Wheat, a vital food source for over 50% of the global population and a contributor of 20% of global calorie intake, faces many challenges today, the biggest being droughts and heat stress. Grains produced under heat-stress conditions often have lower protein content and are smaller in size. As the global demand for wheat continues to rise, these yield reductions pose a significant challenge to food security and the agricultural economy.
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Wheat Growth Stages
Wheat undergoes various growth stages, each with its unique characteristics and importance. Understanding these stages is crucial for optimising wheat production.
Optimal Growth Conditions and Temperatures for Wheat
The optimal temperature for wheat during its reproductive stages lies between 15 and 20°C. However, many wheat-growing regions are experiencing temperatures above this optimum, leading to significant decreases in grain yield. With the increasing frequency of high daytime temperatures, understanding the effects of heat stress on wheat growth and yield becomes even more crucial.
Stages of Wheat Growth Affected by Heat Stress
- Germination and Tillering: Heat stress during the germination stage can delay or inhibit the sprouting process. During the tillering stage, excessive heat can reduce the number of tillers, which can impact the overall yield.
- Stem Elongation and Booting: High temperatures during stem elongation can lead to shorter plants, which might reduce grain yield. During the booting stage, heat stress can cause the head to emerge prematurely, leading to potential yield losses.
- Flowering: This is a critical stage where heat stress can severely impact grain formation. High temperatures can cause sterility in the flowers, leading to fewer grains.
- Grain-filling Stage: Heat stress during this stage can reduce the size and quality of the grains. The grains might not fill properly, leading to shriveled grains with lower nutritional content.
Physiological Effects of Heat Stress on Wheat
Photosynthesis and Lipid Composition: Heat stress during the anthesis or grain filling stages can decrease the photosynthetic rate by 17% and 25%, respectively. This reduction is associated with thylakoid membrane damage, changes in leaf lipid composition, and oxidative damage to cell organelles. For instance, heat stress during these stages increases the molar percentage of less unsaturated lipid species, affecting the overall health and efficiency of the plant.
Thylakoid Membrane Stability: Maintaining thylakoid membrane stability is crucial for optimal photosynthesis. Heat stress can cause significant damage to the thylakoid membrane, leading to decreased photosynthetic electron transfer, ATP synthesis, and alterations in photochemical reactions.
Reactive Oxygen Species (ROS) Production: High temperatures increase the production of ROS, including superoxide radicals and hydrogen peroxide. This results in increased membrane damage, affecting the overall health of the plant.
Molecular Mechanisms in Response to Heat Stress
- Protein Homeostasis: Protein homeostasis is vital for wheat’s thermotolerance. Heat stress affects protein quality control, the elimination of toxic proteins, and translational regulation. Maintaining protein homeostasis under heat stress is crucial for the plant’s overall health and productivity.
- Transcriptional Regulation: In response to heat stress, wheat plants activate complex transcriptional regulatory networks. These networks play a pivotal role in the plant’s ability to adapt and respond to increased temperatures.
- Lipid Composition: Changes in membrane lipid composition and unsaturation levels are proposed as a crucial mechanism of thermotolerance in wheat. For instance, heat stress during the anthesis stage can significantly alter the levels of various lipids, affecting the plant’s overall health and efficiency.
Impact on Wheat Yields Due to Heat Stress
Heat stress has a profound impact on wheat yields, with studies indicating that for every 1°C increase in temperature above the optimal during the grain-filling period, wheat yields can decline by as much as 6%. This is particularly concerning given the current trajectory of climate change. In regions where temperatures frequently surpass the optimal range for wheat growth, yield reductions of up to 20-30% have been reported. Furthermore, not only is the quantity of the yield affected, but the quality also diminishes. Grains produced under heat stress conditions often have lower protein content and are smaller in size.
Read more about how other crops are impacted by heat stress here.