
• Decreased respiration, reduced radiant energy                  saturation had better heat tolerance (Rao et al., 2006)
absorption
• Heat shock proteins/thermal proteins
• Reflectance of radiant energy by leaves

• Transmissivity- pale green leaf > dark green leaf            • Hormones (cytokinins, etc.) and solutes (sugars, glycine
betaine, etc.)
• Absorption by protective layer – require an external layer
of cells with higher heat tolerance
• Antioxidants and violaxanthin/zeaxanthin (scavenge
ROS and dissipate heat)
• Transpiration cooling

Some strategies/practices for                                   Cytokinin (trans-zeatin riboside) improved
improving heat stress resistance                                 creeping bentgrass tolerance to heat stress

• Select genotypes that mature or pass critical
stages of growth or development before the onset
of heat stress (heat avoidance) and heat tolerant
genotypes

• Crop management practice

– Irrigation (for transpiration cooling)
Control     0.1 M      1.0 M     10 M        100 M    1000 M

– Plant growth regulators/hormones                          The treatments were applied to foliage every 2 weeks (4 applications)
and the treated creeping bentgrass was subjected to heat stress
(35/25 ºC) 8 weeks.
Summary                                                Summary (cont.)
• Crops can only grow in certain temperature ranges (45-     • Starvation, protein denaturalization, membrane damage,
50 C for most crops)                                         and metabolic changes are associated heat injury to
crops
• GDD accumulation is related with crop selection and
planting dates                                             • Heat resistance (heat avoidance and heat tolerance) is
associated with membrane lipid saturation, protein, and
• Seasonal changes in growth patterns of C3 and C4             metabolic changes
crops are related to the responses of photosytnhesis and
respiration to temperatures
• Cultivar/genotype selection and some crop management
practices may alleviate heat injury (to certain degree)
• High stress reduces crop growth, development, grain
formation and yield

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