Breeding for heat tolerance in Wheat
Authors: Vikas Gupta, Chandra Nath Mishra and Satish Kumar
Indian Institute of Wheat and Barley Research, Karnal-132001 Haryana

Wheat is the second most important winter cereal after rice. Bread wheat contributes approximately 95% to total production where as the remaining 4 and 1% is contributed from from durum wheat and Dicoccum. Ideal Temperature for wheat crop cultivation is 150C. Worldwide, around 36 mha area is affected by heat stress. UN report suggests earth will be warmer by 2.40 C by 2020 which will reduce crop yield by 50% in India. UN report predicted global wheat production to be reduced by 14% compared to demand. According to Dr R.K. Pachauri, there is possibility that food production of many countries including India would be badly affected by the climate change, particularly wheat, paddy and other crops would be directly affected and these are the primary food of common Indian people. We can only estimate how the climate change would impact upon the livelihood of majority of people. Breeding programmes mainly focussed on improving yield with emphasis on resistance and to some extent on quality traits. Recent advancements in research showed physiological traits contributing to heat stress tolerance in field and can be used for selecting genotypes in breeding programmes.

Wheat under climate change

CIMMYT study has claimed: The Earth will be 2.4 degree Celsius warmer by 2020 if the world continues with the business-as-usual approach to climate change and India would be one of the hardest hit countries witnessing upto 30 per cent reduction in crop yields. The rising temperatures will adversely affect the world's food production and India would be the hardest hit, according to the analysis by the Universal Ecological Fund (FEU-US), the US subsidiary of FEU founded in Argentina in 1990. The New Delhi-based Indian Agricultural Research Institute (IARI) has now warned that the country's annual wheat output could plunge by 4-5 million tonnes with every 1 degree Celsius rise in temperature. These projections cannot and should not be brushed aside as long term issues that have no immediacy. By 2050 about half of India’s prime wheat production area could get heat-stressed, with the cultivation window getting shorter, affecting productivity. For each 10C rise in mean temperature, wheat yield losses in India are likely to be around 7 million tonnes per year, or around $1.5 billion at current prices.


Heat and its impact on phenology of Wheat

1. Heat stress affects growth at different developmental stages

2. Low temperature at initial stages or high temperature at later stages leads to 70% of growth of wheat during anthesis.

3. If temperature is high during initial stages, many genotypes show less than 70% growth before anthesis.

4. Main cause of reduction in yield is the reduction in duration of grain growth.

5. Heat stress at critical stage (Spikelet number) affects yield as this is one of the yield contributing character

6. Changing phenology of wheat plant for early filling of grain in early maturing genotypes is a solution to heat stress.

7. Grain growth is limited by sink size and faster growth due to high temperature resulting in reduction in grain size

8. Time period from seedling to anthesis is photoperiod and vernalization sensitive is genetically controlled and can be manipulated

9. Genes for photoperiod sensitivity, vernalization and earliness have been reported on many chromosomes and can be can be manipulated to control the time of anthesis in different environments. Time taken to heading can be manipulated by above mentioned gene groups.


Heat stress and Physiological characters for heat tolerance

Membrane thermostability

1. Cell membrane is the site of physiological injury during heat stress (Blum 1988, Hall 1992).

2. High genetic correlation of membrane thermostability with yield and is highly heritable (Fokar et al., 1998).

3. Positive correlation have been reported between grain yield and MT in spring wheat (Blum et al., 1989).

4. Genetic variablitiy in Indian condition was reported in Spring wheat (Rane et al., 2002).

5. Association of MT with leaf tolerance during grain filling stage has also been reported (Shanhan et al., 1990)

6. Flag leaf and seedlings can be used for analysis when there is heat stress.

7. Large number of samples can be analysed faster for seedlings

8. Genetic variance for MT is additive and heritability of 89% is observed (Fokar et al., 1989)

Stomatal conductance

1. When temperature and RH are high leaves permits absorption of CO2 and fixation rates varies among genotypes.

2. Significant correlation between yield and flag leaf photosynthesis have been reported. (Reynolds et al., 1994)

3. Weak correlation between flag leaf photosynthesis with yield but significant correlation with biomass.

4. Leaf conductance can be measure in on individual plants and can be used in selecting plants. (Reynolds et al., 2001)

5. Stomatal frequency is heriatble trait

6. Genes for increase in stomatal frequency have been identified on chr. 3AL and 6AL (Varghese et al., 1992)

7. When temperature and RH are high leaves permits absorption of CO2 and fixation rates varies among genotypes.

8. Significant correlation between yield and flag leaf photosynthesis have been reported. (Reynolds et al., 1994)

9. Weak correlation between flag leaf photosynthesis with yield but significant correlation with biomass.

10. Leaf conductance can be measure in on individual plants and can be used in selecting plants. (Reynolds et al., 2001)

11. Stomatal frequency is heriatble trait

12. Genes for increase in stomatal frequency have been identified on chr. 3AL and 6AL (Varghese et al., 1992)

Heat Shock proteins

1. HSP are synthesised during heat stress in plants and protect plants during stress

2. HSP not expresses at temperature of 250C while variation at 320C was observed and expression doubled when temperature raised to 370C. (Chinaswamy and Chopre 2004)

3. Temperature above 350C affects polymeric fraction of Gluten during grain filling period. (Ciaffi et al., 1996)

4. Seven different types of proteins were expreesed when plants were exposed to heat stress. (Skylas et al., 2002)

5. Heat shock proteins can be used in breeding for selection of heat tolerant genotypes.

Canopy temperature depression

1. Plants maintain a low temperature than that of air by evaporation of water from leaf surface.

2. Transpiration rate is determined by stomatal conductance which itself governed by rate of carbon fixation.

3. This measure is used for yield prediction in rainfed dry environments.

4. High correlation between yield and CTD and direct impact on stress tolerance has been reported by Reynolds et al., 1994.

5. Positive correlation of CTD with grain yield and grain weight per spike. (Rane et al., 2002)

6. Pierre et al., 2010 have concluded that relation between CTD and grain yield could be repeated across environments over the years.

7. CTD can be used as a selection criterion for heat stress tolerance.

Stem Reserves

1. Stem reserves support grain filling during heat stress. (Blum 1988)

2. Genetic variability has been determined fro this trait and had significant affect on grain filling for late sown crop.

3. Grain filling during normal course of development:

4. Current photosynthesis (90-95%)

5. Stem reserves (5-10%)

6. Photosynthetic apparatus damages during high temperature and stem reserves contribute to grain filling. (Blum et al., 1998)

Important traits a breeder has to focus for heat tolerance

1. Greater biomass

2. Higher 1000 grain weight

3. Canopy temperature depresssion

4. Stomatal conductance

5. Stem reserve mobilization

6. Grain growth rate

Indian wheat program has released a few varieties possessing moderate level of heat tolerance: RAJ 3765, UP 2425 and DBW 16, NW1014, HW2045, HD2643, HP1744, DBW14and NW 2036.

Research priority

1. Evaluation of traditional varieties valuable traits like tolerance to higher temperatures, drought and salinity, disease resistance, for use in breeding new varieties.

2. Participatory plant breeding to develop climate resilient crop varieties that can tolerate higher temperatures, drought and salinity.

3. Developing short duration crop varieties (especially wheat) that can mature before the peak heat phase sets in.

4. Selecting genotypes in crops that have a higher per day yield potential, to counter the yield loss from heat induced reduction in growing periods.

5. Developing (the more heat tolerant) durum wheat varieties for North India, to supplement the diminishing wheat yield from existing wheat cultivars and breeding for chapatti making qualities in durum wheat.




About Author / Additional Info:
I am working as a Scientist in Indian Institute of Wheat and Barley Karnal, Haryana under ICAR, New Delhi.