Heat Stress Induced Accelerated Senescence of Flag Leaves, Spikes and Awns in Winter Wheat Using Non-Invasive Chlorophyll Fluorescence.

Future climatic scenarios are predicted to encounter increased frequency of extreme weather events, which pose a severe threat to wheat (Triticum aestivum), a globally important staple cereal. Among the major drivers of changing climate, rising temperature can lead to heat stress events that can severely affect wheat production mainly when stress coincides with reproductive and grain filling stage. Application of chlorophyll fluorescence (Chl-F) to temporally track response of wheat leaves, spikes and awns exposed to heat stress was investigated. Seven winter wheat varieties cultivated in Kansas (Everest, WB-Cedar, Zenda, Larry, SY Monument, WB-4458 and Joe) were exposed to post-flowering heat stress in (i) growth chambers [35/15 (heat stress) and 25/15 °C (control) day/night) and (ii) field based heat tents (daytime temperature increased by 7 °C throughout grain filling). Effective quantum yield of photosystem II (QY) were recorded temporally on leaves, spikes and awns; while these values were simultaneously compared with changes in chlorophyll content also measured non-invasively. The decrease in chlorophyll content during grain filling was accelerated under heat stress. Both under chambers and field conditions, QY measured on leaves, spikes and awns tended to decrease with progressive senescence over time in both control and heat stress treatment. The extent of this reduction was however more significant in the case of heat stress, suggesting stress induced impairment of photosystem II reaction centers. Differential responses observed across investigated varieties in chambers and field conditions will be discussed with emphasis on the investigated approach as a potential high-throughput phenotyping method. The demand for increased and sustainable production under the predicted consequences of warming climate has necessitated developing fast and accurate phenotyping approaches that can help capture large genetic diversity and introduce greater heat tolerance into ongoing wheat breeding programs.