Seeds of annual and perennial populations of Mimulus guttatus exhibit differential responses to cold stratification.

Abstract

Premise: Life history theory predicts that temporal variation in fitness can select for either persistence in the seed bank or iteroparity, but rarely both simultaneously. However, this prediction has not been explicitly evaluated by testing for a negative relationship between these two life history traits.

Methods: We conducted a germination experiment to measure the extent of seed persistence in semelparous and iteroparous populations of Mimulus guttatus across a factorial combination of cold stratification and water potential treatments. We performed a tetrazolium assay on all seeds that did not germinate to quantify seed persistence (i.e., viable seeds that did not germinate).

Results: Semelparous seeds were more likely to persist than iteroparous seeds only in the absence of cold stratification in two of the three water potential treatments; there was no difference at the lowest water potential treatment because almost all seeds persisted. These differences in seed persistence reversed or disappeared when seeds were cold stratified because cold stratification surprisingly increased seed persistence in iteroparous seeds. There was substantial variation in cold stratification responses among seed families within populations of both ecotypes.

Conclusions: Contrasting responses to cold stratification between semelparous and iteroparous plants and notable within-population variation in these responses complicated the otherwise simple relationship that life history theory predicts between seed persistence and iteroparity. In addition, this study has practical implications for M. guttatus research, which often uses cold stratification to increase germination proportions; this treatment, our results show, does not always have this effect.