Species‐specific thermal thresholds for postdispersal embryo growth in Apiaceae

Premise Temperature is a primary regulator of seed development. In seeds with morphological (MD) or morphophysiological (MPD) dormancy, embryo elongation represents a distinct postdispersal developmental phase that precedes germination. However, the thermal thresholds governing this embryo growth phase remain poorly quantified. We quantified species‐specific embryo‐growth thermal niches to provide a mechanistic framework for understanding regeneration timing and its evolutionary constraints. Methods We estimated cardinal temperatures—base ( T b ), optimum ( T o ), and maximum ( T m )—for embryo growth in 10 Apiaceae species, a family in which MD and MPD are very common due to the presence of underdeveloped embryos. Seeds were incubated at five temperatures (5–25°C) with and without gibberellic acid (GA 3 ). Embryo growth rates were modeled using nonlinear thermal performance curves within a multimodel inference framework, followed by phylogenetic signal analyses. Results Substantial interspecific variation was found, with T b ranging from 0 to 6.5°C, T o from 5 to 25°C, and T m from 21.5 to 31°C. GA 3 generally increased growth rates and widened thermal ranges by reducing T b and raising T m , though responses were strongly species‐specific. Phylogenetic analyses revealed significant signal for all thermal thresholds ( T b , T o , T m ), indicating that evolutionary history constrains these thermal niches for postdispersal embryo growth. Conclusions Postdispersal embryo growth constitutes a distinct, quantifiable thermal niche rather than a mere proxy for germination. By explicitly treating postdispersal embryo growth as a distinct thermal niche rather than a proxy for germination, this study extends thermal threshold theory to an overlooked developmental phase and provides a mechanistic framework for understanding dormancy release and regeneration timing under variable climatic conditions.