Durum wheat germplasm response to high temperatures, the role of small HSP26 in the defense response

Heat stress is a major constraint on wheat productivity, and short-term heat-shock studies often fail to capture the complexity of field conditions. Here, we assessed heat responses in contrasting durum wheat genotypes (SSD69, SSD397, Svevo, and Kronos) exposed to prolonged high temperatures throughout their life cycle under field-like conditions. Physiological evaluations at flowering and post-flowering showed that the tolerant lines SSD69 and Svevo maintained higher PSII efficiency (Fv/Fm), stomatal conductance, and canopy cooling, thereby sustaining photosynthesis and grain set. In contrast, SSD397 exhibited PSII damage, impaired transpiration, and reduced fertility. Kronos showed intermediate behavior. Biochemical analyses revealed tissue-specific variation in lipid peroxidation: SSD397 accumulated more malondialdehyde (MDA) in spikes, while SSD69 displayed reduced MDA over time, suggesting more efficient detoxification of reactive oxygen species (ROS). Molecular assays identified differential regulation of TdHsp26 alleles. Tolerant lines showed strong TdHsp26-A1 expression, associated with PSII protection, stomatal regulation, and reduced oxidative damage. By contrast, SSD397 exhibited TdHsp26-A1 downregulation and relatively higher TdHsp26-B1 expression, correlating with stress sensitivity. Overall, our results demonstrate that natural sequence variation in TdHsp26 underpins key physiological and biochemical mechanisms of thermotolerance in durum wheat. SSD69, originating from arid North Africa, displayed traits consistent with adaptation to hot environments, including a stay-green phenotype that supports transpirational cooling and yield stability despite reduced biomass. These findings highlight the adaptive value of durum wheat germplasm and provide targets for breeding cultivars resilient to future climate scenarios.