Single versus repeated heat stress in wheat: What are the consequences in different developmental phases?

With a possible reference to heat priming and to characterize the extent and variation in the heat stress responses in wheat, the effects of single vs. repeated heat stresses were examined by measuring the changes in morphological and grain yield-related traits and photosynthetic parameters. To achieve these objectives, 51 winter wheat cultivars of various geographic origins were included in two independent experiments covering different phenological stages. In Experiment I, a single heat stress event was applied at stem elongation (SE) and booting (B), and the repeated heat stress was applied at both of these stages (SE+B). In Experiment II, the single heat stress was applied at stem elongation (SE) and full heading (CH), while the repeated heat stress was applied at both stages (SE+CH). While genotype was a more important factor for determining the morphological and yield-related traits, it was the treatment effect that mostly influenced the photosynthetic parameters, with the exception of the chlorophyll content. The heading stage was more sensitive to heat stress than the booting stage, which was primarily due to the larger decrease in the average seed number. The importance of biomass in contributing to grain yield intensified with the heat stress treatments. There was a large variation between the wheat cultivars not only in yielding abilities under control conditions but also in sensitivities to the various heat stresses, based on which 7 distinct groups with specific response profiles could be identified at a highly significant level. The 7 wheat groups were also characterized by their reaction patterns of different magnitudes and directions in their responses to single vs. repeated heat stresses, which depended on the phenological phases during the second cycle of heat stress. The possible association between these findings and heat priming is discussed.