Impact of post-flowering heat stress in winter wheat tracked through optical signals

Increasing temperatures can severely affect wheat (Triticum aestivum L.) production, particularly when it coincides with the grain-filling period. Heat stress induces rapid senescence resulting in early maturity and shortened grain-filling period. In this study, the applicability of in vivo chlorophyll fluorescence (Chl-F) and chlorophyll index to track rate of senescence in flag leaves and spikes exposed to heat stress were investigated. Seven winter wheat varieties were exposed to post-flowering heat stress using growth chambers [35/15 degrees C (heat stress) and 25/15 degrees C (control) day/night] and unique field-based heat tents (imposed +6 degrees C higher than ambient). Effective quantum yield of photosystem II (PSII) (QY) was recorded temporally in flag leaves and spikes, and compared with in vitro chlorophyll-a (Chl-a) concentration and non-invasive estimation of chlorophyll and flavonoids index. Time point indicating the start of senescence (change-point, CP) for QY was advanced by 0-8 and 0-6 d in flag leaves and spikes, respectively, under heat stress. In the chamber experiment, sustained heat stress induced accelerated decline of QY, particularly in wheat cultivars Larry and WB4458. Stronger positive relationship between days to senescence in spikes and thousand kernel weight indicated an extended period of assimilate supply from sink compared to the source tissue, during grain filling. Capturing heat stress-induced changes in photosynthetic pigments and QY at high temporal frequency presents an effective phenotyping approach for testing genetic diversity in large-scale field experiments involving different crops.