Wheat heat stress causes the greatest yield losses when heat waves strike before flowering, field experiments show. However, early exposure may also prime wheat to better withstand a second heat wave later in the growing season.
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Field experiments show that heat before flowering causes the greatest yield losses, but can also prime wheat to withstand a second heat wave later in the season.
Wheat is more vulnerable to yield losses from heat waves before flowering than from those occurring later, according to research published in the Journal of Experimental Botany. However, early heat exposure may also trigger a protective response that reduces the impact of subsequent heat waves.
Researchers at the University of Lleida in Catalonia, Spain, investigated how the intensity, frequency and timing of heat waves interact to affect wheat production. Their findings suggest that predicting yield losses requires consideration of both crop development and previous exposure to heat.
Testing Heat Stress Under Field Conditions
The team conducted field experiments over two growing seasons using two modern wheat varieties with contrasting strategies for building yield. Both responded similarly to the heat treatments.
“Most studies have examined heat stress as a single event, and have done so under controlled conditions rather than under field conditions,” said study co-author Gustavo Slafer. “We investigated whether an earlier heat wave could alter how wheat responds to a later one during reproductive development”.
Researchers used transparent tents to raise daytime temperatures through a greenhouse effect. They compared heat exposure before and after flowering using a standardized measure of heat load relative to control plots, giving greater weight to temperatures above 32 C.
The approach allowed the researchers to assess how the timing of heat exposure influenced crop responses, according to a press release.
Earlier Heat Reduced Grain Number
Heat waves before flowering caused greater grain yield losses than those occurring after flowering.
The timing also affected how those losses developed. Earlier heat reduced the number of grains produced, while later heat reduced grain weight.
The findings highlight the vulnerability of the reproductive stages when wheat establishes grain number. They also show why heat intensity alone cannot explain the eventual effect on yield.
Repeated Heat Waves Revealed a Priming Effect
Wheat exposed to heat both before and after flowering experienced less damage from the later event than wheat exposed only to post-flowering heat.
The pattern suggests a protective priming effect, in which an initial stress triggers physiological responses that remain partly active and help the plant respond more effectively to another stress.
The earlier heat exposure still carried a production cost through reduced grain number.
“To our knowledge, this is the first evidence of this type of antagonistic interaction between successive heat waves in field-grown crops,” Slafer said.
Protecting Sensitive Growth Stages
The findings could improve crop yield models and help identify when adaptation measures would provide the greatest benefit.
“This study reinforces the importance of protecting wheat during the stages when grain number is determined, particularly around flowering,” Slafer said. “The most effective adaptation measures are therefore likely to be those that reduce exposure during the most sensitive reproductive stages.”
The research also highlights the need to consider multiple heat waves within a growing season when estimating future yield losses. Treating each event independently could overlook how earlier exposure changes a crop’s response to later heat.
Potential Relevance for Other Cereal Crops
The researchers suggest that similar responses may occur in other temperate crops, including barley, oats and rye. However, temperature thresholds and responses are likely to differ among species.
“Wheat is one of the world’s most important staple crops, so safeguarding its yield under climate change is a major priority,” Slafer said. “As a crop of temperate origin, wheat is generally less adapted to high-temperature episodes than crops with tropical origins, such as maize or rice.”
Breno Bicego and Roxana Savin also authored the study.
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