<i>PHOTOPERIOD 1</i> enhances stress resistance and energy metabolism to promote spike fertility in barley under high ambient temperatures

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 resistance are largely unknown. Here, we demonstrate that PHOTOPERIOD 1 (PPD-H1), homologous to PSEUDO RESPONSE REGULATOR genes of the Arabidopsis (Arabidopsis thaliana) circadian clock, controls developmental stability in response to HT in barley (Hordeum vulgare). We analyzed the 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 favorable conditions. HT delayed inflorescence development and reduced grain number in ppd-h1 mutant lines, while the wild-type Ppd-H1 genotypes exhibited accelerated reproductive development and showed a stable grain set under HT. CRISPR/Cas9-mediated genome editing of Ppd-H1 demonstrated that the CONSTANS, CO-like, and TOC1 domain of Ppd-H1 controls developmental stability, but not clock gene expression. Transcriptome and phytohormone analyses in developing leaves and inflorescences revealed increased expression levels of stress-responsive genes and abscisic acid levels in the leaf and inflorescence of the natural and induced mutant ppd-h1 lines. Furthermore, the ppd-h1 lines displayed downregulated photosynthesis- and energy metabolism-related genes, as well as decreased auxin and cytokinin levels in the inflorescence, which impaired anther and pollen development. In contrast, the transcriptome, phytohormone levels, and anther and pollen development remained stable under HT in the wild-type Ppd-H1 plants. Our findings suggest that Ppd-H1 enhances stress resistance and energy metabolism, thereby stabilizing reproductive development, floret fertility, and grain set under HT.