Morphological dormancy, embryo growth and pericarp restraint during crop and wild Apiaceae mericarp germination in response to ambient temperature

Apiaceae morphological dormancy and germination differ between crop and wild species, and among crop cultivars in the mechanical, hormonal and thermal mechanisms that control pericarp (fruit coat) weakening and pre-gremination embryo growth. The Apiaceae disperse morphologically (MD) or morphophysiologically dormant mericarps, indehicent fruits in which the single seed is encased by the pericarp (fruit coat) and the underdeveloped (small) embryo is embedded in abundant living endosperm tissue. Pre-germination embryo growth from an initial to a critical relative embryo size (embryo:fruit or embryo:seed length ratio) is a requirement for the completion of germination by radicle emergence. The roles and mechanisms of pre-gremination embryo growth and pericarp constraint were investigated by embryo-growth imaging, pericarp ablation/biomechanics, tissue-specific hormone analytics, and population-based thermal-time threshold modelling. Comparison of Apiaceae crop cultivars, including Pastinaca sativa (parsnip), Apium graveolens (celery) and Daucus carota (carrot) with > 50 wild Apiaceae species revealed that the initial relative embryo sizes of crop species are significantly larger compared to wild species. Interestingly, the critical relative embryo sizes of the phylogenetic group that contains parsnip, were smaller for the crop compared to wild species. ABA-insensitive and auxin-promoted pre-germination embryo growth was blocked by heat (thermoinhibition), while the completion of germination by radicle emergence was inhibited by ABA. The thick pericarp of parsnip decreased in thickness and mechanical tissue resistance in parallel with the pre-germination embryo growth, while the thin pericarps of celery and carrot did not change. Parsnip pericarp contained significantly higher contents of the germination-inhibiting hormones abscisic acid (ABA) and cis-(+)-12-oxo-phytodienoic acid (cis-OPDA) compared to celery pericarp. Pericarp ablation experiments revealed that it acts as a mechanical and chemical (ABA, cis-OPDA) constraint (coat component of MD), and has a key role in narrowing the permissive temperature window for germination.