Ecological correlates of seed dormancy differ among dormancy types: A case study in the legumes

Abstract

Comment on Rubio de Casas et al. (2017) ‘Global biogeography of seed dormancy is determined by seasonality and seed size: a case study in legumes’. Seed dormancy, and seed dormancy classes. Seed germination and seedling survival are crucial in determining species distributions, and plant population and community dynamics (Larson & Funk, 2016). By controlling the timing of germination, seed dormancy plays an important role in seedling survival, particularly in seasonal environments (Willis et al., 2014). The recent paper by Rubio de Casas et al. (2017) examined the seed size and global distributions of dormant compared with nondormant (ND) members of the Fabaceae, finding that dormant seeds are typically smaller than ND seeds, and are clearly associated with, and evidently have adaptive value in, more seasonal environments. This paper has made a valuable contribution to our understanding of the ecology and evolution of seed dormancy within the Fabaceae. However, the term ‘seed dormancy’ (defined at its simplest as a ‘block to the completion of germination of an intact viable seed under favourable conditions’ (p. 502); Finch‐Savage & Leubner‐Metzger, 2006) encompasses a diverse range of different physiological and structural mechanisms, which can be broadly split into discrete dormancy ‘classes’ (e.g. Baskin & Baskin, 2014) and can differ in prevalence among habitats, climatic zones, and taxonomic lineages (Finch‐Savage & Leubner‐Metzger, 2006). Within the Fabaceae, species with ‘dormant’ seeds can be assigned one of three dormancy classes: physiological dormancy (PD; where dormancy is broken in response to particular environmental cues such as warm or cold temperature), physical dormancy (PY; where the seed coat is impermeable to water thereby preventing imbibition; i.e. ‘hard seededness’), and combinational dormancy (PYPD; where a seed has both physical and physiological dormancy) (Baskin & Baskin, 2014). PY is the dominant dormancy class within the Fabaceae, with PD and PYPD making up a small minority of species.