RESEARCH PAPER Dependency of seed dormancy types on embryo traits and environmental conditions in Ribes species E. Mattana 1,2 , W. H. Stuppy 2 , R. Fraser 2 , J. Waller 2 & H. W. Pritchard 2 1 Centro Conservazione Biodiversita (CCB), Dipartimento di Scienze della Vita e dell’Ambiente, Universita degli Studi di Cagliari, Cagliari, Italy 2 Seed Conservation Department, Wellcome Trust Millennium Building, Royal Botanic Gardens, Kew, Ardingly, West Sussex, UK Keywords Dormancy index; endospermic seeds; Grossulariaceae; morphophysiological dormancy; predictive model; under developed embryos. Correspondence E. Mattana, Department of Seed Conservation, Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly, West Sussex RH17 6TN, UK. E-mail: e.mattana@kew.org Editor D. Byers Received: 29 May 2013; Accepted: 10 September 2013 doi:10.1111/plb.12115 ABSTRACT The hypothesis that seed dormancy may be dependent on environmental conditions and seed morphological traits was tested for six Ribes species, across an altitudinal gradient of 1300 m and a longitudinal separation of 120°. Embryo measurements and seed germination experiments were conducted for R. alpinum L., R. hudsonianum Richardson var. petiolare (Douglas) Jancz., R. nevadaense Kellogg, R. roezlii Regel var. cruentum (Greene) Rehder and R. speciosum Pursh, and data taken from the literature for R. multiflorum Kit. ex Schult. ssp. sandalioticum Arrigoni. Germination was com- pared with seed viability to reveal proportional seed dormancy, which was then corre- lated to seed/embryo morphological traits and these traits related to the seed provenance environment. The embryos of all the investigated species are linear under- developed and all had a morphological component of seed dormancy (MD). Seeds of R. roezlii, R. hudsonianum and R. nevadaense required a temperature and/or hormone pre-treatment in order to germinate, highlighting morphophysiological seed dor- mancy (MPD). Seed dormancy was found to be strongly negatively correlated with embryo length, but not with embryo to seed (E:S) ratio or seed mass. Initial embryo length was positively related to mean annual temperature. Seed dormancy in the investigated Ribes species could be quantified and predicted by the interaction of embryo traits and environmental conditions. This approach may be helpful in assess- ing and predicting seed dormancy in the Ribes genus and in other genera and families with underdeveloped embryos. INTRODUCTION The timing of germination is determined to a large extent by the depth of seed dormancy, a physiological process preventing seed germination in a specified period of time, under any com- bination of physical environmental factors (temperature, irra- diance, etc.) that otherwise is favourable for its germination (Baskin & Baskin 2004). Large variations, which are considered adaptations to particular environments, exist for this seed trait among and within plant species (Baskin & Baskin 1998). Plants in environments that experience frost, drought, or both frost and drought are more likely to possess some form of seed dor- mancy than species of milder environments (Jurado & Flores 2005). Moreover, populations from habitats with severe win- ters tend to have a higher fraction of dormant seeds (Allen & Meyer 1998; Pendleton & Meyer 2004). This pattern can be highlighted by altitudinal (e.g., Orr u et al. 2012) or latitudinal (e.g., Wagmann et al. 2012) gradients on the thermal require- ments for seed dormancy release and germination. As reported by Forbis et al. (2002), the idea that embryo morphology has important implications for dormancy and ger- mination was firstly recognised by Goebel (1898), who identi- fied a group of angiosperms with ‘incomplete embryos’ requiring a period of ‘after-ripening’ prior to germination. According to the seed dormancy classification system (Baskin & Baskin 1998, 2004), morphological (MD) or morphophysio- logical (MPD) dormancy may occur in seeds with embryos underdeveloped at dispersal (i.e. endospermic seeds with rudi- mentary, linear-underdeveloped or spatulate-underdeveloped embryos; Baskin & Baskin 2007). Embryos which need time to grow and germinate (in ca. 30 days) are reported to have MD, whereas, if they require a dormancy-breaking treatment to overcome their physiological component (PD) of seed dor- mancy, they are reported to have MPD (Baskin & Baskin 2004; Baskin et al. 2008). Nine levels of MPD have been identified on the basis of the protocols for embryo growth and removal of seed dormancy, such as cold and warm stratifications (and combinations of them) and treatment with exogenous gibberel- lic acid (Baskin & Baskin 2004; Baskin et al. 2008). MPD levels may vary among species belonging to the same genus, as docu- mented for Erythronium L. (Baskin & Baskin 1985, 1998; Ba- skin et al. 1995; Kondo et al. 2002; Mondoni et al. 2012) and Aristolochia L. (Adams et al. 2005; Alves-da-Silva et al. 2011). Generally, embryo size increases within the angiosperms from basal families having small embryos to the most derived families having larger embryos (Martin 1946; Forbis et al. 2002). However, there are a few eudicot taxa with underdevel- oped embryos that represent reversals to this pattern, such as Santalales, Paeoniaceae, Grossulariaceae and Parnassiaceae, suggesting that their embryo morphology is closely tied to ecol- ogy, as a convergent response to similar environmental condi- tions (Forbis et al. 2002). The beneficial effect for seedling Plant Biology 16 (2014) 740–747 © 2013 German Botanical Society and The Royal Botanical Society of the Netherlands 740 Plant Biology ISSN 1435-8603