Post-Dispersal Embryo Growth Is a Thermal Checkpoint for Seed Regeneration.

Many seeds with underdeveloped embryos, particularly those exhibiting morphological (MD) or morphophysiological dormancy (MPD), must complete a phase of post-dispersal embryo growth before germination can occur. Yet this developmental stage is typically treated as a passive delay within dormancy frameworks rather than as an independent ecological process. We propose that embryo growth constitutes a distinct thermal performance process governed by species-specific base, optimum, and ceiling temperatures that are mechanistically decoupled from germination thresholds. As such, embryo growth functions as a developmental checkpoint that filters regeneration under variable climatic conditions. We introduce the concept of the embryo-growth thermal niche (EGTN), defined by (i) a temperature-dependent initiation threshold and (ii) a thermal performance curve governing elongation rates once growth has begun. The EGTN determines not only the pace of embryo development but the climatic window within which regeneration is possible. This framework generates counterintuitive predictions under climate warming. Because embryo growth may decline or arrest at supra-optimal temperatures, warming can reduce cumulative developmental progress and delay seedling emergence, even when germination rates increase, producing phenological inversion relative to linear thermal-time expectations. Viewing embryo growth as a functional trait reveals distinct regeneration strategies structured by thermal breadth and upper thermal limits, independent of dormancy class. We further predict strong phylogenetic signal in embryo-growth thermal parameters, potentially constraining adaptive responses to rapid climate change. By integrating developmental biology with functional trait ecology, the EGTN framework positions post-dispersal embryo growth as a central determinant of regeneration timing, climate sensitivity, and evolutionary constraint. Incorporating embryo-growth thermal niches into regeneration models will improve forecasts of plant responses in a warming world.