Species-specific thermal thresholds for postdispersal embryo growth in Apiaceae

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.

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 (GA₃). Embryo growth rates were modeled using nonlinear thermal performance curves within a multimodel inference framework, followed by phylogenetic signal analyses.

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. GA₃ 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.

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.