The initiation of embryo growth in imbibed celery mericarps marks a key mechanism by which temperature signals are integrated to regulate germination timing.

Relative embryo size (embryo:seed length ratio) is a key trait in which the internal morphology of mature seeds differs. It has shaped the angiosperm history at major evolutionary and climatic events, but its adaptive significance and role in dormancy are unknown. We investigated Apium graveolens (celery) morphologically dormant (MD) fruits, which have underdeveloped (small) embryos embedded in abundant endosperm tissue, for their mechanisms in response to non-optimal colder and warmer temperatures. To germinate, the underdeveloped embryo must first grow inside the endosperm to reach a critical relative embryo size. Distinct hormone-temperature interactions and molecular mechanisms underpinned the reduced embryo growth in response to suboptimal and supraoptimal temperatures. Thermoinhibition (29 °C) inhibited germination by surpressing the initiation of embryo growth in a gibberellin (GA)-abscisic acid (ABA)-regulated manner. This included inhibited endo-β-1,4-mannanase, expansin, and auxin biosynthesis gene expression. In contrast to this, during chilling and across the entire suboptimal temperature range (6-20 °C), the initiation of embryo growth was delayed in a thermal time-compliant manner, as was the expression of GA-induced genes important for ABA-insensitive endosperm degradation and embryo growth. The thermal-hormonal control of germination in seeds with underdeveloped embryos (MD) constitutes a unique programme distinct from seeds with fully developed embryos.