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High ambient temperature (HT) impairs reproductive development and grain yield in temperate crops. To ensure reproductive success under HT, plants must maintain developmental stability. However, the mechanisms integrating plant development and temperature resilience are largely unknown. Here, we demonstrate that *PHOTOPERIOD 1* (*PPD-H1*), homologous to *PSEUDO RESPONSE REGULATOR* genes of the Arabidopsis circadian clock, controls developmental stability in response to HT in barley. We analyzed HT responses in independent introgression lines with either the ancestral wild-type *Ppd-H1* allele or the natural *ppd-h1* variant, selected in spring varieties to delay flowering and enhance yield under favourable conditions. HT delayed inflorescence development and reduced grain number in *ppd-h1* mutant lines, while the wild-type *Ppd-H1* genotypes accelerated reproductive development and showed a stable grain set under HT. Using a CRISPR/Cas9-induced *ppd-h1* mutant, we confirmed that the CCT domain of *Ppd-H1* controls developmental stability, but not clock gene expression. Transcriptome and phytohormone analyses in developing leaves and inflorescences revealed increased stress gene expression and abscisic acid levels in the leaf and inflorescence of the natural and induced mutant *ppd-h1* lines. Furthermore, the mutant ppd-h1 lines downregulated photosynthesis-and energy metabolism-related genes, and reduced auxin and cytokinin levels in the inflorescence, which impaired anther and pollen development. By contrast, in the wild-type *Ppd-H1* plants, the transcriptome and phytohormone levels and anther and pollen development remained stable under HT. Our findings suggest that *Ppd-H1* enhances stress resistance and energy metabolism, thereby stabilizing reproductive development, floret fertility and grain set under HT.
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