Introduction
Wheat (Triticum spp.) is a fundamental staple crop, globally cultivated on 219 million hectares in 2022, and producing over 808 million tons, ranking as the third most important crop by total world production.  Adapted to a broad latitudinal range (67° N to 45° S), wheat  thrives under widely varied  environmental conditions. In Iran, wheat remains the foremost priority of agricultural development efforts, with recent years focused on attaining self-sufficiency. However, maximizing grain yield requires breeders to address both biotic and abiotic stresses, particularly terminal heat stress, which critically affects grain development. Among the phenological traits, heading date is pivotal in breeding and genotype selection, directly impacting adaptation and final yield under stress conditions. 
Materials and Methods
This study analyzed the effects of temperature during critical developmental phases on grain yield across six growing seasons (2012-2018) in Northern Khuzestan. Data were complied from 14 international and 12 national wheat experiments conducted in a warm southern climate, comprising a total of 1,226 genotypes. For each genotype, the average minimum and maximum temperatures were recorded and analyzed for two distinct periods: from sowing to heading and from heading to physiological maturity. The impact of thermal factors especially during pre- and post-heading stages, on grain yield was assessed. Post-heading data from the first replicate in each experiment were used to calculate correlation coefficients between phenological traits and yield. Analysis of variance and estimation of genotypic and phenotypic correlation coefficients were conducted using META-R software.
Results and Discussion 
Thermal amplitude (Tmax-Tmin) was employed as an environmental index to assess its impact on phenological development, growth, and yield. The variation in average minimum and maximum temperatures from sowing to heading ranged from 7.61 to 18.96°C in the early-heading genotypes and 7.62 to 21.46°C in late-heading genotypes. During the grain-filling period (heading to physiological maturity), the minimum and maximum temperature averages ranged from 11.1 to 15.74°C and 24.51 to 31.39°C, respectively. Although correlation coefficients between grain filling and thermal variables did not exhibit consistent trends, a significant relationship was observed between yield and thermal amplitude during the pre-heading stage. In contrast, correlations between yield and days to heading, as well as with average minimum and maximum temperatures from sowing to heading, were generally not statistically significant. Similarly, relationships observed during the reproductive phase were inconsistent, underscoring the pre-heading stage as a critical period of sensitivity to heat stress.
Conclusion
These results provide deeper insights into the complex interplay between temperature fluctuations and wheat developmental stages. The findings suggest that thermal amplitude before heading is a critical determinant of yield stability under terminal heat stress, guiding both breeders and farmers. For breeders, this underscores the value of selecting for optimal heading dates and heat resilience, while for farmers, understanding temperature effects can inform variety choice and sowing times.  Ultimately, adopting such strategies has the potential to mitigate crop losses, support sustainable agriculture, and enhance food security amid climate variability.