This document provides information about heat stress in wheat crops. It discusses how heat stress reduces wheat yields and quality by affecting grain filling. Several traits for measuring heat tolerance are described, including canopy temperature depression, membrane thermo-stability, chlorophyll content, and stay green ability. Breeding approaches to develop heat tolerant wheat varieties are summarized, such as selection, mutation breeding, and hybridization. Research on heat tolerant wheat varieties developed in India is also mentioned.

1. RAJASTHAN COLLEGE OFAGRICULTURE
UDAIPUR (RAJASTHAN)
Department of Plant Breeding and Genetics
Dev Hingra
Ph. D Scholar
Mail Id-
mail2devhingra@gmail.com

2. INTRODUCTION
 Wheat (Triticum aestivum L.) is one of the world’s
leading cereal crop.
 In India wheat is the Second most important winter
cereal after rice.
 Bread wheat contributes approximately 95% to total
production. Remaining 4% from durum wheat and 1%
from Dicoccum.
 India is the second largest producer of wheat in the world and
the area and production during year 2013-14 recorded was
Area (m. ha.) Production m. tonnes
India 31.34 95.91
Rajasthan 2.81 8.92
(Anonymous, 2014a).

3.  Wheat is a thermo sensitive crop mostly grown in
temperate environment. However, it is predominantly
consumed in tropical and subtropical regions of the
world. In subtropical regions it is cultivated in winter
season but it exposed to high temperature stress at the
end of the season i.e at grain filling stage. Heat stress is
one of the major limiting factors for growth and
productivity in wheat crop particularly in warmer region.
Exposure to higher than normal temperature or heat
stress reduces yield and decreases quality. Hence, now
breeding for heat tolerance has become an integral
component of wheat improvement at both National and
International level.

4.  Temperature is basic to life processes, which increase with
temperature within a limited range. This effect is expressed as
Q10, which is the ratio of the rate at one temperature to that at a
temperature 10°C lower.
 When the temperature rises beyond the upper limit of the range,
i.e., it goes above the optimal temperature, the relation between
life processes and temperature is disturbed. Similarly, when the
temperature goes below a threshold, which is often close to zero,
life processes are disturbed enough to cause injury and death in
sensitive genotypes.
Heat stress

5. •Each plant species, more particularly each genotype, has an
optimum range of temperatures for its normal growth and
development, the specific temperatures would depend not only on
the genotype but also on the stage of growth and development of
a given genotype. When temperature moves beyond this optimal
range, it generates temperature stress.

6. Heat-stress threshold
It is a value of daily mean temperature at which a detectable
reduction in growth begins / the temperature at which growth and
development of plant cease.
• Upper threshold: is the temperature above which growth and
development cease.
• Lower threshold (base temperature): is the temperature below
which plant growth and development stop.
• Cool season and temperate crops often have lower threshold
temperature values compared to tropical crops.

7. Crop plants Threshold
temperature (0C)
Growth stage References
Wheat 26 Post -anthesis Stone and Nicolas (1994)
Corn 38 Grain filling Thompson (1986)
Cotton 45 Reproductive Rehman et al., (2004)
Pearl millet 35 Seedling Ashraf and Hafeez (2004)
Tomato 30 Emergence Camejo et al., (2005)
Brassica 29 Flowering Morrison Stewart (2002)
Cool season pluses 25 Flowering Siddique et al., (1999)
Ground nut 34 Pollen production Vara Prasad et al., (2000)
Cow Pea 41 Flowering Patel and Hall (1990)
Rice 34 Grain yield Morita et al., (2004)
Threshold high temperatures for some crops plants

8. Research findings of ICAR ( Indian Council for Agricultural Research) on wheat crop
has indicated that there is about 3 to 4 per cent decrease in grain yield with 1 degree
Celsius rise in temperature during grain filling stage.
Out of 28 million hectare area under wheat in India, about 9 million hectare in North
Eastern plain zone, Central zone and peninsular zone is prone to terminal heat stress.
Heat stress due to increased temperature is a very important problem globally

9.  The adverse effects on plants of temperatures higher than the optimal is
considered as heat stress. Heat stress is an increased temperature level sufficient
to cause irreversible damage to plant growth and development.
 Occasional or prolonged high temperatures cause different Morpho-anatomical,
physiological and biochemical changes in plants.
 The ultimate effect is on plant growth as well as development and reduced yield
and quality.
 Generally a temperature rise, above usually 10 to 15°C above ambient, can be
considered heat shock or heat stress.
 Breeding for heat stress tolerance can be mitigated by breeding plant varieties that
have improved levels of thermo-tolerance using different conventional or
advanced genetic tools.

10. Heat would effect
on
Survival
Cell and
tissue
survival
Physiological effects
Respiration
Photosynthesis
Photosynthate
translocation
Protein denaturation
Membrane
composition and
stability
Heat shock proteins
Growth and
devlopment
Singh B.D.2014
Physiological Effects

11.  HSP is a group of proteins that normally exist in cells, but
their synthesis is accelerated by heat.
 Most of the stress proteins are soluble in water and
therefore contribute to stress tolerance presumably via
hydration of cellular structures.
 In higher plants, HSPs is usually induced under heat shock
at any stage of development.
Heat shock proteins
Proteinclass Size(kDa) Location
HSP100 100-114 cytoplasm
HSP90 80-94 cytoplasm,ER
HSP70 69-71 ER,cytoplasm,mitochondria
HSP60 10-60 chloroplasts,mitochondria
smHSP 15-30 cytoplasm,chloroplast,ER,mitochondria
Momamad and Whabi 2011

12. PLANT RESPONSES TO HEAT STRESS
shoot and root growth inhibition
scorching of leaves and twigs
 sunburns on leaves branches and stems
Leaf senescence and abscission
fruit discoloration and damage and reduced yield
• Reproductive phases most sensitive to high temperature are gametogenesis (8–9 days before
anthesis) and fertilization (1–3 days after anthesis) in various crop plants (Foolad, 2005).
• In wheat, both grain weight and grain number appeared to be sensitive to heat stress, as the
number of grains per ear at maturity declined with increasing temperature (Ferris et al.,
1998).
Morphological symptoms

13.  Anatomical changes : At the whole plant level
 Reduced cell size
 Closure of stomata and curtailed water loss
 Increased stomatal and trichomatous density
 Greater xylem vessels of both root and shoot
 Damaged the mesophyll cells and increased
permeability of plasma membrane
 High temperatures reduced photosynthesis by changing
the structural organization of thylakoids
 Loss of grana stacking or its swelling
The cumulative effects of all these changes under high temperature stress may
result in poor plant growth and productivity.
Karim et al., 1997).

14. Measurement of heat tolerance parameters
 Different physiological mechanisms may contribute to heat tolerance in the field—
for example, heat tolerant metabolism as indicated by higher photosynthetic rates,
stay-green, and membrane thermo-stability, or heat avoidance as indicated by
canopy temperature depression. Several physiological and morphological traits
have been evaluated for heat tolerance - Canopy temperature, leaf chlorophyll, stay
green, leaf conductance, spike number, biomass, and flowering date.
 Canopy temperature depression (CTD): There is a clear association of CTD
with yield in both warm and temperature environments. CTD has been used as
seletion criteria for tolerance to high temperature in wheat ( Munjal and Rana,
2001) reported that lower canopy temperature during grain filling period is an
important physiological principle for high temperature stress tolerance. Indicating
that the trait is heritable and therefore amenable to early generation selection.
.

15.  CTD measurements were made by infrared thermometer
which was focused to 10:1 meter and at the morning to early
afternoon cloudless periods. Measurements were taken at three
different growth stages ( pre anthesis, anthesis and post
anthesis)
Canopy temperature depression = air temperature – canopy
temperature.

16.  Membrane thermo-stability:
 Heat tolerance usually improves membrane stability under heat
stress. Membrane stability may be determined as
 (1) lipid fluidity. Lipid fluidity reaches at a temperature equal to
100c higher in heat-hardened plant then in unhardened plant.
 (2) Electrolyte leakage. It is measured by conductometrically from
leaf tissue segments taken from properly heat hardened plants
subjected to a period of heat exposure.

17.  Chlorophyll content and stay green
Chlorophyll content and stay green traits have been
found to be associated with heat stress tolerance.
 Photosynthesis: Declined photosynthesis is suggested as
measure of heat stress sensitivity in plants.
 Stem reserve remobilization : In cereals , this seems to
be an important component of grain yield under heat
stress.
 Osmoregulation: osmoregulators like proline and
glycine-betaine may have a protective role in heat stress.

18.  Stomatal conductance: A porometer was used to measure
stomatal conductance across the upper and lower side of the leaf. It
works by creating a seal on the leaf surface and actively pumping the
air and moisture from the chamber. By doing so it is able to calculate a
conductance rate across the leaf. The SPAD chlorophyll meter is also
shown above and calculates an indexed value of chlorophyll content
when a leaf is inserted between the sensors.
Figure. Left- The model of Porometer used to calculate stomatal conductance.
Right- A SPAD 502 chlorophyll meter used to measure leaf chlorophyll content.

19. Breeding approaches for heat tolerance
 Introduction
 Selection (ex. IC 29007A, IC 321889)
 Mutation-
 Hybridization
Interspacific e.g.- C-306, Raj-3765, PBW-343, Raj-4037,HD-2935
 Genetic engineering
Garg et al 2012

20. Mechanisms
 Heat stress resistance may be defined as ability of some genotypes to perform
better than others when they are subjected to the same level of heat stress.
 The various mechanisms of heat resistance may be grouped into two categories :
(1) heat avoidance (2) heat tolerance
 Heat avoidance : The ability of a genotype to dissipate the radiation energy and
there by , to avoid a rise in plant temperature to a stress level. The primary
mechanism of energy dissipation is transpiration.
 Heat tolerance: Ability of some genotypes to withstand / perform better than
others when their internal temperatures are comparable and in the realm of heat
stress.

21. Selection environment for heat tolerance
 Selection for heat tolerance can be carried out under the following four types of
environment
1. Normal field environment : The natural field environment is the simplest and
cheapest to use but its effectiveness depends mainly on the repeatability of the heat
stress profile over years, and on the nature of heat tolerance being selected for it is
unsuitable for the selection of such traits , which require a critical temperature. when
the critical temperature is reached in the field, all the plants at the specific
development stage, i.e., antheis may be marked by, say, a paint spray. Only the
marked plants are evaluated at maturity, and the remaining population is regarded as
escapes.

22. 2.Abnormal field environments :
When normal field environment does not provide the
suitable heat stress conditions, abnormal field
environments available at certain locations or during
the off- season may be used. e.g., wheat is grown
during summer, which is the off- season.

23. 3.Programmed environments : such environments are available
either in growth chambers or in green houses. The temperature
programme should be such that the plants are subjected to
appropriate heat hardening before their heat tolerance is evaluated.
4. In vitro environments : certain assays for heat tolerance can be
performed in test tubes, e.g., membrane thermostabilty by the
electro conductivity method.

24. SELECTION
CRITERIA
FOR HEAT
RESISTANCE
Yield under
heat stress
Growth under
heat stress
Recovery after
heat stress
Sensitivity of the
photosynthetic
process
Flower formation
and pollen fertility
Seed
germination
under heat
stress

25. Characteristic Measured As Usefulness As Selection Criterion
Germination Percent germination under
stress
Useful when crop faces heat stress at
germination
Growth during heat
stress
Yield , biomass Most commonly used selection criterion
Membrane stability Solute leakage
(con-ductivity)
Reasonable correlation ( r= .07) with yield
under heat stress or with drought resistance
Photosynthesis
sensivity
Chlorophyll fluorescence at
685 nm
Becoming increasingly important : difficult to
assay and , especially , interpret
Recovery after heat
stress
Yield , biomass etc. Used whenever relevant for the target
environment
Sensitivity of
reproductive phase
Pollen fertility , grain filling Useful selection criterion ; accounted for in
selection based on yield under heat stress
Different selection criteria for heat tolerance
DWR( 2009-10)

26.  Heat stress at late growth stages is a problem in 40% of wheat areas in the temperate
environnent (Reynolds et al., 2001).
 Grain weight is affected by high temperatures, especially those above 34 ºC, that reduce the
duration of grain filling owing to the limited photosynthesis (Al Khatip and Paulsen, 1984),
and inhibit starch biosynthesis in the endosperm (Keeling et al., 1993; Jenner, 1994).
 Heat stress during post-anthesis (grain-filling stage) affects availability and translocation of
photosynthates to the developing kernels and starch synthesis and deposition within the
kernel, thus resulting in lower grain weight and altered grain quality (Bhullar and Jenner,
1985, Mohammadi et al., 2004). It has been observed that each degree rise in ambient
temperature reduces the yield by 3-4% (Mishra, 2007).
 The period from onset of spike initiation to flowering in wheat is very sensitive to
temperature, and acceleration of this phase seems to be the main cause of yield reduction
under hot conditions.
Research findings

27. Work done in heat stress breeding
 Wheat Variety Golden Halna (K0424 ) is a terminal heat tolerant variety
developed by Wheat breeder Dr. L.P. Tiwari and others CSAUA&T, Kanpur.
 It has tolerance up to 36 °C with yield potential of 40-45 quintal / ha. It
matures in 90-110 days. The grain colour of this variety is golden bright and
contains 12 to 12.5 % proteins. Good for Chapati making, it is found suitable
for central and western part of U.P.( ICAR , 2012).
 Heat tolerant varieties like DBW 14, DBW 16, Raj 3765, Lok 1, GW 322,
Parbhani - 51, PBW – 373 are popular in India.
DBW 16

28.  Attempts have been made to induce heat tolerance in a range of plant species using
different approaches.
-preconditioning of plants to heat stress
-exogenous applications of osmo protectants or plant growth-
regulating compounds on seeds or whole plants.
 Physiological mechanisms of heat tolerance are relatively well understood, further
studies are essential to determine.
-physiological basis of assimilate partitioning from source to
sink, plant phenotypic flexibility which leads to heat tolerance,
-factors that modulate plant heat-stress response.
-an understanding of root responses to heat stress,
-most likely involving root–shoot signaling, is crucial and
warrants further exploration.
future prospects

29.  Molecular knowledge of response and tolerance mechanisms will give the
way for engineering plants that can tolerate heat stress and could be the
basis for production of crops which can produce economic yield under
heat-stress conditions.