Early Heat Waves Hit Wheat Yields Hardest, Spanish Field Study Finds
25 September 2026, Catalonia, Spain: Heat waves that strike wheat crops before flowering cause significantly greater yield losses than heat waves that arrive after flowering, according to a field study from researchers at the University of Lleida in Spain published in the Journal of Experimental Botany on September 18. The same study found an unexpected silver lining: an early heat wave appears to make the crop more resilient to a second heat wave later in the season.
The findings matter because heat waves are becoming both more frequent and less predictable in their timing as global temperatures rise, and wheat is grown across a wide range of climates where growers have limited ability to shift sowing dates enough to avoid every possible heat event. Understanding which growth stage is most vulnerable helps plant breeders and agronomists decide where to focus limited resources, whether that means breeding for heat tolerance at a specific developmental window or timing irrigation and other interventions around forecast heat events.
How the study was conducted
Led by Dr. Gustavo Slafer, with co-authors Breno Bicego and Roxana Savin, the research team ran field trials over two growing seasons using two modern wheat varieties. Rather than relying on climate chambers, which can produce artificial results that do not hold up in real field conditions, the researchers used transparent tents placed over field plots to create genuine heat wave conditions through a greenhouse effect. Heat exposure was measured precisely as the number of hourly temperature degrees above 32 degrees Celsius, allowing the team to compare heat waves of similar intensity but different timing relative to the crop’s flowering stage.
The results showed a clear pattern: heat waves before flowering reduced the total number of grains the plant produced, which tends to have an outsized effect on final yield since grain number is largely locked in before grain filling even begins. Heat waves after flowering instead reduced the weight of individual grains, a loss that is generally smaller in overall yield terms because the plant has already committed to a given grain number by that stage.
A newly documented “priming” effect
The more novel finding involved what happens when wheat experiences two heat waves in sequence. Rather than compounding damage, an early heat wave appeared to prime the plant, making it somewhat better able to cope with a second heat event later on. Dr. Slafer described this as “the first evidence of this type of antagonistic interaction between successive heat waves in field-grown crops,” a distinction that matters because most existing research on heat stress interactions comes from controlled chamber studies rather than crops grown under real field conditions with natural variability. He added that protecting wheat specifically during the developmental window when grain number is determined remains the most important target for adaptation strategies, since that is where the largest and least recoverable yield losses occur.
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