Plant Response to Heat Stress

1. ‫هجاد‬‫إ‬‫ل‬‫ا‬ ‫حتت‬ ‫حيوي‬ ‫ياء‬‫مي‬‫ك‬‫بييئ‬‫ل‬‫ا‬
Present by

2. Major
Effect of
High
Temperature
on Plants
Reduction of
plant growth
Yield
reduction
Reduction of
crop quality
Inhibition of
seed
germination
Improper
development
Alteration in
photosynthesis
Alteration in
phenology
Alteration in
dry matter
partitioning
Water loss
Oxidative
stress

3. Plant Response to Heat Stress
• Growth
• Photosynthesis
• Reproductive Development
• Yield
• Oxidative Stress

4. Growth
• Among the growth stages of plant the germination is affected first of
all.
• major impacts caused by heat stress documented in various cultivated
plant species are (Reduced germination percentage, plant emergence,
abnormal seedlings, poor seedling vigor, reduced radicle and plumule
growth of geminated seedlings)
• High temperature causes loss of cell water content for which the cell
size and ultimately the growth is reduced
• Reduction in net assimilation rate (NAR) is also another reason for
reduced relative growth rate (RGR)
Plant Response to Heat Stress
1

5. The morphological symptoms of heat stress include :
• scorching
• sunburns of leaves and twigs, branches and stems,
• leaf senescence and abscission,
• shoot and root growth inhibition,
• fruit discoloration
At extreme heat stress plants can show programmed cell death in specific cells or
tissues may occur within minutes or even seconds due to denaturation or aggregation of
proteins, on the other hand moderately HTs for extended period cause gradual death; both
types of injuries or death can lead to the shedding of leaves, abortion of flower and fruit, or
even death of the entire plant
Growth1
Plant Response to Heat Stress

6. Plant Response to Heat Stress
Photosynthesis2
Photosynthesis is one of the most heat sensitive physiological
processes in plants.
High temperature has a greater influence on the photosynthetic
capacity of plants especially of C3 plants than C4 plants.
In chloroplast, carbon metabolism of the stroma and
photochemical reactions in thylakoid lamellae are considered as
the primary sites of injury at HTs ,Thylakoid membrane is highly
susceptible to HT.

7. Plant Response to Heat Stress
Photosynthesis:2
Effect of HT on
Photosynthesischloroplast
photochemical
reactions
carbon metabolism
of the stroma
loss of grana stacking
swelling of grana
photosystem II (PSII)
activity is greatly reduced
or even stops
reduces the amount of
photosynthetic pigments
the primary sites of injury
Closure of
stomata under HT
is another reason
for impaired
photosynthesis
that affects the
intercellular CO2
Heat markedly
affects the leaf water
status, leaf Stomatal
conductance (gs)
and intercellular CO2
concentration
altered structural organization of
thylakoids
The decline in chl pigment also is a
result of lipid peroxidation of
chloroplast and thylakoid membranes

8. Plant Response to Heat Stress
Photosynthesis2
Some other reasons believed to hamper photosynthesis under heat stress
 reduction of soluble proteins, Rubisco binding proteins (RBP), large-
subunits (LS), and small-subunits (SS) of Rubisco in darkness, and
increases of those in light.
 High temperature also greatly affects starch and sucrose synthesis, by
reduced activity of sucrose phosphate synthase, ADP-glucose
pyrophosphorylase, and invertase.
 Heat imposes negative impacts on leaf of plant like reduced leaf water
potential, reduced leaf area and pre-mature leaf senescence which have
negative impacts on total photosynthesis performance of plant.
 Under prolonged heat stress depletion of carbohydrate reserves and
plant starvation are also observed.

9. Plant Response to Heat Stress
Reproductive Development3
Although all plant tissues are susceptible to heat stress at almost all the
growth and developmental stages, the reproductive tissues are the most
sensitive, and a few degrees elevation in temperature during flowering time
can result in the loss of entire grain crop cycles .
During reproduction, a short period of heat stress can cause significant
decrease in floral buds and flowers abortion although great variations in
sensitivity within and among plant species and variety exists [76]. Even heat
spell at reproductive developmental stages plant may produces no flowers or
flowers may not produce fruit or seed

10. Plant Response to Heat Stress
Reproductive Development3
 Impaired meiosis in both male and female organs.
 Impaired pollen germination and pollen tube growth.
 Reduced ovule viability.
 Anomaly in stigmatic and style positions.
 Reduced number of pollen grains retained by the stigma.
 Disturbed fertilization processes, obstacle in growth of the endosperm,
proembryo and unfertilized embryo
The reasons for increasing sterility under the HT are:

11. Plant Response to Heat Stress
Yield4
Elevated temperatures are raising apprehension regarding crop productivity
and food security
Its affect is so terrible that even a small (1.5°C) increase in temperature have
significant negative effects on crop yields
It was demonstrated that increase of the seasonal average temperature 1°C
decreased the grain yield of cereals by 4.1% to 10.0%
The sensitive crop varieties are more severely affected by heat stress
relative to tolerant varieties.

12. Plant Response to Heat Stress
Yield4
 Loss of productivity in heat stress is chiefly related to decreased assimilatory
capacity which is due to reduced photosynthesis by altered membrane stability
and enhanced maintenance respiration cost, reduction in radiation use efficiency
(RUE, biomass production per unit of light intercepted by the canopy).
An example: Grain quality characteristics in barley significantly changed
under heat stress. In barley grain several proteinogenic amino acids
concentrations and maltose content increased, where the
concentrations of total non-structural carbohydrates, starch, fructose
and raffinose, lipids and aluminum were reduced
 Elevated temperature affects the performance and crop quality
characteristics.

13. Plant Response to Heat Stress
Oxidative Stress5
Production of ROS
 The main source of ROS production in plants and the reaction centers of PSI and
PSII is in chloroplasts, also generated in other organelles viz mitochondria,
plasma membrane, cell wall, apoplast and peroxisomes
 ROS include : O2 , O2
•− , H2O • , H2O2 , OH • , RO• organic hydroperoxide
(ROOH), excited carbonyl (RO • ), etc.
 They cause damage to biomolecules like proteins, lipids, carbohydrates, and
DNA, which ultimately results in cell death

14. Production of ROS

15. ROS

16. Classification of plants on the basis of their heat tolerance
Plant Adaptation to Heat Stress

17. Different adaptation mechanisms of Plant Adaptation to
Heat Stress
The ability of the plant to grow and produce economic yield under HT
Under HT conditions, plants exhibit various mechanisms for surviving
which include long-term evolutionary phonological and morphological
adaptations and short term avoidance or acclimation mechanisms
 AvoidanceMechanisms(A)
 ToleranceMechanisms(T)

18. Avoidance Mechanisms(A)
Transpirational cooling.
Leaf rolling.
Alteration of membrane lipid compositions.
early maturation.
Increased stomatal and trichomatous densities
Changing leaf orientation
Closure of stomata and reduced water loss
larger xylem vessels.

19. Tolerance Mechanisms(T)
Some major tolerance mechanisms, including:
 Ion transporters,
late embryogenesis abundant (LEA) proteins,
 Osmoprotectants,
Antioxidant defense,
 And factors involved in signaling cascades and transcriptional
control are essentially significant to counteract the stress effects

20. Enzymatic Antioxidants
Superoxide Dismutase,
Catalase,
Ascorbate Peroxidase,
 and Glutathione
Reductase
Antioxidative Responses to Environmental Stress
Nonenzymatic Antioxidants
Ascorbic Acid,
 α –Tocopherol,
 and Reduced Glutathione
Tolerance Mechanisms(T)

21. Enzymatic Antioxidants
Glutathione Reductase
 GR is an important enzyme of the
ascorbate–glutathione system
 maintains the balance between
reduced glutathione (GSH) and the
ascorbate pool
AscorbatePeroxidase,
 helps to detoxify H 2 O 2 in the ascorbate
glutathione pathway.
 utilizes ascorbic acid and reduces H 2 O 2
to water and monodehydroascorbate
(MDA).
Catalase
 Catalases decompose hydrogen
peroxide to water and
molecular oxygen without
consuming reductants
Superoxide Dismutase
 plays a key role in cellular
defense mechanisms against ROS
Tolerance Mechanisms(T)

22. Non-enzymatic Antioxidants
Ascorbic Acid
 Ascorbate peroxidase utilizes
ascorbic acid and reduces H 2 O 2
to water, thereby generating
monodehydroascorbate (MDA) in
the ascorbate– glutathione cycle
 ascorbate plays a role in the defense
against ozone
 AsA has the capability of donating
electrons in various enzymatic and
nonenzymatic reactions and is thus
a powerful radical scavenger.
α –Tocopherol
 α -tocopherol scavenges free
radicals in combination with
other antioxidants
 protects the structure and
function of PSII as it chemically
reacts with O 2 in chloroplasts
 helps in membrane stabilization
and alleviates the tolerance of
plants during oxidative stress.
Reduced Glutathione
 protects cells from the toxic
effects of many ROS
 regulation of protein and DNA
synthesis, protein activities, and
maintaining membrane integrity
 oxidation of glutathione (GSSG)
are necessary for controlling H 2
O 2 levels in cells and have an
important role in redox signaling
Tolerance Mechanisms(T)

23. Mechanism of Signal Transduction and Development
Tolerance Mechanisms(T)

24. Tolerance Mechanisms(T)
Heat-Shock Proteins (HSPs)
• High temperature stress causes extensive denaturation and
aggregation of cellular proteins, which, if unchecked, lead to cell
death.
• Through their chaperoning activity, hsps help cells to cope with
heat-induced damage to cellular proteins.
• During stress, hsps function primarily to prevent aggregation and
promote proper refolding of denatured proteins, but because
protein conformation is important right from the time a protein is
synthesized,
• hsps play important roles under normal conditions as well. The
principal role of hsps under non-stress conditions is to assist in the
synthesis, transport, and proper folding of the target proteins.

25. Major families of heat shock proteins
Tolerance Mechanisms(T)
Heat-Shock Proteins (HSPs)
• Hsp100
• Hsp90
• Hsp70
• Small hsps
• The Chaperonins

26. Diagram representing integrated circuit of different “omics” approaches that are connected to each
other at molecular genetic level associated with heat stress tolerance in plants.