Trait stasis versus adaptation in disjunct relict species: evolutionary changes in seed dormancy-breaking and germination requirements in a subclade of Aristolochia subgenus Siphisia (Piperales)

Abstract

There are two ideas regarding changes in the physiological and ecological tolerances and requirements within plant lineages through geological time. One is that these attributes have changed very little, or not at all (trait stasis), and the other is that they have undergone considerable change (adaptation), as plants shifted to new climatic and vegetation zones. We tested these ideas for seed dormancy-breaking and germination requirements of four species in a subclade ofAristolochiasubgenusSiphisia: the three temperate species,A. macrophyllaandA. tomentosa(the basal species in the subclade) of eastern USA andA. manshuriensisof East Asia, and the Mediterranean-climate speciesA. californica endemic to California, USA. A long period at cold-stratifying temperatures was required for growth of the underdeveloped embryo and seed germination inA. californica, whereas embryos grew and seeds germinated in the other three species at warm temperatures, either before or after they were cold stratified. Thus, seeds ofA. californicahave either intermediate or deep complex morphophysiological dormancy (MPD), whereas those of the three temperate species have either morphological dormancy or non-deep simple MPD. Further, there were quantitative differences in temperature requirements for dormancy-break and germination between the AppalachianA. macrophylla, which did not differ from its sister speciesA. manshuriensis, and the lowlandA. tomentosa. Thus, within this lineage there has been both trait stasis and divergence (adaptation) in the physiology and ecology of seed dormancy and germination.