Annals of Botany 89: 559-562, 2002
© 2002 Annals of Botany Company
Seed Germination and Reproductive Features of Lysimachia minoricensis (Primulaceae), a Wild-extinct Plant
1Jardín Botánico de Valencia, Universidad de Valencia c/Quart 80, E-46008, Valencia, Spain and 2Centre de Recerca Ecològica i Aplicacions Forestals (CREAF), Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain
* For correspondence. Fax: +34 963156826, e-mail rossello{at}uv.es
Received: 25 May 2001 Returned for revision: 2 September 2001; Accepted: 3 December 2001 .
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Lysimachia minoricensis is one of the few Mediterranean endemic plants (Minorca, Balearic islands) that has gone extinct in the wild but which persists as extant germplasm or cultivated plants in several botanical gardens. Reproductive features (seed set, number of seeds per capsule, seed weight) and germination responses to constant temperatures, sea water and dry-heat pre-treatments were investigated to determine the extent to which they may have influenced the extinction of the species. Seed set in Lysimachia is not dependent on pollinators, suggesting a functional selfer breeding system. Most plants produced a large mean number of fruits (23·2) and seeds (46·6), and the mean production of seeds per individual was estimated to be almost 1100. Overall, no highly specific requirements were observed for seed germination. Seed germination was not inhibited in the dark, and a high germinability (over 87 % in all cases) was recorded in most experiments, with the exception of those performed at low temperatures (5 and 10 °C). These data suggest that fertility and seed viability were not the major causes of extinction. The high reproductive performance of L. minoricensis is in striking contrast to its status as a wild-extinct plant, suggesting that extrinsic factors were responsible for its extinction.
Key words: Seed germination, Mediterranean flora, extinct taxa.
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Islands are fragile ecosystems that often have a remarkably high number of endemic taxa, most of them being confined to a few geographically very restricted populations (Carlquist, 1974). The risks of extinction for plant and animal island species are usually higher than those for other taxa growing in continental areas (Alcover et al., 1999; Hylton-Taylor, 2000). Amongst the 30-plus species of vascular plants to have gone extinct in the Mediterranean basin in the last two centuries (Greuter, 1994) were three insular endemics: Diplotaxis siettiana Maire (Alboran islet), Dianthus multinervis Vis. (Jabuka islet) and Lysimachia minoricensis J.J. Rodr. (Minorca). Fortunately, two of these wild-extinct taxa (D. siettiana and L. minoricensis) persist in cultivation and/or seed banks (Greuter, 1994).
The reasons for the extinction of L. minoricensis are not clear. The plant was believed to be very rare since its description (Rodríguez, 1868), and it was assumed to have disappeared from the single known location between 1926 and 1950 (Ibáñez et al., 1999). Molecular screening (using isozyme and RAPD markers) of germplasm accessions of L. minoricensis from 12 European botanical gardens has shown that the remaining living stocks of the species are genetically depauperate since no variation was detected (Calero et al., 1999; Ibáñez et al., 1999). Several attempts aimed at reintroducing L. minoricensis in the wild have failed (Gómez Campo, 1987; Mayol, 1997), but whether or not this is due to some underlying biological factor is unclear.
Molecular analysis of extant Lysimachia accessions cannot determine whether the amount of genetic variability played any role in the decline of the species in the wild. Survival of endangered species may not necessarily be compromised by the absence of significant levels of genetic diversity, and low levels of genetic variation are not necessarily associated with low population viability (Arden and Lambert, 1997).
The low genetic variability (and thus reduced potential to adapt to changing environmental conditions) and/or inbreeding due to selfing or a limited number of mating partners could have decreased the fitness of the ex situ cultured populations of L. minoricensis, lowering plant performance through a reduction in growth, survivorship, seed set, total mass of seeds, average seed mass and/or percentage of germination. Germination requirements and germination rate are amongst the most important seed traits associated with plant fitness (Harper, 1977), and are key components in the ecology and evolution of plant life histories. Knowledge of the performance of these stages may help ascertain whether failure in germination success has contributed to population decline in endangered plant species. Unfortunately, there are no data concerning any critical elements of the reproductive biology of L. minoricensis. These data are essential, not only to improve recovery programmes, but also to determine the extent to which they may have influenced the wild extinction of this species.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Seeds of L. minoricensis were obtained from ten botanic gardens (Cambridge, Brest, Berlin, Paris, Belgium, Nancy, Córdoba, Sóller, Copenhagen and Edinburgh) and grown in a glasshouse at Valencia University, Spain. Ultimately, all accessions probably derived from a single source (Calero et al., 1999; Ibáñez et al., 1999). Several reproductive features (number of capsules per scape, number of seeds per capsule, seed weight) were recorded at the fruiting stage. To avoid any bias concerning the origin of the samples in the germination experiments, seeds were collected from plants belonging to all accessions and pooled together. Seeds were air-dried immediately upon collection and then stored at room temperature (20 °C) until they were used in the germination tests. Seeds were placed on wet filter paper in Petri dishes and then placed in a germination chamber. First, the responses of seeds to various constant temperatures (5, 10, 15, 20, 25 and 30 °C) were investigated. The influence of fire as a possible inhibitor of germination in Lysimachia was investigated by pre-treating seeds at 80 °C for 10, 60 and 90 min and recording subsequent germination success at 25 °C. For salt tolerance tests, seeds were imbibed in sea water for 1, 3 or 10 d before being allowed to germinate at 25 °C. Given the high percentage of seed germination, no further treatments involving lower salt concentrations were attempted. For each germination test, two batches of 50 seeds were used. Seeds were considered to have germinated when the radicle penetrated the seed coat. Germination was recorded every day for 30 d.
Data analysis
Germination data were arcsine-transformed and analysed using ANOVA. Tukey’s LSD multiple comparison was used to test for significant differences among treatment means. The following parameters were determined: germinability (G), or percentage of germinating seeds; time to first observed germinant (T); number of days required for 50 % of the total number of seeds to have germinated (T50); number of days for 50 % of the total number of seeds germinated (T'50); time to maximum germination (T100); germination rate (GR) or germinants per day, calculated as the maximum number of germinants/(days to maximum germination – days to initial germination); index of germination rate (IGR) = (100/n) 
gi/ti, where n is the number of seeds incubated, and gi is the number of seeds germinated at time ti.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Reproductive features
Plants of L. minoricensis are biennial and usually have a single flowering stem per individual. However, departures from this pattern are sometimes observed. Some plants flowered in the first year; exceptionally, some did not die after flowering and so attained a second flowering season; lastly, up to two or three scapes were observed in some large individuals. Isolated plants (grown in an insect-free glasshouse) regularly developed fruits in all flowers of the scape (Table 1), suggesting that within-flower spontaneous selfing (autogamy) is the main breeding system of L. minoricensis. Anthers and stigmas close together at maturity, and anther dehiscence occurs in unopened flower buds, but we have failed to verify pollen tube growth occurring at this stage. No experiments were conducted to test whether or not the high fruit set observed in this species was due to apomixis. However, this seems unlikely since, as far as we know, no apomictic taxa of Primulaceae have been detected. Reproductive features concerning fruit set, seed set, fruit number and seed weight are shown in Table 1. Morphologically, the seeds conformed to earlier descriptions, and the mean weight of batches of 50 seeds indicated that a single seed weighs as little as 0·20 mg on average. From the values shown in Table 1 it is estimated that the mean seed production per individual is approx. 1100.
|
Germination rates
Results of the germination experiments are shown in Table 2. Good germination success was obtained for most treatments. Germination of Lysimachia seeds was neither inhibited nor significantly reduced when seeds were incubated in darkness. High germination rates were also observed when additional temperature tests were conducted under light (data not shown), suggesting that germination is indifferent to light. Germinability was poor at the lower temperatures tested (5 and 10 °C). No signs of germination were observed after 30 d at 5 °C, but at 10 °C germination started after 28 d. When the seeds were incubated with a thermogradient between 15 and 30 °C germinability values were over 98 %, and germination started between 5 and 12 d, with T50 between 9 and 14 d, and T100 within 22 d. Germination rates were also high at 15 and 20 °C; at these temperatures maximum germination occurred 6–8 d after sowing.
|
Incubation of Lysimachia seeds in sea water for up to 10 d had only a minor effect on germination rates. Statistically non-significant lower germination percentages were obtained for pre-soaked seeds (mean 88·3 % compared with 99 % for control seeds germinated at 25 °C). The lowest values were recorded when seeds were immersed in sea water for 24 h only.
The three pre-treatments did not significantly decrease the germination rate when the test was conducted without immersion in sea water. On the contrary, the IGR index increased in all pre-treatments, which indicates faster germination regimes. Pre-heating the seeds at 80 °C resulted in high germinability values (over 90 %) which were not significantly different from control values, irrespective of the time of exposure to high temperatures. Pre-heating also increased the speed of germination, as shown in Table 2. The results after dry-heat treatments were similar to those obtained when the seeds were imbibed in sea water.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
There is no evidence that a drastic decline in the distribution range and population size of L. minoricensis has occurred in recent times since its discovery. The species is known from only a single location (Sa Vall Valley, Minorca), where it was reported as very rare by Rodríguez (1878), and later by other botanists (Porta, 1886; Knoche, 1921) who saw and collected the plant. Although collecting individuals for scientific collections was probably a major cause of the extinction of L. minoricensis in the wild, it is likely that this merely accelerated the inevitable decline of this endemic lineage. All authors dealing with the taxonomic relationships of L. minoricensis have stated that it belongs to an old lineage (Knoche, 1921; Contandriopoulos and Cardona, 1984) on the basis that no close relatives are known. However, evolutionary changes leading to rapidly derived morphological divergences could have obscured sister relationships, as demonstrated by taxa showing a high level of morphological change (e.g. Heterogaura heterandra; Sytsma and Smith, 1992). Some floral features displayed by L. minoricensis seem to represent derived traits (such as small corolla lobes, inconspicuous petal pigmentation and a related autogamous breeding system), which may indicate a young lineage. However, no phylogenetic analysis of sect. Ephemerum (Reichb.) Endl, in which L. minoricensis is currently included (Pax and Knuth, 1905), has yet been performed.
Our reproductive and germination results do not help explain the decline of L. minoricensis or its limited range. Seed production in this species is high (best estimates suggest that a single plant can produce up to 3300 seeds) and should be free of year-to-year fluctuations (in the absence of flower or fruit predators) through its selfing breeding system. The high germinability obtained under different temperature, salt and dry-heat regimes is unexpected for such a narrowly restricted plant. The germination requirements of Lysimachia species have been studied rarely and, as far as we know, are available for just a single annual herb growing in central Africa and Afghanistan, L. ruhmeriana Vatke (Teketay, 1998). These results are in striking contrast with those for L. minoricensis. Germinability was less than 50 % in L. ruhmeriana at the different constant temperatures assayed (between 10 and 30 °C), but better results were achieved when diurnally alternating temperatures (20/12 °C and 30/12 °C) were used. Furthermore, L. ruhmeriana did not germinate in darkness, and when seeds previously incubated in the dark were exposed to light germinability increased very little, suggesting the induction of secondary dormancy in this species (Teketay, 1998). The ability to germinate over a wide range of temperatures, as is the case for both Lysimachia taxa, has been associated with species in which water supply is the main determinant of the timing of germination in the field (Grime et al., 1981). Since our germination experiments were conducted under saturated water conditions, soil moisture content could be a key factor in regulating germination responses in L. minoricensis under natural conditions.
Laboratory germination studies may not adequately predict the ecology of germination in the field (Baskin and Baskin, 1998). However, although the levels of germination in L. minoricensis obtained in these experiments might not be attainable in the field, they are of interest. Absolute and comparative results with other taxa of Lysimachia suggest that fertility and seed viability were not the major causes of extinction of this species. Low fitness values at these stages have been suggested to operate in other rare and endangered insular species, such as Lactoris fernandeziana from the Juan Fernández archipelago (Bernardello et al., 1999) and Schiedea membranacea from the Hawaiian Islands (Culley et al., 1999).
It seems that the low genetic variability present in L. minoricensis has not led to maladaptation causing failure to survive, and there do not appear to be any highly specific requirements for germination. Extrinsic factors acting through the reproductive processes in L. minoricensis (from flowering to germination), which have not been tested in this study, could be associated with successful plant establishment and may have been involved in the overall failure to obtain self-sustaining re-introduced populations in the wild. Field studies investigating flower and fruit predation, allelopathic effects on seed germination and seedling survival, seedling predation, seedling survivorship and establishment in the presence and absence of interspecific competition could be very rewarding. Such studies could help elucidate why a self-compatible plant with a high seed set, unreliant on pollinators and without highly specific germination requirements went extinct in the wild.
|
ACKNOWLEDGEMENTS |
|---|
We are indebted to O. Ibáñez for technical assistance. G. Nieto and J. Thompson made useful criticisms that improved the paper.
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
-
Alcover JA, Seguí B, Bover P. 1999. Vertebrate evolution and extinction on the western Mediterranean islands. In: MacPhee RDE, ed. Explaining Quaternary extinctions: humans and other catastrophes. New York: Plenum Press, 165–188.
Arden SL, Lambert DM. 1997. Is the black robin in genetic peril? Molecular Ecology 6: 21–28.
Baskin CC, Baskin JM. 1998. Seeds: ecology, biogeography, and evolution of dormancy and germination. San Diego: Academic Press.
Bernardello G, Anderson GJ, López SP, Cleland MA, Stuessy TF, Crawford DJ. 1999. Reproductive biology of Lactoris fernandeziana (Lactoridaceae). American Journal of Botany 86: 829–840.
Calero C, Ibáñez O, Mayol M, Rosselló JA. 1999. RAPD markers detect a single phenotype in Lysimachia minoricensis J.J. Rodr. (Primulaceae), a wild extinct plant. Molecular Ecology 8: 2133–2136.[CrossRef][Medline]
Carlquist S. 1974. Island biology. New York: Columbia University Press.
Contandriopoulos J, Cardona MA. 1984. Caractère original de la flore endémique des Baléares. Botanica Helvetica 94: 101–131.
Culley TM, Weller SG, Sakai AK, Rankin AE. 1999. Inbreeding depression and selfing rates in a self-compatible, hermaphroditic species, Schiedea membranacea (Caryophyllaceae). American Journal of Botany 86: 980–987.
Gómez-Campo C. 1987. Libro rojo de especies vegetales amenazadas de España peninsular e islas Baleares. Madrid: ICONA.
Grime JP, Mason G, Curtis AV, Rodman J, Band SR, Mowforth MAG, Neal AM, Shaw S. 1981. A comparative study of germination characteristics in a local flora. Journal of Ecology 69: 1017–1059.[CrossRef]
Greuter W. 1994. Extinctions in Mediterranean areas. Series B 344: 41–46.
Harper JL. 1977. Population biology of plants. London: Academic Press.
Hylton-Taylor C. 2000. 2000 IUCN Red List of threatened plants. Compiled by the World Conservation Monitoring Centre. Gland & Cambridge: IUCN.
Ibáñez O, Calero C, Mayol M, Rosselló JA. 1999. Isozyme uniformity in a wild extinct insular plant, Lysimachia minoricensis J.J. Rodr. (Primulaceae). Molecular Ecology 8: 813–817.[CrossRef]
Knoche H. 1921. Flora Balearica. Etude phytogéographique sur les îles Baléares. Montpellier: Roumégous & Déhen.
Mayol J. 1997. Iniciativas para la conservación de las plantas en las islas Baleares. Conservación Vegetal 2: 4.
Pax F, Knuth R. 1905. Primulaceae. In: Engler A, ed. Das Pflanzenreich. Regni vegetabilis conspectus. IV. 237. (Heft 22). Leipzig: Verlag von Wilhelm Engelmann.
Porta P. 1887. Stirpium in insulis Balearicum anno 1885 collectarum enumeratio. Nuovo Giornale Botanico Italiano 19: 276–324.
Rodríguez JJ. 1868. Additions à la Flore de Minorque. Bulletin de la Societé Botanique de France 25: 238–241.
Sytsma KJ, Smith JF. 1992. Molecular systematics of Onagraceae: examples from Clarkia and Fuchsia. In: Soltis PS, Soltis DE, Doyle JJ, ed. Molecular systematics of plants. New York & London: Chapman & Hall, 295–323.
Teketay D. 1998. The role of light and temperature in the germination of twenty herbaceous species from the highlands of Ethiopia. Flora 193: 411–423.
CiteULike 
Connotea 
Del.icio.us What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||