Comparative Germination Ecology of the Endemic Centranthus Amazonum (Valerianaceae) and Its Widespread Congener Centranthus Ruberpsbi 280 165..172
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Plant Species Biology (2010) 25, 165–172 doi: 10.1111/j.1442-1984.2010.00280.x Comparative germination ecology of the endemic Centranthus amazonum (Valerianaceae) and its widespread congener Centranthus ruberpsbi_280 165..172 EFISIO MATTANA*, MATTHEW I. DAWS† and GIANLUIGI BACCHETTA* *Centre for the Conservation of Biodiversity (CCB), Department of Botanic Sciences, University of Cagliari, 09123 Cagliari, Italy and †Seed Conservation Department, Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly RH17 6TN, UK Abstract Centranthus amazonum Fridl. & A. Raynal (Valerianaceae) is a critically endangered narrow endemic to Central–Eastern Sardinia (Italy), where it occurs in rocky habitats. In the present study, the germination ecology of this species was investigated and compared with the relatively widespread species Centranthus ruber (L.) DC. subsp. ruber. In par- ticular, the effects of a range of constant temperatures (5–25°C) and two alternating temperature regimes (25/10 and 30/15°C) on seed germination were analyzed. For the two species, cardinal temperatures were determined as the reciprocal of time to 50% germi- nation (t50). In addition, seed persistence in the soil was assessed by experimental seed burials. Seeds of both species were non-dormant; however, C. amazonum produced more empty seeds and germination was slower and reached lower final percentages than C. ruber. Moreover, seed and seedling mass of C. ruber were higher than for C. amazonum and the lack of a persistent soil seed bank detected for C. amazonum increases vulner- ability to extinction for this species. The present study provides new data on the germi- nation ecology of the previously unstudied C. amazonum that may explain the rarity of this species when compared with the commonness of C. ruber. Keywords: cardinal temperatures, rarity, Sardinia, seed size, soil seed bank. Received 5 January 2010; accepted 4 March 2010 Introduction Corrasi, Oliena—NU). A few individuals (2–5) have also been found in the locality of Codula di Luna Centranthus DC. (Valerianaceae) is a circum- (Urzulei—OG) at 140 m a.s.l. (Fridlender & Raynal- Mediterranean genus that includes 10 species with Roques 1998; Bacchetta et al. 2008). This rare endemic Mediterranean and European distribution, with nine rec- species is included in the IUCN Red Lists as CR B1ab(iii, ognized by Richardson (1975) and one additional species iv)+2ab(iii, iv); D (Fridlender 2006). Centranthus ruber is described more recently by Fridlender and Raynal- probably native only in the Balkan Peninsula and parts of Roques (1998). Species in this genus are glabrous, usually the Central Mediterranean region (Richardson 1975). glaucous, annual or rhizomatous perennial herbs (Rich- However, this species has been cultivated for ornamental ardson 1975). Two perennial species belonging to this purposes and is now naturalized in many regions of genus are reported for the flora of Sardinia (Italy): Cen- Western and Central Europe, South-Western Asia, Maca- tranthus amazonum Fridl. & A. Raynal and Centranthus ronesia and Australia, as well as in North and South ruber (L.) DC. ssp. ruber (Conti et al. 2005). Centranthus America, where it grows in disturbed habitats and rocky amazonum is a narrow endemic occurring predominately places (López Martínez & Devesa 2007). Currently in Sar- (less than 100 individuals) in Central–Eastern Sardinia at dinia this species is only found in the south part of the 1300 m a.s.l. on a limestone karstic mountain (Monte island. Species in the Valerianaceae have either non-dormant Correspondence: Efisio Mattana (ND) or physiologically dormant (PD) seeds (Baskin Email: mattana.efi[email protected] & Baskin 1998). Branca (1998) reported over 78% © 2010 The Authors Journal compilation © 2010 The Society for the Study of Species Biology 166 E. MATTANA ET AL. germination at temperatures ranging between 10 and common Baptisia lanceolata (Walt.) Ell. (Fabaceae) (Young 20°C for C. ruber, suggesting that this species has ND et al. 2007). Lavergne et al. (2004) conducted a comparative seeds. Fridlender et al. (1999) compared the germination analysis of 25 ecological and biological traits in 20 conge- ecology of C. amazonum and C. trinervis (Viv.) Bég. (a neric pairs of endemic and widespread plant species Corsican endemic). They found that C. trinervis seeds occurring in the French Mediterranean flora. The authors germinated faster and to a higher level than seeds of found that morphological and ecophysiological traits of C. amazonum. However, their experimental design and narrow endemics indicate that they are often more stress- common garden approach do not enable conclusions to be tolerant than their widespread congeners and have a deduced regarding final germination percentages or the lower investment in pollen transfer and seed production. optimal temperature ranges for the two species. In addition, at a global level, invasive species generally Among environmental factors, temperature is arguably have both a greater seed production (Mason et al. 2008) the single most important factor governing the maximum and seed mass than natives (Daws et al. 2007), confirming germination percentage and rate of germination (Hey- that reproductive traits may contribute to commonness/ decker 1977), as well as the success or failure of plant rarity. establishment (Kader & Jutzi 2004). Seed mass also affects In the present study, reproductive traits such as germi- seedling survival: larger seeds generally result in larger nation requirements, seed and seedling size and seed per- seedlings, which often have a higher probability of sur- sistence in the soil of the narrow endemic species C. vival (Daws et al. 2005) as a result of a greater ability to amazonum were investigated and compared with the withstand either low levels of resources or various closely related species C. ruber subsp. ruber. The aims of hazards (Leishman et al. 2000). Seed size is also related to this study were: (i) to investigate the germination ecology persistence in the soil seed bank, with persistent seeds and provide new data for the previously unstudied, in generally being smaller and more spherical than transient terms of seed biology, C. amazonum; and (ii) to identify seeds (Cerabolini et al. 2003). The ability to form a persis- differences in seed biology, with respect to the wide- tent soil seed bank (PSB) is crucial to the survival of many spread congener C. ruber, that might contribute to both rare or declining species (Quilichini & Debussche 2000; explaining their respective rarity and commonness. Eckstein et al. 2006). For example, British species that are increasing in abundance are twice as likely to have a PSB as those that are declining (Hodgson & Grime 1990). Materials and methods In their review, Bevill and Louda (1999) found that Seed lot details pair-wise comparison studies between rare and common species provide a method of gathering intensive informa- Achenes (hereafter referred to as seeds) were collected in tion on key similarities and differences between closely late spring and summer 2007 after obtaining permits as related species that differ in abundance. A range of repro- required by European and national laws for the species ductive attributes such as seed size, seed viability, germi- listed in the appendices of the Habitat Directive (DIR.92/ nation rate, seedling growth and the presence of a soil 43/EEC). Seeds were collected at the point of natural seed bank have been analyzed in comparative studies dispersal by placing fine mesh nylon bags over the between rare and common species in studies of plant infructescences to avoid seed dispersal by wind. Seeds of rarity (e.g. Rabinowitz & Rapp 1981; Rapp & Rabinowitz C. amazonum were collected in July 2007 from the natural 1985; Aizen & Patterson 1990; Ingersoll & Wilson 1993; population in the Monte Corrasi of Oliena (Central Baskin et al. 1997). Different authors have more recently Eastern Sardinia, 1300 m a.s.l., 40°14′N, 09°25′E), whereas investigated reproductive and ecophysiological attributes C. ruber ssp. ruber (hereafter C. ruber) seeds were collected of congeneric pairs of rare and common species. For from May to July 2007 from plants cultivated in the Botanic example, Osunkoya and Swanborough (2001) studied dif- Gardens of Cagliari (South Sardinia, 50 m a.s.l., 39°13′N, ferences in the rare Gardenia actinocarpa Puttock and the 09°06′E). Seeds were stored at the Sardinian Germplasm common co-occurring species Gardenia ovularis F.M. Bank (BG-SAR) where, once cleaned by removing the Bailey (Rubiaceae) and found that the rarity of G. acti- papus, they were placed in a dry room at 15°C and 15% nocarpa might be explained by its lower germination per- relative humidity (RH). For the germination tests, three centage and its loss of seed viability after 3 months (less replicates of 25 seeds per treatment were sown on the than 15% of viable seeds compared with 37% of G. ovularis surface of 1% solid water agar, which provided a solid, viable seeds). Similarly, among reproductive attributes, non-sterile medium for germination, in 90 mm plastic seed traits such as germination percentages after heat Petri dishes and incubated at both constant (5, 10, 15, 20 shock treatments provide the greatest insight into the and 25°C) and alternating temperatures (25/10 and potential mechanisms contributing to commonness/ 30/15°C) in November 2007. In all treatments, seeds were rarity between the rare Baptisia arachnifera Duncan and the exposed to irradiance for 8 h per day (the ratio of red : far © 2010 The Authors Plant Species Biology 25, 165–172 Journal compilation © 2010 The Society for the Study of Species Biology COMPARATIVE GERMINATION IN CENTRANTHUS 167 red light was ~2.0 and the light quantity was ~7 mmol/m2/ crevices. Consequently, it was not possible to carry out s). In the alternating temperature regimes the 8 h light burials at the site of the original population. Therefore, period coincided with the elevated temperature period. both trials were conducted in the Botanic Gardens of Low numbers of seeds per replicate (25) were used Cagliari on a limestone substrate (similar to the collection because of limited seed availability for C.