Diverse alleles of <i>Photoperiod-H1</i> directly and indirectly affect barley yield-related traits under contrasting photoperiods and <i>PHYTOCHROME C</i> backgrounds

Barley traits that determine grain number are established between jointing and flowering. The timing of flowering is critical for adaptation and yield as it affects the fertility of both the main shoot and tiller spikes. The Photoperiod-H1 (PPD-H1) gene controls flowering time and impacts spike fertility and yield in barley; however, it is not known if these effects are truly pleiotropic or indirect. Recent findings indicated that under extremely long photoperiods, barley near-isogenic lines (NILs) with contrasting PPD-H1 alleles tend to flower simultaneously, allowing any genetic effects to be determined independently of flowering time. We examined the direct impact of PPD-H1 on barley spike fertility and yield in two PHYC genetic backgrounds. Experiments combined (i) two NILs for PPD-H1 alleles (ppd-H1 or Ppd-H1) under two contrasting PHYTOCHROME C genetic backgrounds (PhyC-l and PhyC-e), and (ii) two photoperiod conditions (12 h and 24 h). Under a 24 h photoperiod, the PPD-H1 gene had a minimal impact on flowering time. The ppd-H1 allele increased grain number by enhancing spike fertility, irrespective of the PHYC background, through better resource allocation to juvenile spikes and more efficient production of fertile florets. These findings support a pleiotropic effect of ppd-H1 in determining flowering time and establishment of yield component traits.