Pre‐Flowering Heat Wave Sensitivity on Yield and Its Components in Two Spring Barley Near‐Isogenic Lines for PPD‐H1 Alleles

ABSTRACT Rising temperatures associated with climate change increasingly threaten cereal yields. Most previous studies have focused on chronic high temperatures, which primarily affect yield indirectly through modifications of crop phenology. In contrast, the effects of short‐term heat waves on reproductive development and yield formation remain poorly understood. In barley ( Hordeum vulgare L.), development and reproduction are tightly regulated by photoperiod sensitivity, mainly controlled by the PPD‐H1 gene, which may modulate plant responses to thermal stress. Here, we quantified the impact of a pre‐flowering heat wave on yield formation and spike fertility in near‐isogenic lines of spring barley differing at the PPD‐H1 locus, across four contrasting environmental backgrounds combining sowing date and photoperiod conditions. Heat waves were imposed for 10 days at the flag leaf stage under field and outdoor conditions, increasing daily maximum temperatures with minimal effects on mean temperature and phenology. Across environments, the heat wave consistently reduced grain yield. Yield losses were primarily driven by reductions in grain number rather than grain weight and were markedly greater in the photoperiod‐insensitive ppd‐H1 line than in the photoperiod‐sensitive Ppd‐H1 line. Differences in flowering time between near‐isogenic lines were small or absent under most conditions, indicating that the contrasting heat wave responses were largely independent of phenological escape and instead reflected direct effects on reproductive development. The lower resilience of ppd‐H1 was associated with both reduced floret fertility at flowering and increased grain abortion, particularly in apical spikelets, whereas spike fertility in Ppd‐H1 plants was largely preserved. Our results demonstrate that the wild‐type Ppd‐H1 allele confers enhanced tolerance to short‐term heat stress immediately before flowering by stabilising spike fertility and grain set. This study highlights the importance of photoperiod genes not only in shaping phenology but also in conferring intrinsic resilience of reproductive processes to heat waves, providing valuable insights for breeding barley cultivars adapted to increasingly variable thermal environments.