Introduction
Dispersal of angiosperm plant families to new parts of the world often is followed by speciation, which leads to the question: are changes in seed dormancy-breaking and germination requirements part of the adaptation of a species to its new habitat? One way to help address this question is to evaluate the dormancy-breaking and germination requirements of plant families that have species growing in a diversity of vegetation regions. This approach has been used to examine seed dormancy/germination in the large families Asteraceae (Baskin and Baskin, Reference Baskin and Baskin2023), Rubiaceae (Baskin and Baskin, Reference Baskin and Baskin2024), Myrtaceae (Baskin and Baskin, Reference Baskin and Baskin2025a) and Malvaceae (Baskin and Baskin, Reference Baskin and Baskin2025b) from a global perspective. In each of these families, there is a unique combination of ways in which seed dormancy and germination are related to the growth form, habitats and distribution of the species.
The Solanaceae was chosen for study of seed dormancy/germination from a phylogenetic and world vegetation perspective because it (1) has a diversity of embryo morphologies including straight linear to spiralled linear and spatulate (Martin, Reference Martin1946; Gunn, Reference Gunn1974) and (2) is cosmopolitan but not species rich in northern latitudes with cold winters, e.g. northeastern USA and adjacent Canada (Gleason and Cronquist, Reference Gleason and Cronquist1991). The family is distributed on all continents except Antarctica, and species grow in rainforests, savannas, steppes, deserts and boreal/subalpine forests and meadows. The highest number of genera and species is in tropical/subtropical zones, especially Central and South America (Orejuela et al., Reference Orejuela, Wahlert, Orozco, Barboza and Bohs2017; Palchetti et al., Reference Palchetti, Cantero and Barboza2020b). Sixty-one genera and 1595 species of Solanaceae are native to South America, and species grow from sea level to the subalpine zone on mountains. Peru has 41 genera and 631 species of Solanaceae, of which 295 species are endemic to that country (Palchetti et al., Reference Palchetti, Cantero and Barboza2020b). Argentina has 315 species (80 endemic) of Solanaceae in 32 genera and Chile has 170 species (89 endemic) in 21 genera (Moreira-Muñoz et al., Reference Moreira-Muñoz, Palchetti, Morales-Fierro, Duval, Allesch-Villalobos and González-Orozco2022).
High numbers of genera and species of Solanaceae occur in other South American countries, e.g. Bolivia, Brazil, Columbia and Ecuador (Palchetti et al., Reference Palchetti, Cantero and Barboza2020b). Other centres of Solanaceae species richness include Africa, 9 genera and 202 species (Jaeger and Hepper, Reference Jaeger, Hepper, Jiao and D’Arcy1986); Australia, 6 genera and 132 species (Purdie et al., Reference Purdie, Symon and Haegi1982); China, 16 genera and 78 species (Lu, Reference Lu and D’Arcy1986) and Mesoamerica, 27 genera and 282 species (Gentry and D’Arcy, Reference Gentry, D’Arcy and D’Arcy1986).
In our consideration of the Solanaceae, we present an overview of the family and address various questions. (1) When and where did the family originate? (2) When did diversification and distribution to new habitats occur? (3) What kinds of embryos are found in seeds of Solanaceae? (4) What class(es) of seed dormancy is (are) in the family in various parts of the world? (5) What is the relationship between seed dormancy and embryo morphology? (6) How is seed dormancy distributed in the eight subfamilies of Solanaceae and in the various vegetation regions on earth? (7) What are the dormancy-breaking and germination requirements? (8) Do species of Solanaceae form persistent soil seed banks? (9) Why do so few species of Solanaceae grow in northern latitudes with cold winters?
Overview of Solanaceae
The Solanaceae belongs to the Solanales, which consists of five families in two clades: (1) Hydroleaceae, Montiniaceae and Sphenocleaceae, and (2) Convolvulaceae and Solanaceae (Refulio-Rodríguez and Olmstead, Reference Refulio-Rodríguez and Olmstead2014). The Convolvulaceae and Solanaceae differ from the other members of the Solanales in that they have tropane alkaloids instead of iridoid compounds (Wang et al., Reference Wang, Tian, Yu, Li, Xua, Chen, D’Auriad, Huang and Huang2023). The Solanaceae has eight subfamilies (Cestroideae, Duckeodendroideae, Goetzeoideae, Nicotianoideae, Petunioideae, Schizanthoideae, Schwenckioideae and Solanoideae), 18 tribes, 103 genera and 2729 species (POWO, website, 2024). Solanum (Solanoideae) is the largest genus with 1237 species (POWO, website, 2024).
The family includes trees, shrubs, lianas/vines, herbaceous perennials, biennials and annuals (Barboza et al., Reference Barboza, Hunziker, Bernardello, Cocucci, Moscone, García, Fuentes, Dillon, Bittrich, Cosa, Subils, Romanutti, Arroyo, Anton, Kadereit and Bittrich2016). Trees (e.g. Duckeodendron) grow only in wet tropical forests (Fay et al., Reference Fay, Olmstead, Richardson, Santiago, Prance and Chase1998), while shrubs are found in a diversity of habitats from deserts (Lycium and Solanum) to wet tropical forests (Lycianthes and Solanum) (Baskin and Baskin, Reference Baskin and Baskin2014). Various species of lianas/vines occur in the tropics. In the Neotropics, climbing species of Solanaceae occur in Capsicum, Cestrum, Dyssochroma, Hawkesiophyton, Juanulloa, Lycianthes, Lycium, Markea, Merinthopodium, Poortmannia, Salpichroa, Schultesianthus, Schwenckia, Solandra, Solandum, Trianaea and Witheringia (Acevedo-Rodríguez, Reference Acevedo-Rodríguez2020). In wet tropical forest, some species of Dyssochroma, Hawkesiophyton, Juanulloa, Lycianthes, Markea, Merinthopodium, Poortmannia, Schultesianthus and Trianaea grow as epiphytic shrubs and lianas/vines (Acevedo-Rodríguez, Reference Acevedo-Rodríguez2020). A new genus (Doselia) of Solanaceae with four species of hemiepiphytic lianas has been found in moist premontane forests of the Andes in Colombia and Ecuador (Orejuela et al., Reference Orejuela, Villanueva, Orozco, Knapp and Särkinen2022). Many species of tribe Juanulloeae (e.g. Markea) not only are neotropical epiphytes, but they also are myrmeophilous with swollen leaf bases (Knapp et al., Reference Knapp, Persson and Blackmore1997).
Leaves lack stipules and are simple, pinnate or cylindrical and alternate but sometimes opposite or in whorls of three to six. Leaves of some herbaceous plants (Nolana, Sclerophylax) are succulent (Barboza et al., Reference Barboza, Hunziker, Bernardello, Cocucci, Moscone, García, Fuentes, Dillon, Bittrich, Cosa, Subils, Romanutti, Arroyo, Anton, Kadereit and Bittrich2016). Trichomes are simple, branched or stellate and sometimes glandular. Stems and leaves of many species are smooth, but those of Solanum subgenus Leptostemonum (350–450 species) have sharp epidermal prickles. Members of this subgenus also have stellate trichomes and tapered anthers with a terminal pore (Wahlert et al., Reference Wahlert, Chiarini and Bohs2014). Vegetative reproduction can occur via formation of stolons (Nierembergia), tubers (Solanum) and root buds (Bouchetia, Leptoglossis and Solanum) (Barboza et al., Reference Barboza, Hunziker, Bernardello, Cocucci, Moscone, García, Fuentes, Dillon, Bittrich, Cosa, Subils, Romanutti, Arroyo, Anton, Kadereit and Bittrich2016).
Flowers occur in a cyme, lax panicle or a fascicle of two to many flowers, but they can be solitary. The bisexual flowers are actinomorphic, rarely zygomorphic. The five-lobel calyx persists during fruit development. The corolla has 5 [4, 6 or 9] petals that are fused at least at the base, resulting in tubular, rotate or salverform flowers that rarely are bilabiate (Schizanthus). The 5 [4 or 8] stamens are adnate to the base of the petals but alternate with corolla lobes. The ovary is superior, and the single pistil is 2-carpellate and usually 2-[3 to 5] loculate. The stigma has two lobes. The ovary has axile placentation, rarely basal, with numerous ovules (Zomlefer, Reference Zomlefer1994; Hunziker, Reference Hunziker2001; Simpson, Reference Simpson2006; Barboza et al., Reference Barboza, Hunziker, Bernardello, Cocucci, Moscone, García, Fuentes, Dillon, Bittrich, Cosa, Subils, Romanutti, Arroyo, Anton, Kadereit and Bittrich2016).
Flowers of Solanaceae are visited by various kinds of insects (bees, butterflies and moths), birds (Knapp, Reference Knapp2010) and bats (Sazima et al., Reference Sazima, Buzato and Sazima2003). Insect-pollinated flowers have evolved many times; bird-pollinated, ten times; moth-pollinated, eight times; and bat-pollinated, two times (Knapp, Reference Knapp2010). Insects, birds and bats collect nectar and/or pollen (Knapp, Reference Knapp2010). Flowers of the epiphytic shrub Dyssochroma viridiflorum in the Atlantic rainforest of southeastern Brazil are visited by bats that collect nectar and fruits (Sazima et al., Reference Sazima, Buzato and Sazima2003). Depending on the species, the zygomorphic flowers of the early-diverging Schizanthus are pollinated by bees, moths and hummingbirds (Pérez et al., Reference Pérez, Arroyo, Medel and Hershkovitz2006). Species of Nierembergia have glands (elaiophores) in the flowers that secrete oil, which is collected by insects (Tapinotaspis spp.) to provision their larvae (Cocucci, Reference Cocucci1991). Oil flowers do not produce nectar, are zygomorphic and bisexual and have elaiophores located at the base of the filaments (Possobom and Machado, Reference Possobom and Machado2017). These authors found that only the Solanaceae and 10 other families (Calceolariaceae, Cucurbitaceae, Iridaceae, Krameriaceae, Malpighiaceae, Orchidaceae, Plantaginaceae, Primulaceae, Scrophulriaceae and Stilbaceae) secrete non-volatile oil that serves as a reward for pollinators.
The fruit is a berry, septicidal capsule or rarely a drupe, pyxidium or mericarp (Knapp, Reference Knapp2002). Capsules are plesiomorphic in Solanaceae, and berries have arisen at least three times (Knapp, Reference Knapp2002; Pabón-Mora and Litt, Reference Pabón-Mora and Litt2011). The endosperm is fleshy with oil and is abundant but scarce in most Juanulloideae. The embryo is straight, curved or coiled and the cotyledons flat or folded (Hunziker, Reference Hunziker2001; Barboza et al., Reference Barboza, Hunziker, Bernardello, Cocucci, Moscone, García, Fuentes, Dillon, Bittrich, Cosa, Subils, Romanutti, Arroyo, Anton, Kadereit and Bittrich2016). Ovules are unitegmic (Corner, Reference Corner1976). The seed coat is multiplicative or not, is generally reduced and does not have a palisade layer of Malpighian cells (Souèges, Reference Souèges1907; Corner, Reference Corner1976; Bhati et al., Reference Bhati, Singh, Saini, Maheshwari and Sharma2017). According to Bhati et al. (Reference Bhati, Singh, Saini, Maheshwari and Sharma2017), the seed coat of Atropa, Capsicum, Datura, Solanum and Withania consists of ‘the epidermis, hypodermis and parenchymatous layers’.
The Solanaceae contains important food plants for humans such as brinjal eggplant (Solanum melongena), bush tomato (S. centrale), cocona (S. sessiliflorum), Gboma eggplant (S. macrocarpon), lulo (S. quitoense), pepino (S. muricatum), peppers (Capsicum spp.), potato (S. tuberosum), scarlet eggplant (S. aethiopicum), tomatillos (Physalis philadelphica), tomato (S. lycopersicum) and tree tomato (S. betaceum) (Hunziker, Reference Hunziker2001; Hilgenhof et al., Reference Hilgenhof, Gagnon, Knapp, Aubriot, Tepe, Bohs, Giacomin, Gouvêa, Martine, Orejuela, Orozco, Peralta and Särkinen2023). Solanum tuberosum is the third most important food crop in the world, after rice and wheat (Birch et al., Reference Birch, Bryan, Fenton, Gilroy, Hein, Jones, Prashar, Taylor, Torrance and Toth2012), and its production is a vital part of global food security for humans (Devaux et al., Reference Devaux, Goffart, Petsakos, Kromann, Gatto, Okello, Suarez, Hareau, Campos and Ortiz2020). Further, S. lycopersicum is one of the world’s most important cultivated vegetables (Ramírez-Ojada et al., Reference Ramírez-Ojada, Peralta, Rodríguez-Guzmán, Sahagún-Castellanos, Chávez-Servia, Medina-Hinostroza, Rijalba-Vela, Vásquez-Núñez and Rodríguez-Pérez2021).
Many kinds of chemical compounds in the Solanaceae, including tropane, pyrrolidine and phyrrolic alkaloids (Hanuš et al., Reference Hanuš, Řezanka, Spížek and Dembitsky2005; Shelar et al., Reference Shelar, Ghogare, Gaikwad, Pardeshi and Katkale2022), can cause sickness and possibly death if ingested by humans (Dauncey and Larsson, Reference Dauncey and Larsson2018). Thus, species such as Atropa belladonna, Datura stramonium, Hypscyamus niger, Mandragora officinarum and Solanum dulcamara are considered to be poisonous to humans (Hunziker, Reference Hunziker2001). Some tropane alkaloids, however, long have been used as medicines (Shah et al., Reference Shah, Shah and Patrekar2013), and research is ongoing to find additional medicinal uses of the diversity of tropane alkaloids, as well as the steroidal lactones, saponins, terpenes, flavonoids and phenolics found in the Solanaceae (Evans, Reference Evans and D’Arcy1986; Parr et al., Reference Parr, Payne, Eagles, Chapman, Robins and Rhodes1990; Hunziker, Reference Hunziker2001; Kohnen-Johannsen and Kayser, Reference Kohnen-Johannsen and Kayser2019; Jan et al., Reference Jan, Iram, Bashier, Shah, Kamal, Rahman, Kim and Jan2024). Many species of Solanaceae such as Brugmansia, Petunia, Salpiglossis, Schizanthus and Solanum are grown as ornamentals.
Palaeohistory: origin
Various crown ages for Solanaceae have been found/proposed: 30 Ma (Särkinen et al., Reference Särkinen, Bohs, Olmstead and Knapp2013), 41 Ma (Wikström et al., Reference Wikström, Savolainen and Chase2001), 73 Ma (Huang et al., Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023), 81.99 and 109.99 Ma (Zuntini et al., Reference Zuntini, Carruthers, Maurin, Bailey, Leepoel, Brewer, Epitawalage, Freançoso, Gallego-Paramo, McGinnie, Negrão and Roy2024). In a study on the evolution of Ipomoea aquatica, Hao et al. (Reference Hao, Bao, Li, Gagoshidze, Shu, Yang, Cheng, Zhu and Wang2021) concluded that Convolvulaceae diverged from the Solanaceae 71.7 Ma. However, Carruthers et al. (Reference Carruthers, Muñoz-Rodríguez, Wood and Scotland2020) placed the divergence of Convolvulaceae and Solanaceae at 130–97 Ma and the crown age of Solanaceae at 99–67 Ma.
Fossils of Physalis infinemunfi with a highly inflated five-lobed calyx have been collected from Early Eocene (52.2 Ma) deposits in Gondwanan Patagonia (Argentina), leading Wilf et al. (Reference Wilf, Carvalho, Gandolfa and Cúneo2017) to conclude that the Solanaceae was well diversified long before final breakup of Gondwanan. Also, fossil berries of Eophysaloides inflata collected in Columbia from Middle to Late Eocene (47.3–33.9 Ma) and Lycianthoides calycina collected in Colorado (USA) from Early Eocene (51.5–49.5 Ma) suggest that the Solanaceae was distributed from South American to North America by the Early Eocene (Deanna et al., Reference Deanna, Martínez, Manchester, Wilf, Campos, Knapp, Chiarini, Barboza, Bernardello, Sauquet, Dean, Orejuela and Smith2023). Thus, there is considerable support for the older crown ages of Solanaceae, which occurred after the separation of South America and Africa between 135 and 105 Ma. Huang et al. (Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023) stated that ‘Solanaceae may have originated in the Late Cretaceous and diversified before the Cretaceous-Paleogene (K-Pg) boundary ….’ Särkinen et al. (Reference Särkinen, Bohs, Olmstead and Knapp2013) concluded that Solanaceae originated and subsequently diversified in South America.
A whole-genome duplication (WGD) event is associated with the origin of angiosperms (De Bodt et al., Reference De Bodt, Maere and Van de Perr2005), and Jiao et al. (Reference Jiao, Wickett, Ayyampalayam, Chanderbali, Landherr, Ralph, Tomsho, Hu, Liang, Soltis, Soltis, Clifton, Schlarbaum, Schuster and Ma2011) dated it at 190–130 Ma. A whole-genome triplication (WGT), i.e. two closely occurring WGDs or hexaploidization, known as the gamma event, is associated with the origin of the eudicots (Jiao et al., Reference Jiao, Leebens-Mack, Ayyampalayam, Bowers, McKain, McNeal, Rolf, Ruzicka, Wafula, Wickett, Wu, Zhang, Wang, Zhang and Carpenter2012). In the long history of evolution of the eudicots, many WGD/WGT events have occurred, and the origin of various families is correlated with the time of a polyploidization event (Soltis et al., Reference Soltis, Smith, Cellinese, Wurdack, Tank, Brockinngton, Refulio-Rodriguez, Walker, Moore, Carlsward, Bell, Latvis, Crawley, Black and Diouf2011, Reference Soltis, Soltis, Endress, Chase, Manchester, Judd, Majure and Mavrodiev2018; Almeida-Silva and Van de Peer, Reference Almeida-Silva and Van de Peer2023).
After the divergence of the Solanaceae and Convolvulaceae (see above), a triplication event occurred in the Solanaceae (Bombarely et al., Reference Bombarely, Moser, Amrad, Bao, Bapaume, Barry, Bliek, Boersma, Borghi, Burggmann, Bucher, D’Agostino, Davies, Druege, Dudareva, Egea-Cortines, Delledonne, Fernandez-Pozo, Franken, Grandont, Heslop-Harrison, Hintzsche, Johns, Koes, Lv, Lyons, Malla, Martinoia, Mattson, Morel, Mueller, Muhlemann, Nouri, Passeri, Pezzotti, Qi, Reinhardt, Rich, Richert-Pöggeler, Robbins, Schatz, Eric Schranz, Schuurink, Schwarzacher, Spelt, Tang, Urbanus, Vandenbussche, Vijverberg, Villarino, Warner, Weiss, Yue, Zethof, Quattrocchio, Sims and Kuhlemeier2016; Wu et al., Reference Wu, Lau, Cao, Hamilton, Sun, Zhou, Eserman, Gemenet, Olukolu, Wang, Crisovan, Godden, Jiao, Wang and Kitavi2018; Cao et al., Reference Cao, Schöttner, Halitschke, Li, Baldwin, Rocha and Baldwin2021a; Zhang et al., Reference Zhang, Zhang, Xiao, Wu, Zhang, Xu, Bao, Wang, Li, Wang and Wang2022). Zhang et al. (Reference Zhang, Zhang, Xiao, Wu, Zhang, Xu, Bao, Wang, Li, Wang and Wang2022) concluded that a common Solanaceae hexaploidization occurred 49–43 Ma and was an allohexaploidization event. However, the Tomato Genome Consortium (2012) estimated that the hexaploidization occurred at 71 Ma, and Huang et al. (Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023) placed this event at c. 81 Ma, before the Schizanthoideae diverged from the other subfamilies of Solanaceae at c. 73 Ma. On the phylogenetic tree of Solanaceae (minus the branch for Duckeodentron), Huang et al. (Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023) found seven nodes with high gene duplications bursts (i.e. ≥300 gene duplications), suggesting that WGD/WGT events have occurred at different times.
Many molecular phylogenetic studies have been conducted on the Solanaceae (e.g. Olmstead and Palmer, Reference Olmstead and Palmer1992; Spooner et al., Reference Spooner, Anderson and Jansen1993; Santiago-Valentin and Olmstead, Reference Santiago-Valentin and Olmstead2003; Martins and Barkman, Reference Martins and Barkman2005; Olmstead et al., Reference Olmstead, Bohs, Migid, Santiago-Valentin, García and Collier2008; Särkinen et al., Reference Särkinen, Bohs, Olmstead and Knapp2013; Huang et al., Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023). The genus Schizanthus (Schizanthoideae) is at the base of the phylogeny and is sister to all other Solanaceae (Olmstead et al., Reference Olmstead, Bohs, Migid, Santiago-Valentin, García and Collier2008). The clade next to Schizanthus is the subfamily Goetzeoideae with Duckeodendron unplaced (Särkinen et al., Reference Särkinen, Bohs, Olmstead and Knapp2013; Huang et al., Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023). These authors refer to Schizanthus as Clade I and Goetzeoideae as Clade II. Clade III is Schwenckieae plus Cestroideae. Clade IV is Petunieae, Clade V Nicotianoideae and Clade VI Solanoideae. Duckeodendron is now placed in the Duckeodrendroideae (Reveal, Reference Reveal2012), but its relationship to other subfamilies has not been resolved (Huang et al., Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023). Divergence of Solanaceae Clade II is 68.9 Ma; Clade III, 64.2 Ma; Clade IV, 63.5; Clade V, 61.3 MA; and Clade VI, 61.3 Ma (Huang et al., Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023). The rapid divergence of Clade VI (Solanoideae) occurred during the Paleocene-Eocene Thermal Maximum and Early Eocene Climate Optimum (Huang et al., Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023).
Palaeohistory: diversification and dispersal
The results of various molecular phylogenetic studies have revealed that diversification occurred in numerous lineages and genera of Solanaceae during the Eocene, Miocene and Pliocene and that often the appearance of new species was accompanied by establishment in new habitats.
Brunfelsia
This genus of mostly medium-size shrubs has a crown age of 18–16 Ma (Miocene), and its ancestral region is in ‘eastern Brazil, southeastern South America and the Amazon Basin’ (Filipowicz and Renner, Reference Filipowicz and Renner2012). These authors determined that dispersal to Cuba, Hispaniola, Jamaica and Puerto Rico (9–4 Ma) was followed by diversification of new species. In addition to the 23 species of Brunfelsia in the Antilles, species of this genus grow in eastern Brazil, southeastern South America and the Amazon Basin; Pacific coastal region of South American; foothills and montane zone of the Andes; and the Guiana Shield (Filipowicz and Renner, Reference Filipowicz and Renner2012).
Capsicum
The genus is endemic to the Americas, and the likely ancestral range is the Andes. Diversification of the nine clades of Capsicum began in the mid- to upper Miocene, after which each lineage diversified in a different region of South American in the Pliocene through the Pleistocene. Lineages giving rise to the domesticated species of Capsicum grew in the central Andes, i.e. Bolivia (García et al., Reference García, Barboza, Palombo and Weiss-Schneeweiss2022). Today, species of Capsicum are widely cultivated and are used as food in both the New and Old World (Eshbaugh, Reference Eshbaugh and Russo2012; García et al., Reference García, Barboza, Palombo and Weiss-Schneeweiss2022); 22 landraces of C. annum have been identified in Mexico (Taitano et al., Reference Taitano, Bernau, Jardón-Barbolla, Leckie, Mazourek, Mercer, McHale, Michel, Baumler, Kantar and van der Knaap2018).
Lycium
This genus of woody plants is found in temperate and dry tropical zones of Africa, Asia, Australia, North America and South America; it is not found in wet tropical habitats. Cao et al. (Reference Cao, Li, Fan, Li, Yoshida, Wang, Ma, Wang, Mitsuda, Kotake, Ishimizu, Tsai, Niu, Zhang and Sun2021b) determined that the first diverging species in the phylogeny was L. ruthenicum in northern Africa. However, these authors were not sure if the genus originated in South America and migrated to Africa or if it originated in Africa. Their results indicate that there was migration from northern Africa to Asia where speciation occurred, after which the genus migrated to North America. The Asian and North American species belong to the same clade. Migration from Asia to Australia and from North America to South America may have occurred, but these migrations have not been confirmed. Results from phylogenetic studies involving the 15 species of Lycium in China indicated that Lycium diverged from its sister genus 17.7 Ma, and speciation in China mostly occurred in the Pliocene (Zhang et al., Reference Zhang, Zhang, Wei and Zheng2024).
Mandragora
Tribe Mandragoreae is monogeneric with four species: M. autumnalis, Mediterranean to W Iran; M. caulescens, Nepal to China; M. officinarum, N Italy to NW Balkan Peninsula; and M. turcomanica, NNE Iran to S Turkmenistan (POWO, website, 2024). The genus originated in the Eocene in the Mediterranean region (Volis et al., Reference Volis, Fogel, Tu, Sun and Zaretsky2018). The Mediterranean/Turanian and Tibetan clades diverged c. 20.5 Ma, and the western Mediterranean and near east Turanian clades diverged 11.1 Ma (Volis et al., Reference Volis, Fogel, Tu, Sun and Zaretsky2018).
Petunia
The pampas region of Brazil is the ancestral area for Petunia, and Reck-Kortmann et al. (Reference Reck-Kortmann, Silva-Arias, Segatto, Mäder, Bonatto and Brandão de Freitas2014) concluded that the ancestor may have had bee-pollinated flowers with a short corolla. Two clades developed: Clade I flowers are purple with a short corolla and bee pollinated, and Clade II (three subspecies of P. axillaris) flowers are white with a long corolla (except subspecies occidentalis with a short corolla) and bee, bird or moth pollinated, depending on the subspecies (Reck-Kortmann et al., Reference Reck-Kortmann, Silva-Arias, Segatto, Mäder, Bonatto and Brandão de Freitas2014). Phylogenetic studies of Petunia by Soares et al. (Reference Soares, Stehmann and Freitas2025) revealed that the two clades of Petunia originated c. 2.5 Ma in the early Pleistocene, and divergence in Clades I and II began about 1.0 and 0.8 Ma, respectively.
Solanum
This genus has a crown and stem age estimated at 53.2 and 44.98 Ma, respectively, (early Eocene) with ‘most clades diverging between 35 and 27 Ma’ (Messeder et al., Reference Messeder, Carlo, Zhang, Tovar, Arana, Huang, Huang and Ma2024). The centre of diversity of Solanum is the Andes Mountains in South America with small centres of diversity in the Atlantic rainforest of Brazil, Amazon Basin and mountains of Central America (Echeverría-Londoño et al., Reference Echeverría-Londoño, Särkinen, Fenton, Purvis and Knapp2020; Tovar et al., Reference Tovar, André, Wahlert, Bohs and Giacomin2021). Divergence of extant species of Solanum begins in the middle Eocene. Clades I and II diverged at 42.49 Ma, and the Petota (potato and close relatives) and Tomato clades split c. 16.85 Ma (Messeder et al., Reference Messeder, Carlo, Zhang, Tovar, Arana, Huang, Huang and Ma2024). According to Echeverría-Londoño et al. (Reference Echeverría-Londoño, Särkinen, Fenton, Purvis and Knapp2020), dispersal of Solanum from the Neotropics to the Old World was followed by diversification. The spiny clade of Solanum has had its highest rate of diversification in the Old World, i.e. 0.68 lineages Myr−1. The global mean rate of Solanum speciation is estimated to be 0.25 lineages Myr−1 (Echeverría-Londoño et al., Reference Echeverría-Londoño, Särkinen, Fenton, Purvis and Knapp2020).
The six most evolutionary labile traits (>100 transitions) of Solanum are as follows: (1) growth form (herb, shrub, tree, herbaceous vine, woody vine and epiphyte), (2) type of glandular trichomes (absent, simple, glandular and stellate), (3) sympodial unit structure (plurifoliate, trifoliate, defoliate non-geminate, defoliate geminate and unifoliate), (4) corolla shape (deeply stellate, broadly stellate and rotate), (5) corolla colour (white, green, purple and yellow) and (6) fruit colour (green, white, purple, yellow, orange and red) (Hilgenhof et al., Reference Hilgenhof, Gagnon, Knapp, Aubriot, Tepe, Bohs, Giacomin, Gouvêa, Martine, Orejuela, Orozco, Peralta and Särkinen2023). On the other hand, conserved traits (<10 to 10 to 49 transitions) of Solanum with high taxonomic value are as follows: (1) pseudostipules (absent, present-single and present-pair), (2) corolla bilateral symmetry (present and absent), (3) anther shape (cylindrical, tapered and cordate), (4) pedicel insertion (flat and cup-shaped), (5) pedicel articulation (basal, near-basal, basal ¼ to ½, distal ½ and absent) and (6) stone cells (absent and present).
Due to the critical role that Solanum tuberosum plays in the food supply for humans, much research has been conducted on its origin and evolutionary history (e.g. Spooner et al., Reference Spooner, McLean, Ransay, Waugh and Bryan2005, Reference Spooner, Núñez, Reujillo, Herrara, Guzmán and Ghislain2007, Reference Spooner, Ghislain, Simon, Jansky and Gavrilenko2014; Sotomayor et al., Reference Sotomayor, Ellis, Salas, Gomez, Sanchez, Carrillo, Giron, Quispe, Manrique-Carpintero, Anglin and Zorrilla2023; Zhang et al., Reference Zhang, Zhang, Ding, Wang, Ma, Gagnon, Jia, Cheng, Bao, Liu, Wu, Hu, Lian, Lin, Wang, Ye, Wang, Zhang, Zhou, Liu, Li, Lucas, Särkinen, Knapp, Rieseberg, Liu and Huang2025). The cultivated potato and its wild relatives belong to section Petota of Solanum, and this is a monophyletic lineage in the genus Solanum derived from a hybridization event c. 8.6 Ma (Zhang et al., Reference Zhang, Zhang, Ding, Wang, Ma, Gagnon, Jia, Cheng, Bao, Liu, Wu, Hu, Lian, Lin, Wang, Ye, Wang, Zhang, Zhou, Liu, Li, Lucas, Särkinen, Knapp, Rieseberg, Liu and Huang2025). Phylogenetic studies by Zhang et al. (Reference Zhang, Zhang, Ding, Wang, Ma, Gagnon, Jia, Cheng, Bao, Liu, Wu, Hu, Lian, Lin, Wang, Ye, Wang, Zhang, Zhou, Liu, Li, Lucas, Särkinen, Knapp, Rieseberg, Liu and Huang2025) reveal that the hybridization occurred between a diploid species of section Tomato and a diploid species of section Etuberosum (of Solanum), which have underground re-sprouting organs. These authors estimate that in the six species of Petota that they analysed c. 60 and 40% of the ancestry were from Etuberosum and Tomato, respectively. All Petota species produce tubers, which are thought to be a major innovation resulting from the hybridization.
After the hybrid with tubers was formed, rapid speciation occurred, especially in cool and relatively dry habitats (Zhang et al., Reference Zhang, Zhang, Ding, Wang, Ma, Gagnon, Jia, Cheng, Bao, Liu, Wu, Hu, Lian, Lin, Wang, Ye, Wang, Zhang, Zhou, Liu, Li, Lucas, Särkinen, Knapp, Rieseberg, Liu and Huang2025). Today, there are four cultivated species of potato (Solanum ajanhuiri, S. curtilobium, S. juzepczukii and S. tuberosum) (Spooner et al., Reference Spooner, Núñez, Reujillo, Herrara, Guzmán and Ghislain2007) and 107 wild tuber-producing Solanum species (Spooner et al., Reference Spooner, Ghislain, Simon, Jansky and Gavrilenko2014). Depending on the species, wild potato relatives grow in Argentina, Bolivia, Chile, Columbia, Ecuador, Guatemala, Honduras, Mexico, Peru and Venezuela; Peru also has the highest number of species (51) of wild potatoes (Spooner et al., Reference Spooner, Ghislain, Simon, Jansky and Gavrilenko2014). Further, in addition to the cultivated tomato (S. lycopersicum), there are 12 wild tomato species in western South America ranging from central Ecuador to northern Chile with two species in the Galápagos Islands (Peralta et al., Reference Peralta, Knapp and Spooner2005; Ramírez-Ojada et al., Reference Ramírez-Ojada, Peralta, Rodríguez-Guzmán, Sahagún-Castellanos, Chávez-Servia, Medina-Hinostroza, Rijalba-Vela, Vásquez-Núñez and Rodríguez-Pérez2021).
In summary, a major point about the palaeohistory of the Solanaceae is that there has been much dispersal and diversification, resulting in species that currently grow on all continents except Antarctic and in all major vegetation regions excluding alpine/tundra. Moving forward in this review, we will see that much diversification also has occurred with respect to embryo morphology but not in class of seed dormancy. That is, in this family, species growing in the same vegetation region may have seeds with physiological dormancy (PD), but the seeds vary with regard to embryo morphology.
Embryo morphology
In our consideration of the embryos in Solanales and Solanaceae, we find linear, spatulate and folded embryos (sensu Martin, Reference Martin1946), all of which are fully developed at seed maturity and do not grow inside the seed prior to radicle emergence (Baskin and Baskin, Reference Baskin and Baskin2014, Reference Baskin and Baskin2021). Linear embryos are longer than wide, and the cotyledons are the same width as the hypocotyl/radicle (stalk). Spatulate embryos have cotyledons that are noticeably wider than the stalk, and the full length of the stalk is visible below the cotyledons. In folded embryos, the cotyledons are wider than the stalk and are folded/twisted together, and the full length of the stalk is visible, i.e. not covered by the cotyledons.
On Martin’s (Reference Martin1946) family tree of seed phylogeny, families with a linear embryo are above the base of the tree, and they are at the junction of the two branches of the tree. One branch goes from seeds with a linear embryo to dwarf seeds and then to micro seeds. The second branch goes in sequence from seeds with linear embryos to those with spatulate, bent, folded and investing embryos, i.e. investing embryos are at the top of the tree. At the linear embryo position on Martin’s tree, there are two kinds of linear embryos. One, the linear embryo is underdeveloped (small with organs) and must grow inside the seeds before germination (radicle emergence) occurs. Two, the linear embryo is fully developed and does not grow inside the seed prior to germination. These two kinds of linear embryos are grouped together at the same location on Martin’s tree. That is, his families with linear embryos include the Apiaceae with underdeveloped linear embryos and Solanaceae and Linaceae with fully developed linear embryos. Hereafter, linear fully developed embryos are referred to as linear-full, sensu Baskin and Baskin (Reference Baskin and Baskin2007).
In the Solanales, one clade consists of Montiniaceae with spatulate embryos (Takhtajan, Reference Takhtajan2000; Kirkbride et al., Reference Kirkbride, Gunn and Dallwitz2006) that is sister to Sphenocleaceae with linear-full embryos (Martin, Reference Martin1946; Monod, Reference Monod1980) and Hydroleaceae with linear-full embryos (Martin, Reference Martin1946; Goldberg, Reference Goldberg1986; Watson and Dallwitz, Reference Watson and Dallwitz1992 onward). The second clade in the Solanales consists of Convolvulaceae with linear-full and folded embryos (Martin, Reference Martin1946; Hutchinson, Reference Hutchinson1959; Kirkbride et al., Reference Kirkbride, Gunn and Dallwitz2006) and Solanaceae with linear-full and spatulate embryos (Martin, Reference Martin1946; Hunziker, Reference Hunziker2001).
Literature searches resulted in us finding drawings/photographs/descriptions of the embryo morphology for 236 species in 87 genera of Solanaceae. Some information on embryo morphology has been found for the eight subfamilies of Solanaceae (Table S1): Cestroideae, 9 genera, 20 species; Duckeodendroideae, 1, 1; Goetzeioideae, 2, 2; Nicotianoideae, 7, 17; Petunioideae, 8, 15; Schizanthoideae, 1, 5; Schwenckioideae, 4, 5; and Solanoideae, 58, 169. There are six general types of embryo morphology in seeds of the Solanaceae (Fig. 1): spatulate (Fig. 1A) and five variations of linear-full, i.e. straight, slightly curved, curved, strongly curved and spiralled (Fig. 1B–F).
Illustrations of the diversity of embryo morphology in Solanaceae seeds. Spatulate embryo (A) and linear-full embryos (B–F): B, straight; C, slightly curved; D, curved; E, strongly curved; and F, spiralled. Em, embryo; en, endosperm, sc, seed coat.
For 232 of the 236 species, we examined drawings or photographs of the embryo, but in four species the authors wrote that the embryo was linear but did not describe it in detail. Thirty-one species had a spatulate embryo, and this kind of embryo was in the Cestroideae, Petunioideae, Nicotianoideae and Solanoideae (Table 1). Curved linear embryos are in all subfamilies except Goetzeioideae, which has a straight linear embryo. Straight linear embryos also occur in the Schizanthoideae, Schwenckioideae and Nicotianoideae. On Martin’s (Reference Martin1946) family tree of seed phylogeny, the linear embryo is placed below the spatulate embryo. When we consider the stem ages of the subfamilies, it seems that the spatulate embryo may have appeared in the Solanaceae after the linear embryo, but more studies are needed on embryos of the early diverging subfamilies.
Kind/shape of embryo in 232 of 236 species of Solanaceaea
Linear-full embryos: STR, straight; SLCU, slightly cured; CU, curved; STCU, strongly curved; SP, spiralled; -, not found.
a Four species with linear-full embryos were not identified to general shape of embryo.
b Stem age of subfamily according to Huang et al. (Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023).
c Not placed by Huang et al. (Reference Huang, Xu, Zhai, Hu, Guo, Zhang, Zhao, Zhang, Martine, Ma and Huang2023).
Seed dormancy and germination
In addition to information on seed dormancy and germination of Solanaceae species, we collected from the literature, beginning in about 1988, a web-based search that included the name of each genus and subfamily of Solanaceae and the words seed dormancy, seed germination, semillas, sementes, germinación and germinação was conducted in May–June 2025. The database includes the life form of each species and the vegetation region where it grows. The temperature regime(s) at which the highest percentages of germination was (were) obtained and the most favourable light/dark conditions for germination were recorded. Information was found for 167 species in 32 genera: Cestroideae, 2 genera, 5 species; Duckeodendroideae, 1, 1; Goetzeoideae, 1, 2; Nicotianoideae, 5, 9; Petunioideae, 4, 7; Schizanthoideae, 1, 5; Schwenckioideae, 1, 1; and Solanoideae, 17, 136 (Table S2).
Seeds of Solanaceae readily imbibe water. For example, intact mericarps/seeds of Nolana divaricata, N. jaffuelii, N. linerifolia and N. sedifolia were fully imbibed after 48 hours (Hepp et al., Reference Hepp, Gómez, León-Lobos, Montenegro, Vilalobos and Contreras2021) and intact seeds of Datura ferox after 24 hours (Soriano et al., Reference Soriano, Sánchez and de Eilberg1964), Solanum lycocarpum in c. 10–12 hours, S. aphydendron in 15 hours (Zuloaga-Aguilar et al., Reference Zuloaga-Aguilar, Briones and Orozco-Segovia2010) and eight species of Solanum in Australia in 24–48 hours (Commander et al., Reference Commander, Merritt, Rokich, Flematti and Dixon2008). Thus, if freshly matured seeds of Solanaceae species take longer than about 4 weeks to germinate, we conclude that they have PD (Baskin and Baskin, Reference Baskin and Baskin2014). That is, the embryo in seeds with PD has a germination-inhibiting mechanism or low growth potential. After seeds imbibe water, the embryo does not exert enough force to break the seed coat or any other embryo-covering structures such as endosperm (Nikolaeva, Reference Nikolaeva1969; Baskin and Baskin, Reference Baskin and Baskin2014).
The 167 species of Solanaceae grow in seven tropical and seven temperature vegetation regions with eight subfamilies in the tropics and five subfamilies in the temperate zone (Table 2). Trees (10 species) are found only in the tropics, while 53 and 31 species of shrubs grow in tropical and temperate zones, respectively. In tropical and temperate zones, there are 47 and 25 species of herbs, respectively. Trees, shrubs, vines and herbs are in four, five, one and six subfamilies, respectively (Table 3). PD occurs in 94.5% of the 167 species of Solanaceae. Nondormant (ND) seeds, which germinate quickly after maturation, were recorded in one species of Cestroideae and eight species of Solanoideae (Table S2). Thus, seeds of most Solanaceae have PD, regardless of life form, subfamily, embryo morphology and vegetation region in which the species grows.
Number of species of Solanaceae of each growth form in each vegetation region for which seed dormancy/germination data were found (see Table S2)
-, no information available.
Presence of various growth forms in subfamilies of Solanaceae in tropical and temperate zones of the world
x, yes; -, no.
The three levels of PD are nondeep, intermediate and deep, and nondeep is the most common, occurring in all vegetation zones on earth and in numerous plant families including the Solanaceae (Baskin and Baskin, Reference Baskin and Baskin2014). Nondeep PD can be broken by relatively short periods (8–12 weeks) of moist cold (ca. 0–10°C) or warm (≥15°C) stratification. In temperate regions, intermediate and deep PD are broken by 13–16 weeks and 13–24 weeks of cold stratification, respectively (Baskin and Baskin, Reference Baskin and Baskin2022). However, if seeds with intermediate PD are given c. 4 weeks of warm stratification at summer temperatures, the cold stratification period required to break PD is decreased to about 8–10 weeks (Baskin and Baskin, Reference Baskin and Baskin1995). The amount of cold stratification required to break PD varies with the species: Physalis longifolia, 12 weeks (Bandara et al., Reference Bandara, Finch, Walck, Hidayati and Havens2019), Schizanthus spp., 8 and 13 weeks (Moreno et al., Reference Moreno, Gómez and Contreras2024), Solanum americanum, 3 weeks (Zhou et al., Reference Zhou, Deckard and Messersmith2005b), S. nigrum, 6 weeks (Wagenvoort and van Opstal, Reference Wagenvoort and van Opstal1979; Bithell et al., Reference Bithell, McKenzie, Bourdôt, Hill and Wartten2002) and S. sarrachoides, 12 weeks (Zhou et al., Reference Zhou, Deckard and Ahrens2005a).
Dry storage of seeds with nondeep PD frequently results in dormancy-break, i.e. after-ripening (Baskin and Baskin, Reference Baskin and Baskin2022). During dry storage, dormancy was broken in seeds of Datura ferox (Soriano et al., Reference Soriano, Sánchez and de Eilberg1964), Schizanthus spp. (Moreno et al., Reference Moreno, Gómez and Contreras2024), Physalis angulata (Ozaslan et al., Reference Ozaslan, Farooq, Onen, Ozcan, Bukun and Gunal2017), P. peruviana (Gaier et al., Reference Gaier, Hillebrand, Cuchiara, Bortolotto and Koefender2019), P. philadelphica var. immaculata (Ozaslan et al., Reference Ozaslan, Farooq, Onen, Ozcan, Bukun and Gunal2017), Solanum americanum (Ladeira, Reference Ladeira1997), S. incanum (Joshua, Reference Joshua1978), S. melongena (Yogeesha et al., Reference Yogeesha, Upreti, Padmini, Bhanuprakash and Murth2006), S. sarrachoides (Zhou et al., Reference Zhou, Deckard and Ahrens2005a), S. stramonifolium, S. torvum (Hayati et al., Reference Hayati, Sukprakarn and Juntakool2005) and S. viarum (Vicente, Reference Vicente1972). After cold stratification for 1, 2 and 9 months, seeds of Nicandra physalodes germinated to only 3.9, 7.2 and 0%, respectively (Watanabe et al., Reference Watanabe, Kusagaya and Saigusa2002), and cold stratification (1 week) decreased germination of Withania somnifera seeds (Kambizi et al., Reference Kambizi, Adebola and Afolayan2006). Perhaps, after-ripening would be a good dormancy-breaking treatment to try on seeds of these two species.
Seeds of Solanum species can after-ripen enough to begin to germinate within 1–2 months. Seeds of S. viarum were stored dry in flasks at room temperatures in São Paulo, Brazil, and tested at monthly intervals at 25/18°C (Vicente, Reference Vicente1972). At 2 months, seeds germinated to c. 65 and 20% in light and dark, respectively, and at 5 months they germinated to 95 and 80%, respectively, after which germination began to decline. At 25 months, no seeds germinated; however, the authors did not determine if the seeds had lost viability or entered secondary dormancy. Diploid seeds of potato (S. tuberosum) incubated in water and GA3 1 week after extraction from the fruits germinated to 18.3 and 53.9%, respectively, but after 1 month of dry storage (at room temperatures?) they germinated to 35.6 and 61.7%, respectively (Balderrama et al., Reference Balderrama, Brown-Donovan, Williams, Spencer, Collins and Tan2025).
We have found that a period of drying promotes seed germination of Solanum incompletum from Hawai’i. Seeds incubated on moist sand in Petri dishes at daily alternating temperatures regimes of 15/6, 20/10 and 25/15°C germinated to 53, 17 and 9%, respectively, after 12 weeks and to 57, 31 and 42% after 60 weeks. After 60 weeks, seeds at 15/6 and 20/10°C were allowed to dry at room temperatures for 12 weeks and then returned to 15/6 and 20/10°C, respectively. After an additional 12 weeks at 15/6°C germination had increased from 57 to 90%, and at 20/10°C germinated had increased from 31 to 80%. The wet seeds at 25/15°C remained wet (control) with no increase in germination, i.e. still 42% after a total of 84 weeks (Baskin and Baskin, unpublished).
Nondeep PD also is broken in many species by treatment with gibberellin (GA), whereas GA may or may not promote germination of seeds with intermediate PD. GA has no effect on germination of seeds with deep PD (Baskin and Baskin, Reference Baskin and Baskin2022). Treatment with GA3 promoted seed germination in various species of Solanaceae, including Capsicum annuum (Hernandez-Verdugo et al., Reference Hernandez-Verdugo, Oyama and Vazquez-Yanes2001), Datura ferox (Soriano et al., Reference Soriano, Sánchez and de Eilberg1964), Datura stramonium (Popay, Reference Popay1974), Hyoscyamus niger (Verma et al., Reference Verma, Verma and Verma2014), Jaltomata procumbens (Saldívar-Iglesias et al., Reference Saldívar-Iglesias, Lagua-Cerda, Gutiérrez-Rodríguez and Domínguez-Galindo2010), Nicandra physalodes (Lovey et al., Reference Lovey, Perisse, Molinelli and Scandaliaris2007), Physalis helicacabum (Nosratti et al., Reference Nosratti, Heidari, Muhammadi and Saeidi2016), Salpiglossis sinuata (Rivas et al., Reference Rivas, Aros, Toledo, Torres, Aguirre, Céspedes, Santander, Prat, Facciuto and Torre2021), Schizanthus spp. (Moreno et al., Reference Moreno, Gómez and Contreras2024), Solanum acaule (Bamberg, Reference Bamberg1999), S. betaceum (Neto et al., Reference Neto, Fabiane, Radaelli, Júnior and Moura2015; Torres-González, Reference Torres-González2019), S. quiroense (Torres-González, Reference Torres-González2019), S. scuticum (Araújo et al., Reference Araújo, Lanssanova, Andrade, Neto, Zuchi and Silva2025), S. sessiliflorum (Silva et al., Reference Silva, Zonetti, Stefanello, Menegaes, Stefanello and Nunes2024), S. torvum (Cutti and Kulckzynski, Reference Cutti and Kulckzynski2016; Jena et al., Reference Jena, Vethamoni, Saraswathi, Natesan, Uma, Garnepudi, Sujanthuya, Sreekumar, Chetry and Arunachalam2024; Wu et al., Reference Wu, Si, Yang, Zhang, Zhang, Okita, Yan and Tiam2024; Priya et al., Reference Priya, Ahamed, Vijayalatha and Geethanjali2025), S. trilobatum (Sundaralingam et al., Reference Sundaralingam, Muniyappan, Kavinesh, Kavichakravarthy and Mathivanan2025) and Withania somnifera (Khanna et al., Reference Khanna, Kumar, Chandra and Verma2013; Malavika et al., Reference Malavika, Viji, Soni, Swapna and Beena2020). Seeds of some species of Solanaceae also are stimulated to germinate when treated with KNO3, e.g. Alkekengi officinarum (Aghilian et al., Reference Aghilian, Khajeh-Hosseini and Anvarkhah2014), Capsicum annuum (Flores-Sánchez et al., Reference Flores-Sánchez, Sandoval-Villa and Uscanga-Mortera2022), Iochroma arborescens (Brito et al., Reference Brito, Bezerra and Pereira2016), Solanum betaceum (Torres-González, Reference Torres-González2019), S. crinitum (Dias-Filho, Reference Dias-Filho, Gascon and Moutinho1998), S. physalifolium (Bithell et al., Reference Bithell, McKenzie, Bourdôt, Hill and Wartten2002) and S. torvum (Cutti and Kulckzynski, Reference Cutti and Kulckzynski2016; Jena et al., Reference Jena, Vethamoni, Saraswathi, Natesan, Uma, Garnepudi, Sujanthuya, Sreekumar, Chetry and Arunachalam2024).
Smoke water prepared by bubbling smoke from burning pine-oak forest litter in Mexico increased germination of S. aphydodendron seeds from c. 50 to 60%, while ash increased germination from c. 50 to 78% However, seeds given a dry-heat treatment at 60°C for 5 min plus smoke water germinated to 90% (Zuloaga-Aguilar et al., Reference Zuloaga-Aguilar, Briones and Orozco-Segovia2011). In additional experiments, heat treatments were very effective in promoting germination of S. aphydodendron seeds. About 100% of the seeds germinated after dry heat at 60, 80 and 120°C for 5 min and after dry heat at 60 and 80°C for 60 min. Wet heat at 60 and 80°C for 5 min and wet heat at 60°C for 60 min also resulted in c. 100% germination. Seeds were killed by dry heat at 120°C for 60 min, wet heat at 80°C for 60 and wet and dry heat at 100 and 120°C for 5 and 60 min (Zuloaga-Aguilar et al., Reference Zuloaga-Aguilar, Briones and Orozco-Segovia2010).
Aerial smoke had no effect on germination of S. aculeastrum seeds (Koduru et al., Reference Koduru, Grierson and Afolayan2006). Soaking seeds of Solanum tomentosum in a 1/10 concentration of smoke water promoted germination (82%), while a diluted 1/100 concentration of smoke water had little effect on germination (6.7%) (Aliero and Afolayan, Reference Aliero and Afolayan2010). The effects of smoke water and karrikinolide isolated from smoke were tested on seed germination of eight species of Solanum in Australia (Commander et al., Reference Commander, Merritt, Rokich, Flematti and Dixon2008). At 26/13 and 33/18°C, smoke water and karrikinolide significantly increased germination percentages of S. centrale, S. diocicum and S. orbiculatum. At 26/13°C, smoke water and karrikinolide significantly increased germination of S. cunninghamiii seeds, and at 33/18°C karrikinolide significantly increased germination of S. phlomoides seeds. Seeds of S. chippendalei, S. diversiflorum and S. sturtianum showed no response to either smoke water or karrikinolide, but GA3 promoted some germination at both temperature regimes. In additional experiments, seeds of S. centrale and S. orbiculatum treated with smoke water and karrikinolide were incubated at constant temperatures of 10, 15, 20, 25 and 30°C. Seeds of S. centrale treated with smoke water germinated to ≥10% at 15 and 20°C, and those treated with karrikinolide germinated to ≥10% at 10, 15 and 20°C; maximum germination (c. 35%) was for karrikinolide-treated seeds at 20°C. In contrast, seeds of S. orbiculatum treated with smoke water germinated to c. 50, 45, 75, 85 and 70% at 10, 15, 20, 25 and 30°C, respectively, and those treated with karrikinolide germinated to 95–100% at all temperatures.
Commercially available aqueous smoke extract used to flavour food promoted germination of Nicotiana attenuata seeds (Cao et al., Reference Cao, Schöttner, Halitschke, Li, Baldwin, Rocha and Baldwin2021a). Fractionation of this aqueous smoke and subsequent use of the fractions to test germination of N. attenuata seeds revealed that syringaldehyde was the germination-promoting compound.
When PD is broken in response to warm (≥15°C) and/or cold (c. 0–10°C) moist stratification, dry after-ripening or treatment with GA3 or KNO3, the embryo gains enough ‘push power’ to emerge from the structures that cover it. Thus, if seeds with PD are scarified, they may germinate because the mechanical resistance of the seed coat has been removed/reduced. Both mechanical scarification with a blade, pin or sand paper (Rick, Reference Rick1956; Sousa-Silva et al., Reference Sousa-Silva, Ribeiro, Fonseca, Antunes, Ribeiro, Fonseca and Sousa-Silva2001; Gupta, Reference Gupta2003; Lovey et al., Reference Lovey, Perisse, Molinelli and Scandaliaris2007; Gonzaga et al., Reference Gonzaga, Carvalho, Almeida, Rocha, Braga and Nunes2009; Suthar et al., Reference Suthar, Naik and Malani2009; Oyelana, Reference Oyelana2011; Cabrera et al., Reference Cabrera, Hepp, Gómez and Contreras2015; Hepp et al., Reference Hepp, Gómez, León-Lobos, Montenegro, Vilalobos and Contreras2021) and chemical scarification with conc. H2SO4 (Wei et al., Reference Wei, Zhang, Li, Cui, Huang, Cui, Meng and Zhang2009, Reference Wei, Zhang, Chen, Li, Sui, Huang, Cui, Liu, Zhang and Guo2010; Ramamoorthy et al., Reference Ramamoorthy, Geetha and Sujatha2010; Palchetti et al., Reference Palchetti, Lianes, Reginato, Barboza, Luna and Cantero2020a) have been shown to promote germination of various species of Solanaceae. Further, if the embryo is removed from seeds with nondeep PD, it will grow into a normal seedling. However, the embryo removed from seeds with intermediate or deep PD may, or may not, grow, or it could give rise to an abnormal/stunted plant. Embryos from mature seeds of Datura ferox (Soriano et al., Reference Soriano, Sánchez and de Eilberg1964) and Mandragon autumnalis (Al-Ahmad, Reference Al-Ahmad2020) and immature seeds of Capsicum annuum (Manzur et al., Reference Manzur, Oliva-Alarcón and Rodríguez-Burruezo2014) produced healthy plants.
Dormancy cycling also is a characteristic of seeds of many species with nondeep PD (Baskin and Baskin, Reference Baskin and Baskin2014), and it has been documented in seeds of Datura ferox (Reisman-Berman et al., Reference Reisman-Berman, Kigel and Rubin1991), D. stramonium (Stoller and Wax, Reference Stoller and Wax1974), Physalis physalifolium (Taab and Andersson, Reference Taab and Andersson2009), Solanum nigrum (Roberts and Lockett, Reference Roberts and Lockett1978; Taab and Andersson, Reference Taab and Andersson2009) and S. sarrachoides (Roberts and Boddrell, Reference Roberts and Boddrell1983). In these studies, dormancy was broken during winter when environmental conditions were suitable for cold stratification, and seeds germinated in spring. However, if seeds fail to germinate in spring they were re-induced into dormancy (secondary dormancy) during summer with dormancy-break occurring the following winter.
Changes in the temperature requirements for germination as seeds undergo dormancy-break are characteristic of many seeds with nondeep PD. However, only a few Solanaceae species have been studied in enough detail to determine if seeds exhibit changes in their temperature requirements for germination during dormancy-break. In several species, however, there was a decrease in the minimum temperature at which seeds could germinate, which is Type 2 nondeep PD (Soltani et al., Reference Soltani, Baskin and Baskin2017). Seeds of Datura stramonium exhibited a decrease in the minimum temperature for germination from 30/15°C for freshly matured to 15/6°C for seeds buried 3 months in moist soil at natural temperature regimes in Kentucky (USA) from mid-October to mid-January (Baskin and Baskin, Reference Baskin and Baskin1988; unpublished). Fresh seeds of Solanum dulcamara germinated to 3% at 25°C and to 0% at 10, 15 and 20°C (Roberts and Lockett, Reference Roberts and Lockett1977). After 1 month of cold stratification at 4°C, seeds of S. dulcamara germinated to c. 80% at 25 and 30°C and to c. 28, 20 and 15% at 20, 15 and 10°C, respectively. After 3 months of cold stratification, seeds of this species germinated to 40–85% at all temperatures. As dormancy was broken during dry storage at 4°C, seeds of S. rostratum exhibited a decrease in the minimum temperature at which seeds would germinate. In light, the minimum temperature for germination decreased from 30/15 to 20/10°C and in darkness from 30/15 to 5/2°C (Shalimu et al., Reference Shalimu, Qiu, Tan, Baskin and Baskin2012). As dormancy-break occurred in S. sarrachoides seeds buried in soil in England, they first gained the ability to germinate at 30, 25 and 20°C, then at 15°C and finally at 10°C (Roberts and Boddrell, Reference Roberts and Boddrell1983). We conclude that seeds of S. dulcamara, S. rostratum and S. sarrachoides have Type 2 nondeep PD.
We have presented various kinds of evidence that seeds of Solanaceae have nondeep PD. However, for each category of evidence, e.g. dormancy cycling, only a few examples of species have been presented because only a small number of species of Solanaceae have been investigated. One possible reason for the small number of detailed studies on dormancy-break could be related to the rapid loss of dormancy when seeds of Solanaceae are stored for several weeks or months before the first germination test. These high germination percentages have, no doubt, removed the incentive for many researchers to conduct detailed studies on dormancy-break, e.g. changes in temperature requirements for germination during the dormancy-breaking process.
Temperature and light requirements for germination
For the 167 species of Solanaceae listed in Table S2, the mean temperature at which seeds germinated to a high percentage is 22.6 ± 0.4°C. In some species of Solanaceae, seeds incubated at an alternating temperature regime germinate to a higher percentage than those incubated at a constant temperature. Seeds of Solanum dennekense, S. incanum, S. luteum and S. nigrum germinated to a higher percentage at 27/13 than at 20°C (Teketay, Reference Teketay1998); S. dulcamara, higher at 30/20 than at 30 or 20°C (Roberts and Lockett, Reference Roberts and Lockett1977); S. elaeagnifolium, higher at 25/15 than at 20 or 30°C (Stanton et al., Reference Stanton, Wu and Lemerie2012); S. lycocarpon, higher at 30/20 than at 30 or 20°C (Pinto et al., Reference Pinto, Silva, Davide, Jesus, Toorop and Hilhorst2007); and S. physalifolium, higher at 25/15 than at 25°C (Del Monte and Tarquis, Reference del Monte and Tarquis1997). Seeds of S. carolinense germinated to c. 85 and 97% at 30/20 and 35/20°C, respectively, and to 0 and c. 3% at 15 and 20°C, respectively (Brown and Porter, Reference Brown and Porter1942).
Information on germination in light (L) vs. dark (D) was found for 44 species (Table S2): light (L) required, 7 species; germination higher in light than in dark (L > D), 18; germination equal in light and dark (L = D), 7; and germination higher in dark than in light (D > L), 5. In seven additional species, more than one response was found, e.g. Solanum americanum, L > D, L = D; S. betaceum, L > D, L = D; S. mauritianum, L > D, L = D; S. villosum, L, D>L; Physalis angulata, L = D, L > D; Withania somnifera, L, L > D (Table S2). Seeds of S. americanum incubated in light at 30 and 35/20°C germinated to 92 and 90%, respectively, but in darkness to 82 and 92%, respectively (Zhou et al., Reference Zhou, Deckard and Messersmith2005b). Seeds of S. nigrum incubated in light germinated equally well at 21 and 25/19°C (97 and 96%, respectively), but in darkness they germinated to 89 and 96%, respectively (Wagenvoort and van Opstal, Reference Wagenvoort and van Opstal1979). Seeds of Physalis philadelphica incubated in light at 30 and 30/25°C germinated to c. 95%, but in darkness they germinated to 20 and 45%, respectively (Madrid et al., Reference Madrid, Caligaris and Rincon1989).
Effects of animals on seeds
Dispersal
Seeds of Solanaceae may be dispersed when animals eat fleshy fruits and subsequently discard the seeds. Animals that have been documented to disperse seeds of Solanaceae include bats (Sazima et al., Reference Sazima, Buzato and Sazima2003; Albuquerque et al., Reference Albuquerque, Velázquez and Mayorga-Saucedo2006; Mello et al., Reference Mello, Kalko and Silva2008; Caves, Reference Caves2010; Rossaneis et al., Reference Rossaneis, Dos Reis, Bianchini and Pimenta2015), birds (Barnea et al., Reference Barnea, Yom-Tov and Friedman1990; Nogales et al., Reference Nogales, Delgado and Medina1998; Burrows, Reference Burrows1999; Albuquerque et al., Reference Albuquerque, Velázquez and Mayorga-Saucedo2006; Vasconcellos-Neto et al., Reference Vasconcellos-Neto, Albuquerque and Silva2009; Jordaan et al., Reference Jordaan, Johnson and Downs2011), fox (Canidae) (Bueno and Motta-Junior, Reference Bueno and Motta-Junior2004), lizards (Nogales et al., Reference Nogales, Delgado and Medina1998), tortoises (Rick and Bowman, Reference Rick and Bowman1961) and maned wolf (Canidae) (Lombardi and Motta-Junior, Reference Lombardi and Motta-Junior1993; Bueno and Motta-Junior, 2024). Rodents also disperse seeds of Solanum lycocarpum, but in doing so they predate/destroy some of the seeds (Briani and Guimerães, Reference Briani and Guimerães2007). Seeds of S. rostratum are dispersed when the berry enclosed by the spiny calyx becomes attached to animal fur and wool of sheep; fruits with the attached calyx also float in water in irrigation canals (Eminniyaz et al., Reference Eminniyaz, Qiu, Tan, Baskin, Baskin and Nowak2013).
Germination
Seeds of some species of Solanaceae collected after they passed through the digestive system of animals and tested for germination have germinated to higher percentages than de-fruited (non-eaten) control seeds (Table 4). In some studies, however, the ingested seeds germinate to the same percentage as control seeds, i.e. fruit removed by hand (Lombardi and Motta-Junior, Reference Lombardi and Motta-Junior1993; Vasconcellos-Neto et al., Reference Vasconcellos-Neto, Albuquerque and Silva2009). However, seeds of Solanum thomasiifolium ingested by a fox germinated to a lower percentage than the control (Vasconcellos-Neto et al., Reference Vasconcellos-Neto, Albuquerque and Silva2009). Seeds of S. nigrum in the droppings of the great bustard (bird) in Spain had a lower percentage of germination and lower percentage of viable seeds than the control, but they germinated faster than the control seeds (Bravo et al., Reference Bravo, Velilla, Bautista and Peco2014). Seeds of S. nigrum defecated and regurgitated by blackbirds and starlings in France germinated to a higher percentage than those in the control, and germination percentage of regurgitated seeds was significantly higher than that of defecated seeds (Clergeau, Reference Clergeau1992).
Examples of Solanaceae in which seed germination percentage was increased after seeds passed through the digestive system of an animal
a Seeds that passed through the digestive system of dark-capped bulbuls germinated to the same final percentage as those removed from fruits by hand (c. 73%), but germination of ingested seeds began sooner than that of seeds removed from fruits by hand.
Germination mechanism: the endosperm cap and radicle emergence
After dormancy is broken in seeds of Solanaceae, the radicle emerges from the seed, first by penetration of the endosperm located in the micropylar region of the seed (endosperm cap) and second by penetration of the seed coat. Breakdown/softening of the endosperm cap precedes radicle emergence, but various studies on different species of Solanaceae indicate that this is a complicated (and still not completely understood) process. Softening involves exposure to red light, production of GA by the embryo, increased activity of hydrolysing enzymes such as endo-β-mannanase and expansin, decrease in level of ABA and a decrease in the puncture force required to break the endosperm cap (Soriano et al., Reference Soriano, Sánchez and de Eilberg1964; Groot and Karssen, Reference Groot and Karssen1987, Reference Groot and Karssen1992; Groot et al., Reference Groot, Kieliszewska-Rokicka, Vermeer and Karssen1988; Sánchez and de Miguel, Reference Sánchez and de Miguel1997; Chen and Bradford, Reference Chen and Bradford2000; Nonogaki et al., Reference Nonogaki, Gee and Bradford2000; Pinto et al., Reference Pinto, Silva, Davide, Jesus, Toorop and Hilhorst2007; Lee et al., Reference Lee, Bas, Steinbrecher, Walsh, Bacic, Bentsink, Leubner-Metzger and Knox2012; Martínez-Andújar et al., Reference Martínez-Andújar, Pluskota, Bassel, Asahina, Pupel, Nguyen, Takeda-Kamiya, Troubliana, Bai, Górecki, Fait, Yamaguchi and Nonogaki2012; Steinbrecher and Leubner-Metzger, Reference Steinbrecher and Leubner-Metzger2017; Shi et al., Reference Shi, Li, Yang, He and Wang2022; Chen et al., Reference Chen, Li, Wu, Zhao, Yang, Huang, Huang and Wei2024). Basically, exposure of ND imbibed seeds to red light leads to formation of Pfr, followed by GA production by the embryo, which promotes activity of enzymes. These enzymes hydrolyse cell wall components of the cells in the endosperm cap. In particular, endo-β-mannanase and the genes involved in its activity and how they relate to hormonal regulation have received considerable research attention (Nonogaki et al., Reference Nonogaki, Gee and Bradford2000; Martínez-Andújar et al., Reference Martínez-Andújar, Pluskota, Bassel, Asahina, Pupel, Nguyen, Takeda-Kamiya, Troubliana, Bai, Górecki, Fait, Yamaguchi and Nonogaki2012; Steinbrecher and Leubner-Metzger, Reference Steinbrecher and Leubner-Metzger2017). The puncture force required to break the endosperm cap has been measured after various dormancy-breaking treatments in seeds of Capsicum, Datura, Nicotiana, Petunia and Solanum (see Steinbrecher and Leubner-Metzger, Reference Steinbrecher and Leubner-Metzger2017).
The force required to break the endosperm cap in seeds of Solanum lycocarpum imbibed in water decreased in two major phases: (1) days 1–10, a decrease from 2.12 to 1.82 N, and (2) days 25–30, a decrease of 1.73 to 1.20 N. After 30 days, the radicle emerged (Pinto et al., Reference Pinto, Silva, Davide, Jesus, Toorop and Hilhorst2007). However, for seeds imbibed in 100 µM ABA, there was only one major period of decrease in puncture force, and it was between days 15 and 20; puncture force at days 1 and 40 was 2.33 and 1.84 N, respectively. After 40 days of imbibition in water and ABA, endo-β-mannanase activity was 120 and 90 pmol min−1 cap−1 ec−1, respectively. ABA decreased activity of endo-β-mannanase and thus softening of the endosperm cap, and it decreased the amount of water imbibed and ability of seeds to germinate.
Persistent soil seed banks
In a persistent soil seed bank, seeds remain viable and can germinate in the second or some later germination season (Walck et al., Reference Walck, Baskin, Baskin and Hidayati2005). A persistence soil seed bank not only helps to ensure long-term persistence of a species at a site, but it could increase the ability of a species to become naturalized in new habitats (Gioria et al., Reference Gioria, Pyšek, Baskin and Carta2020, Reference Gioria, Carta, Baskin, Dawson, Essl, Kreft, Pergl, van Kleunen, Weigelt, Winter and Pyšek2021).
In our search for information about soil seed banks of Solanaceae, we identified 193 published research articles in which the methods were such that presence of a persistent seed bank could be detected. That is, soil samples were collected after the germination season but before dispersal of newly produced seeds. Thirty-two of the 193 papers included species of Solanaceae. We found soil seed bank information for four of the eight subfamilies (Cestroideae, Petunioideae, Schwencioideae and Solanoideae), 12 genera and 39 species of Solanaceae (Table 5). Atropa, Cestrum, Datura, Discopodium, Fabiana, Petunia, Quincula, Schwenkia and Vassobia were represented by only one species; Nolana by two species; Physalis by three species; and Solanum by 25 species. Thirteen species were found in two or more locations, and seeds of S. nigrum were found in seven locations, including Australia, Botswana, Ethiopia, France and Germany. Seeds of Solanaceae have been found in soils of forests, coastal dunes, deserts and savannas, and they often are found in disturbed soils, e.g. agricultural soils, gardens, pastures and secondary forests.
Persistent soil seed banks for species of Solanaceae
+ , species present, but no information was provided about seed density; *, listed as a weed by Holm et al. (Reference Holm, Pancho, Herberger and Plucknett1979).
Only a relatively few studies in which seeds were buried in the soil and seed longevity monitored for 1 or more years have been conducted for species of Solanaceae. The longest such experiment (1902–1941) that included Solanaceae is the Duvel buried seed experiment conducted in Rosslyn, Virginia (USA). The experiment included Datura stramonium, Nicotiana tabacum and Solanum nigrum. After 39 years of burial, seeds of each species from burial depths of 55 and 105 cm germinated: D. stramonium, 91 and 88%, respectively; N. tabacum, 22 and 17%, respectively; and S. nigrum, 83 and 70%, respectively (Toole and Brown, Reference Toole and Brown1946).
In a 6-year study of buried seeds in England, Roberts and Dawkins (Reference Roberts and Dawkins1967) disturbed (dug to a depth of 22.5 cm) 540 × 300 cm plots in March and September (first year) and other plots in March, June, September and December in each year of the study. Seeds of Solanum nigrum germinated in each of the 6 years. Since no seed set was allowed to occur in the plots, we can assume that seeds of S. nigrum can live in the soil for at least 6 years.
Four studies have been done in which seeds of Solanaceae have been placed in mesh bags and buried in soil in the field. Solanum elaeagnifolium seeds were buried in a weedy pasture in Australia, and after 36 months seed viability at 10 cm was 59% but only 18–25% viability at 0, 2.5 and 5 cm (Stanton et al., Reference Stanton, Wu and Lemerie2012). Viability of S. hayseii seeds buried in a seasonal rainforest in Panama for 18 months was 45% (Dalling et al., Reference Dalling, Swaine and Garwood1997), and viability of S. mauritianum seeds buried for 2 years in a rainforest in Australia was c. 75% (Hopkins and Graham, Reference Hopkins and Graham1987). However, viability of S. mauritianum seeds buried in a grassland in South Africa for 13 months was 15–35% (Witkowski and Garner, Reference Witkowski and Garner2008).
Freshly matured seeds of Solanum carolinense and S. elaeagnifolium were mixed with soil and placed in vials with perforated caps that in turn were placed in soil in jars with perforated lids; the jars then were buried in a field in Iowa (USA) (Brown and Porter, Reference Brown and Porter1942). Each species was buried at soil depths of 10–15 cm and 40–45 cm. After 3 years of burial at 10–15 and 40–45 cm, exhumed seeds of S. carolinense germinated to 98 and 0.5%, respectively, and those of S. elaeagnifolim to 50 and 66%, respectively.
Seeds of Hyoscyamus niger germinated in soil samples removed from two archaeologically dated sites in Denmark (Sjørrind and Hirsholm) abandoned in 1300 and 1871, respectively, and one site Sweden (Tommarp) abandoned in 1300 (Ødum, Reference Ødum1965). However, the presence of viable/germinable seeds of H. niger in these long-abandoned sites does not prove that seeds have lived in the soil since 1300. Unknown soil disturbances may have resulted in seed burial. For example, seeds may have been buried due to the activities/feeding of earth worms (McTavish and Murphy, Reference McTavish and Murphy2021). In the future, genomic sequencing of presumably long-buried and recently produced seeds of H. niger (and other species found in archaeological sites) might provide more information on how long seeds have lived in the soil (see Pérez-Escobar et al., Reference Pérez-Escobar, Tusso, Przelomska, Wu, Ryan, Nesbitt, Silber, Preick, Fei, Hofreiter, Chomicki and Renner2022).
Many questions remain about Solanaceae
Distribution in temperate zone
In terms of number of species in each of the vegetation regions on earth, the Solanaceae is not evenly distributed. The centres of distribution for this family are in South America, Mesoamerica, China, Africa and Australia (see Introduction). With an increase in distance from the equator in the Northern Hemisphere, there is a decrease in the number of species of Solanaceae. For example, Mexico has 381 species in 34 genera (Martínez et al., Reference Martínez, Vargas-Ponce, Rodríguez, Chiang and Ocegueda2017); northeastern USA and adjacent Canada, 29 species in 10 genera (Gleason and Cronquist, Reference Gleason and Cronquist1991); and Canada, 8 species in 1 genus (Solanum) with only one native species (S. carolinense) (https://thecanadianencyclopedia.ca/en/article/nightshade). Alaska has one species, the weedy S. nigrum (Hultén, Reference Hultén1968). If numerous species of Solanaceae, in particular species of Solanum, live at high elevations in the Andes, why do so few species grow in the temperate latitudes, e.g. central and northern USA and southern Canada?
Heat and cold tolerance of Solanaceae species
Does the ability of Solanaceae plants to tolerate heat and cold play a role in determining global distribution? The search for genes to help improve growth and production of potatoes and tomatoes has resulted in various tests of the response of wild species of Solanum to heat and cold stress.
Tolerance of 16 species of wild potatoes collected between 450 and 4200 m a.s.l. in Bolivia and Pera and one species from 2500 m a.s.l. in Mexico were tested for both heat and cold tolerance (Smillie et al., Reference Smillie, Hetherington, Ochoa and Malagamba1983). Newly expanded (dark-adapted) leaves of each species were cold- and heat-stressed at 0 and 41°C, respectively, then assayed for changes in chlorophyll fluorescence. Cold-stressed leaves were placed at 0°C and assayed at hourly intervals until fluorescence decreased by 50%. For the heat-stressed leaves, fluorescence was measured first at 0°C, after which leaves were warmed to 22°C and then exposed to 41°C for 10 min. After the heat treatment, fluorescence was measured again at 0°C. In general, with an increase in elevation where the species grew cold tolerance increased and heat tolerance decreased.
Nine diploid wild potato species from South American and one from Mexico were tested for production of microtubers in culture media in a cool (1°C) and warm (25°C) environment (Guedes et al., Reference Guedes, Haynes, Vinyard and Pinto2019). Microtuber production was similar at 19 and 25°C for four species, greater at 19 than at 25°C for six species and greater at 25 than at 19°C for one species. Overall, S. kurtzianum (from Argentina) and S. sogarandinum (from Peru) were the most heat tolerant. In another study, S. chacoese (accession BGB444 from South America) produced significantly more tubers at 24–34°C than at 14–27°C (Bashir et al., Reference Bashir, Barros, Castro and Heiden2020).
Li (Reference Li1977) tested plants of 60 tuber-producing species of Solanum at temperatures ranging from −2.5 to −6.5°C (at 0.5°C intervals). The temperature that killed the leaves ranged from −2.5 to −6.5, depending on the species and accession. Leaves of eight species were killed at −2.5°C, while those of S. acuale were killed at −4.5 to −6.5°C, depending on the accession of this species. In another study, natural frosts (two nights of c. −2°C) on plants of 101 species of Solanum from the U.S. Potato Collection growing in field plots in Wisconsin (USA) killed all the leaves on 59 species but had little or no effect on leaves of seven species (Vega and Bamberg, Reference Vega and Bamberg1995).
Hijmans et al. (Reference Hijmans, Jacobs, Bamberg and Spooner2003) evaluated frost tolerance in 87 wild potato species in South America. The species with high frost tolerance grew in regions in which the mean minimum yearly temperature was below 3°C, i.e. in ‘central and southern Peruvian Andes, the lowlands of Argentina and adjacent areas and a small area in the central Chilian Andes’. In the plains of north central India, a 3-day period of day temperatures of 19.5–20.5°C and night temperatures of 1–3°C killed 90% of the leaves of one or more accessions of Solanum alandiae, S. arnexii, S. berthaultii, S. boliviense, S. cardiophyllum, S. microdontum and S. sparipilum (Luthra et al., Reference Luthra, Gopal, Manivel, Kumar, Singh and Pandey2007).
Phylogenetic analyses revealed that Solanum pimpinellifolium originated in Ecuador and subsequently diverged southward along the western side of the Andes into Peru (Lin et al., Reference Lin, Lu and Lee2020). The range of the species now spans from wet tropical rainforest in Ecuador to deserts in Peru. The authors asked why S. pimpinellifolium did not expand northward but did not answer this question. We think the authors have asked an important question about heat-tolerant Solanaceae lineages, and answers to this question could provide insight on the low number of Solanaceae species in cool temperate climates, especially in North America.
Was the northward migration of heat-tolerant Solanaceae from South to North America blocked? For example, did the separation of South and North America during the Miocene when much speciation and dispersal were occurring in Solanaceae prevent northward dispersal? Was migration of Solanaceae to North America, especially to Mexico, promoted when the Isthmus of Panama formed in the Pliocene, c. 2.8 Ma (O’Dea et al., Reference O’Dea, Lessios, Coates, Eytan, Restrepo-Moreno, Cione, Collins, de Queiros, Farris, Norris, Stallard, Woodburne, Aguilera, Aubry and Berggren2016)? On the other hand, was northward migration of cold-tolerant lineages in the Andes blocked by the high temperatures of the rainforests of northern South America and of Central America? Are the native species of Solanaceae in eastern USA derived from heat-tolerant lineages from South America?
Molecular phylogenetic studies of the Carolinense Clade in Solanum subgenus Leptostemonum (the spiny solanums) identified four North American (S. carolinense, S. dimidiatum, S. perplexum and S. pumilum) and five South American (S. aridum, S. comptum, S. juvenale, S. moxosense and S. reineckii) species (Wahlert et al., Reference Wahlert, Chiarini and Bohs2014, Reference Wahlert, Chiarini and Bohs2015). Since the North American species ‘are nested among a large clade of mostly Neotropical species in the overall phylogeny’, Wahlert et al. (Reference Wahlert, Chiarini and Bohs2014) hypothesized that there was a single dispersal event of a South America Solanum to North America. Further, they suggested that diversification of the South American progenitor explains the present-day occurrence of four species in the Carolinense Clade in North America. Three of the four Solanum species in the South American and North American parts of the Carolinense Clade are weedy and grow in disturbed habitats (Wahlert et al., Reference Wahlert, Chiarini and Bohs2015); however, the fourth species S. pumilum is an endemic on outcroppings of Ketona dolomite and amphibolite in Alabama (USA) (Allison and Stevens, Reference Allison and Stevens2001).
PD, timing of germination and life cycle
Does Type 2 nondeep PD promote growth of Solanaceae species in habitats with cold winters and warm summers?
Since dormancy-break in seeds with Type 2 nondeep PD results in a decrease in the minimum temperature at which seeds can germinate, they do so at low temperatures in the habitat in early spring. Thus, seeds with Type 2 could germinate in tropical mountains and in temperate regions at the end of the cold season, and young plants have the full length of the warm season for growth. The number of studies on type of nondeep PD in seeds of Solanaceae species is limited; however, seeds of some species have been shown to have Type 2 (e.g. Roberts and Lockett, Reference Roberts and Lockett1977; Roberts and Boddrell, Reference Roberts and Boddrell1983; Baskin and Baskin, Reference Baskin and Baskin1988; Shalimu et al., Reference Shalimu, Qiu, Tan, Baskin and Baskin2012). More studies in which seeds of Solanaceae are tested over a range of temperatures at various times during the dormancy-breaking are needed before we have a good understanding of the importance of the various types of nondeep PD in the adaptation of Solanaceae species to different kinds of habitats.
Cold-sensitive plant species can grow in winter-cold habitats if they are dormant during winter, i.e. species behave as herbaceous perennials with dormant buds in winter or as summer annuals present as seeds during winter. Species of Solanaceae in temperate regions behave as herbaceous perennials or summer annuals. For example, in northeastern USA and adjacent Canada, Gleason and Cronquist (Reference Gleason and Cronquist1991) list 29 species of Solanaceae in 10 genera, and 14 and 15 of the species are perennials and annuals, respectively. All the annuals behave as summer annuals with flowering and fruiting occurring in summer to autumn. Do seeds of these 29 species have Type 2 nondeep PD?
In Canada, shoots of the perennial Solanum carolinense emerge from the soil by mid-May but die after the first frost in autumn (Bassett and Munro, Reference Bassett and Munro1986). Do seeds of S. carolinense have Type 2, which would promote seed germination in spring? Chandler and Travers (Reference Chandler and Travers2024) found that vegetative and reproductive traits of S. carolinense from a northern population (Minnesota, USA) were more heat tolerant than those of plants from a southern population (Texas, USA). However, plants from the southern population were more cold tolerant than those from the northern population. Do seeds from the northern and southern populations have the same dormancy-breaking and germination requirements?
Type 2 nondeep PD has been found in two summer annual Solanaceae species living in the temperate zone. The weedy summer annual Solanum rostratum is native to central America, Mexico and the Great Plains of the USA and has been introduced into northeastern USA and southwestern Ontario (Canada). It also has become established in Australia, China, Europe, the Middle East and South Africa (POWO, website, 2024). In Ontario, seeds of S. rostratum germinate in spring. Plants flower in late June to early July, and fruits are mature by early September (Bassett and Munro, Reference Bassett and Munro1986). Seeds of this species have Type 2 nondeep PD (Shalimu et al., Reference Shalimu, Qiu, Tan, Baskin and Baskin2012). The weedy summer annual S. sarrachoides is native to South America (Argentina, Bolivia, Brazil, Paraguay and Uruguay), but it now has been introduced into Australia, Europe, South Africa and the USA (POWO, website, 2024). Its seeds not only have Type 2 nondeep PD but dormancy can be broken during dry storage or cold (moist) stratification (Roberts and Boddrell, Reference Roberts and Boddrell1983). It seems reasonable that seeds of many other summer annual species of Solanaceae have Type 2 nondeep PD.
In winter-cold habitats in South America, e.g. upper slopes of the Andes, germination at the beginning of the summer warm season would be adaptive. Seedling establishment at the beginning of the growing season means that plants would have the whole warm season for growth. Thus, do high elevation species of Solanaceae in the Andes produce seeds that have Type 2 nondeep PD?
Many species of Solanaceae, however, grow in climates that are warm throughout the year; some areas are wet and others have an annual wet/dry cycle. The ability of seeds to after-ripen in dry warm conditions and to germinate at high temperatures with the onset of the wet season means that germination occurs after conditions are favourable for seedling establishment and growth. Did Type 2 nondeep PD develop when migrating lineages of Solanaceae came in contact with winter-cold climates or when lineages growing outside the tropical zone in the Eocene began to experience cold winters in the late Eocene and Oligocene (Zachos et al., Reference Zachos, Pagani, Sloan, Thomas and Billups2001; Westerhold et al., Reference Westerhold, Marwan, Crury, Liebrand, Agnini, Anagnostou, Barnet, Bohaty, Vieeschouwer, Florindo, Frederichs, Hodell, Holbourn, Kroon, Lauretano, Littler, Lourens, Lyle, Pälike, Röhl, Tian, Wilekns, Wilson and Zachos2020)?
Concluding thoughts/questions
We found seed dormancy/germination information for only 167 of the 2729 (6.1%) species of Solanaceae; thus, much additional research is needed on members of this family. Most (94.5%) of the 167 species had seeds with PD, and this class of dormancy was found in all subfamilies, vegetation regions and growth forms of Solanaceae. However, spatulate and five kinds of linear-full embryos are found among the species of Solanaceae, but seeds have only PD. Why is there more diversity in embryo morphology than in class of seed dormancy in the Solanaceae? Clearly, there is more to be learned about the evolution of Solanaceae. In general, nondeep PD is easily broken, often by dry after-ripening, and seeds germinate to a high percentage at relatively high temperatures (mean = 22.6°C). Thus, from a seed dormancy-breaking and germination perspective, species of Solanaceae are well suited for living in either wet or seasonally wet/dry tropical and subtropical zones.
The heat and cold tolerances of Solanaceae, especially Solanum, have received much recent attention by scientists involved in crop development, e.g. potatoes. One hypothesis for the lack of high numbers of Solanaceae in northern cold winter climates is that the lineage(s) that migrated north, e.g. to North America, was (were) heat tolerant but not cold tolerant. The heat-tolerant lineage of the Carolinense Clade of Solanum that migrated from South America to North America presumably had PD that was broken by high temperatures with the ND seeds germinating at high temperatures. A lowering of the minimum temperature at which seeds could germinate (i.e. Type 2 nondeep PD) would facilitate survival and speciation of members of the clade in the temperate zone. That is, the ability of seeds to germinate in spring at the beginning of the warm season enhances adaptation to winter-cold, summer-warm habitats.
This scenario for development of temperate species in the Carolinenese Clade of Solanum fits the model for the origin and evolutionary relationships of the various levels and types of PD proposed by Baskin and Baskin (Baskin and Baskin, Reference Baskin and Baskin2014, see their Fig. 12.21) with regard to origin of Type 2 nondeep PD. In this model, after dormancy is broken seeds with Type 4 nondeep PD germinate only at high temperatures and those with Type 2 nondeep PD germinate at both high and low temperatures. Type 4 in seeds of species growing in tropical seasonally wet/dry habitats is broken by warm dry conditions, and seeds germinate during the warm wet season. With climate cooling or dispersal of lineages to seasonally cool climates, the favourable season for germination and subsequent plant growth and establishment has low (spring) temperatures. Thus, the model proposes that Type 4 gave rise to Type 2 via a lowering of the minimum temperature at which seeds can germinate, i.e. the ability to germinate at low temperatures has been ‘added’ to ability to germinate at high temperatures. Thus, seeds germinate in spring at the beginning of the warm wet season.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0960258526100142.
Competing interests
The authors have no conflict to declare.
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