Physical dormancy in Geraniaceae seeds

Abstract

Seed dormancy was analysed in Geraniaceae, testing 29,103 seeds of 35 species for seed-coat impermeability to water. Seeds of wild and cultivated species from a number of different climatic/ecological zones (northern Germany, the Canary Islands and South Africa) were investigated. Seeds from cultivated plants and natural populations had similar (±2%) percentages of water-impermeable seeds. The percentages of impermeable seeds at maturity were also mostly constant (±2%) for two different years. The species analysed contained from 0 to 100% impermeable seeds at maturity, and could be classified into three quantitative types of physical dormancy: four species with no impermeable seeds (PY0), 10 species with a maximum of 80% impermeable seeds (PY80), and 21 species with more than 80% impermeable seeds (PY100). Species with different life cycles and from very different habitats, e.g. Geranium pratense or Pelargonium capitatum, belonged to the PY100 group. However, all of the succulent shrubs investigated from the genus Pelargonium were PY0 species. Five years of dry storage at 20 ± 5°C decreased physical dormancy in a species-dependent fashion. Erodium cicutarium, for example, had 95% impermeable seeds at maturity; all of them became permeable during the dry storage and began to germinate immediately after placing them on a moist substrate. Physical dormancy of other species, e.g. Pelargonium zonale, Pelargonium vitifolium or Geranium pratense, was not affected by dry storage. Thus, physical dormancy is a diversely differentiated feature in Geraniaceae with regard to both percentages of impermeable seeds at maturity and maintenance of dormancy under particular conditions.

A classification system for seed dormancy

“…of dormancy (with regard to maintenance and breakage) in Geraniaceae. It is quite clear, however, that ' … physical dormancy is a diversely differentiated feature in Geraniaceae, with regard to both percentage of impermeable seeds at maturity and maintenance of dormancy under particular conditions' (Meisert, 2002). Thus, lack of a single dormancy-breaking mechanism in plant families with PY suggests a need for subdivision of the PY class into lower layers in the hierarchy (Table 1).…”

“…In a recent study of physical dormancy in 35 species of the family Geraniaceae, including Erodium, Geranium and Pelargonium, Meisert (2002) recognized three categories of dormancy, based on the proportion of water-permeable and water-impermeable coats in samples of fresh seeds. Water-permeability versus water-impermeability of the seeds of 18 of these species was tested again after 2 years of dry storage at 20°C.…”

“…There was a general positive correlation between proportion of fresh seeds with impermeable coats and thickness of the water-impermeable and mechanical layers. Thus, Meisert (2002) concluded that ' … species with a high percentage of impermeable seeds have a closed chalazal slit [see Meisert et al, 1999] and form a thick mechanical and impermeable layer'. Even so, all impermeable seeds of two species in the PY 100 category became permeable during storage, while no impermeable seeds in seven species in this category did so.…”

Physical dormancy in a changing climate

“…Projections for future changes to rainfall patterns are highly variable between regions, but in general there is consensus that more extreme rainfall events, combined with longer drought intervals, will occur (HeislerWhite et al, 2009). Kimball et al (2010) showed that delayed arrival of winter rainfall in the south western US over a 25 year period altered community composition, with physically dormant Erodium cicutarium one of the species benefiting from this change, possibly due to PY loss being related to summer conditions rather than winter (Meisert, 2002). However, whilst some species may benefit, an increase in sporadic rainfall may increase the prevalence of 'false start' rain events.…”

The crypsis hypothesis explained: a reply to Jayasuriya et al. (2015)

“…the ratio of hard and soft seeds in dimorphic plants. Since plants naturally produce different proportions of hard and soft seeds (Morrison et al, 1992;Thanos et al, 1992;Meisert, 2002), we can predict under which environmental conditions selection would favour one strategy over another. We base our predictions on clearly outlined assumptions based on experimental findings and evolutionary modelling.…”