Tackle heat stress to sustain productivity

PK Kingra

CLIMATE change is drastically affecting sustainable crop production globally. Extreme weather events, especially temperature and rainfall fluctuations, are threatening the cultivation of field crops in many regions of the world, endangering food security. High temperature is one of the most recurrent forms of abiotic stresses. Heatwaves and droughts are expected to become more frequent and severe in many parts of the world. South Asia is specifically threatened by climate extremes, especially a heatwave, as it includes mainly tropical/sub-tropical areas, which are already experiencing higher temperatures. Most of these regions have very high population density; thus, increased frequency and intensity of climate extremes may pose a severe threat to food security for the burgeoning population. Studies have predicted that 1°C rise in temperature may lead to 6% reduction in wheat production globally. However, these reductions can enhance locally as local climates can be more severely affected by climate extremes.

Such conditions are more worrisome for the Indo-Gangetic plains, the major wheat-growing region in India, due to its vulnerability to terminal heat stress for rabi (winter) crops. Under current and future climate change scenarios, heat and water stress are the major abiotic constraints to crop productivity. Their impacts are quite severe when occurring even individually, but significant yield reductions are observed when they coincide. Although they affect crops at every growth stage, severe yield reductions are observed during reproductive growth, especially grain-filling, thus reducing seed size and number. Studies have shown that yield losses can be as high as 50% due to increased temperatures and drought.

A heatwave is a period of abnormally high temperature, usually above 40°C in the plains and 30°C in hill regions. A heatwave is said to occur when the temperature is higher than normal by 4.5°C to 6.4°C and a severe heatwave occurs when the temperature exceeds the normal by more than 6.5°C. Heatwaves typically occur in north-west India from March to June. As the rabi crops are at their reproductive stage during March-April, heatwaves in these months are of utmost importance for rabi crops as they lead to terminal heat stress and have an adverse effect on their productivity. Although the occurrence of heatwaves is not a new phenomenon in our region, the increase in their frequency and intensity is a great concern.

Effect on crop production

A sudden rise in temperature in March leads to enhanced grain maturity with significant reduction in the yield of rabi crops. The increase in night-time temperature increases respiratory losses, thus creating an imbalance between source and sink and decreasing net photosynthesis. As a result of reduction in the grain formation period along with loss of carbohydrates in enhanced respiration under higher maximum and minimum temperatures, proper grain development cannot take place, leading to shrivelling of grains and consequently reduced crop yields. Such hot-weather conditions, experienced during March 2022, resulted in terminal heat stress with an adverse impact on wheat productivity in the region.

Management strategies

To combat the impact of global warming and terminal heat stress on crops, there is a dire need to identify and develop varieties which are heat-tolerant. There is also a need to evolve management practices which can decrease the heat load on crops. The following strategies should be adopted for protection of crops from heat stress:

• Provide light irrigation to the crop as maintaining adequate soil moisture is crucial to minimise heat stress. Providing water during early morning or evening is desirable to avoid evaporation losses or minimise losses due to high wind speed during daytime.
• Time of sowing of the crops should be adjusted in such a way that growth stages most sensitive to high temperature do not coincide with the period of heat stress.
• Soil management by inorganic or organic amendments should be made to improve its moisture retaining capacity.
• Practices such as tillage intervention should be adopted to manage heat stress in crops.
• Fertilisers should not be applied to the crop during a heatwave unless there is adequate availability of moisture, otherwise they can have a detrimental effect on the crop.
• Appropriate method and frequency of irrigation should be followed.
• Use of crop residue and mulch application should be practised to improve moisture status of the soil.
• Selection of suitable crop varieties tolerant to heat stress should be made as per the prevailing weather conditions in a particular region.
• Comprehensive research efforts are required to better understand the plant responses to abiotic stresses for their effective management.
• As heatwaves and moisture deficit are likely to intensify in the future, developing abiotic stress-tolerant varieties using modern breeding techniques is the need of the hour.
• In view of global warming and changing climatic scenarios, a holistic approach dealing with the management of different abiotic stresses needs to be followed to sustain crop productivity and ensure food security in the years to come.

Promoting heat-resistant varieties of wheat

• Wheat varieties DBW187 and DBW222 have been found superior to HD-3086 as far as heat tolerance is concerned. During the crop season 2021-22, DBW187 and DBW222 have shown heat tolerance with yield gain of 3.6% and 5.4%, respectively, compared to HD-3086 (as per AICRP on Wheat and Barley report, 2020-21 & 2021-22).
• The govt aims to promote the use of heat-resistant varieties among farmers through public & private partnership and providing seed directly to them. The Indian Institute of Wheat and Barley Research (IIWBR), under ICAR, has signed pacts for DBW187 and DBW222 with pvt firms for seed production.
• The ICAR-IIWBR has initiated a project, ‘Breeding high-yielding wheat genotypes for stress conditions of warmer regions of India’. It is also collaborating with the International Maize and Wheat Improvement Centre, Mexico.

The author is Professor & Head, Dept of Climate Change and Agricultural Meteorology, PAU

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