Impact of Post-Flowering Heat Stress on Kansas Winter Wheat.

Post-flowering heat stress is a major environmental constraint for wheat (Triticum aestivum L.) production in Kansas. Wheat is the most widely grown grain crop in Kansas. Studies have shown the optimal temperature for grain development of wheat to be approximately 21°C. It is not uncommon for temperatures in Kansas to reach 30°C and above during the grain-fill period. The major objective of our studies was to quantify the impact of post-flowering heat stress on grain yield, yield components, and physiological performance of prominent Kansas winter wheat varieties. In 2016 and 2017 two field experiments and one growth chamber experiment were conducted. Heat stress was imposed in the field experiment by placing heat tents over the plots, ten days after flowering and remained until maturity. Plants grown in the growth chamber experiment were tagged on the first day of flowering and subjected to heat stress (35/15^oC; day/night temperature) treatment 10 days after the main stem spike had initiated flowering and remained until maturity. In all three experiments, temporal physiological measurements were recorded throughout the grain filling period - chlorophyll concentration and fluorescence. Yield and yield components were recorded at maturity. Chlorophyll content and photochemical efficiency of PSII showed rapid reduction under heat stress over time compared to control plants, indicating early senescence. Across all three experiments SY-Monument was consistently tolerant while WB-4458 was susceptible. Heat stress reduced thousand kernel weight and harvest index in nearly all the genotypes compared to control. Varietal response documented for heat stress response indicates the need to further improve heat stress resilience in Kansas varieties to sustain future climatic changes.