title: Humidity as a determinant of heat stress tolerance in wheat (Triticum aestivum L.)
Creators: name: Seyhun, Rahme
Publisher: name: University of Reading
Year: 2024

description: Wheat is one of the most common crops, grown worldwide on about 220 million ha of farmland. However, ongoing climate change has raised concerns about the effects of environmental variation on wheat yields. Studies on grain crops show that high spikelet (or panicle) temperature is the main reason for sterility and can be distinct from ambient air temperature. Transpirational cooling commonly lowers spikelet temperature, but the capacity of plants to cool their tissues may be affected by relative humidity. This thesis reports on findings from three consecutive years of pot and field-based experiments where two wheat cultivars, namely Blasco (bred for warm and dry conditions) and Paragon (bred for wet and cool conditions), were exposed to combinations of temperature (T), relative humidity (RH) and soil moisture treatments. The cooling capacity of Blasco was higher than Paragon under most conditions in controlled environment experiment, while the temperature of the spike was higher than that of the flag leaf in both cultivars in all three experiments. Even though we have not observed RH effect on tissue temperature in the semi-controlled pot-based and field experiments, controlled environment experiment in the cabinets showed that both RH and air T has an impact on tissue temperature, while the main driver of tissue T in both cultivars was the temperature of surrounding air regardless of the growth stage. Higher RH at low air temperature resulted in a decrease of the cooling capacity in Paragon during early grain filling in controlled climate cabinets, but a complete reversal was observed in Blasco. RH did not modify tissue temperature of the cultivars at higher air T at flowering, but high RH increased spike temperatures in both cultivars during early grain filling. Increased tissue temperatures at flowering triggered lower grain number in Paragon. Lower grain weight under high RH conditions caused lower grain yield which triggered higher total protein and gluten protein content in both cultivars in the controlled environment and field experiments. In pot-based semi-controlled experiment, higher soil moisture caused higher grain yield in Blasco and Paragon, and protein content tended to be higher at lower soil moisture conditions. In the controlled environment experiment, we have observed that high tissue temperature under high temperature caused decrease in alpha amylase activity of Paragon. Gluten protein composition in Blasco was affected from high RH and high air temperature as ω-gliadins % glutenin subunits in Blasco increased under these conditions. There was a negative correlation between grain yield and protein content in both cultivars in all three Experiments. In the semi-controlled field experiment, total nitrogen and gluten protein content in both cultivars increased under high RH and high air temperature conditions, while in pot-based semi-controlled experiment, increase in soil moisture resulted in lower SDS-sedimentation. Blasco responds to the stress factors with altering its stomata area, while Paragon altered its stomata number. High RH, high air T and low soil caused an increase in stomata are in Blasco, while High RH and high air T resulted in higher stomata number in Paragon. Lower RH caused higher stomatal conductance in Blasco under ambient temperatures. The main finding of the thesis is that atmospheric humidity affects the capacity of wheat plants to regulate tissue temperature which impacts grain quantity and quality.