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Germination ecology of two closely related taxa in the genus Oenanthe: Fine tuning for the habitat?
Aquatic Botany 89 (2008) 345–351
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Germination ecology of two closely related taxa in the genus Oenanthe: Fine tuning for the habitat? Doris Jensch a,b, Peter Poschlod a,* a b
Institute of Botany, Faculty of Biology and Preclinical Medicine, University of Regensburg, D-93040 Regensburg, Germany Wilhelm-Leuschner-Straße 39, D-61169 Friedberg, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2007 Received in revised form 26 March 2008 Accepted 28 March 2008 Available online 18 April 2008
For two closely related amphibious plant taxa of the genus Oenanthe (Apiaceae) germination traits were examined. Habitats of the two taxa differ in hydroregime. The endemic Oenanthe conioides of the river Elbe estuary experiences daily tidal fluctuations whereas the widespread Oenanthe aquatica grows at the edge of ponds and in freshwater wetlands with rare and unpredictable fluctuations of water table. Seeds of both taxa could be characterized as non-dormant and light dependent. Under temperature fluctuations, germination percentage was higher than under constant temperatures. Salinity was tolerated to up to 3.3%. At 10% there was a strong decrease in germination percentage, which did not result from ionic toxicity, as experiments with a gradient in osmolarity showed. Differences between the taxa were found concerning hydrological and temperature fluctuations. While O. aquatica showed no reduction in germination percentage under permanent 1 cm flooding, O. conioides did. On the other hand, germination under an artificial tidal hydroregime was better in O. conioides than in O. aquatica. During fluctuating temperatures of 3/22 and 6/22 8C O. conioides germinated much quicker and had a higher final germination percentage. Differences between the taxa could be correlated with differences of the hydrological regime in the specific habitat. Taking into account that O. conioides is phylogenetically a relatively young taxon, it may be hypothesized that a quick adaptation to the tidal conditions might have taken place. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Keywords: Germination ecology Amphibious habitat Tidal water regime Oenanthe aquatica Oenanthe conioides
1. Introduction An important life-history trait of plant species in flood plains is the timing of germination. Strong selection for gap detection mechanisms and for the detection of a suitable position in relation to the average water-level (hence to frequency of flooding) can lead to different germination characteristics and to different realized niches of closely related species (e.g. Rumex spp.: Voesenek and Blom, 1992; Voesenek et al., 1992). As selection on germination in space and time is important in flood plains we suspect ‘‘fine-tuned’’ differences between closely related taxa or ecotypes which are restricted to specific hydro-regimes. We analysed the germination characteristics of Oenanthe aquatica (L.) Poiret and Oenanthe conioides (Nolte) Lange. O. conioides has probably arisen sympatrically from O. aquatica through ecological divergence (Kadereit and Kadereit, 2005). Both taxa are annual or biennial, germination hence being an important
* Corresponding author. Tel.: +49 941 943 3108; fax: +49 941 943 3106. E-mail address: [email protected] (P. Poschlod).
life stage. O. aquatica is wide-spread, mostly found in freshwater wetlands, edges and mud flats of ponds and ditches and in backwaters. Its habitat is characterized by low-frequency (erratic or regular) flooding events. The taxon is able to endure more or less long-time flooding. A clear morphological adaptation are specific submerged leaves to which already the primary leaves can develop under flooded conditions (Fig. 1). O. conioides is restricted to the tidal freshwater flood plains of the Elbe estuary (Germany). This is a habitat with regular tidal floodings (hence of high frequency) and erratic high (long-time) floodings by storm tides or high water of the river (several times per year). O. conioides is morphologically adapted to these conditions by the absence of specific submerged leaves and by less pinnated leaves, thus being probably more desiccation-resistant and less vulnerable towards currents than O. aquatica. Leaf differences between the taxa are most distinct in the primary leaves (Fig. 1). The shape of the adult leaves shows high plasticity, although both taxa might be distinguished by the relation of circumference to leaf area (Neubecker, in press; Poppendieck, in press). Therefore, hydroregimes of habitats of both Oenanthe taxa differ mostly in the predictable (tidal) component of frequent floodings
0304-3770/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2008.03.013
D. Jensch, P. Poschlod / Aquatic Botany 89 (2008) 345–351
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lakes). Such an alteration must be tolerated because successfully reproducing individuals of the taxon can be found at 1.0 m below to 0.1 m above average high-tide water level. Since the river Elbe has a high load of suspended and dissolved matter and the distribution of O. conioides reaches to the zone of light brackish water (psu up to >3%), the influence of osmolarity and salinity could be another factor which might significantly affect the germination (e.g. Ignaciuk and Lee, 1980; Redondo et al., 2004). Therefore, we analysed in a series of laboratory experiments the germination responses of these two related taxa to a range of conditions reflecting habitat variability which were light, constant temperatures as well as temperature fluctuations, flooding, tidal hydroregime, osmolarity and salinity. 2. Methods
Fig. 1. Typical seedlings of Oenanthe conioides and Oenanthe aquatica from the Elbe estuary and its surroundings under flooding of 1 cm (above) and without flooding (below).
while long-term erratic floodings are more or less frequent. Until recently, O. aquatica was not found in the Elbe estuary (Poppendieck, in press). In the most recent studies, some populations of O. aquatica were found in this habitat (Below et al., 1996) and three of these were confirmed as genetically different from O. conioides or intermediate (Kadereit and Kadereit, 2005). Both Oenanthe species set fruit in summer to early autumn (Hroudova´ et al., 1992; Below, in press) and probably form a seed bank of several years (Hroudova´ et al., 1992; Jensch and Poschlod, in press). As regulation mechanisms of germination stratification, light (red/far-red ratio), temperature, temperature fluctuations, and the tolerance to flooding are widely described for plant species in flood-plains (Morinaga, 1926a,b; Shamsi and Whitehead, 1974; Washitani and Masuda, 1990; Voesenek et al., 1992; Clevering, 1995; Leck, 1996; Baskin and Baskin, 1998; Schu¨tz, 1998; Poschlod et al., 1999). In the case of O. conioides, the alteration of flooding (often correlated to anoxic conditions) and terrestrial conditions may also be suitable to detect the position of a seed on the river bank (see Arts and Van der Heijden, 1990: Littorella uniflora in
All experiments were performed with seeds originating from the river Elbe estuary and its surroundings, collected in two successive years within 2 weeks in August and September 1999 and September and October 2000, respectively; hence, excluding differences in seed ripening due to interannual differences in habitat conditions. Differences caused by maternal effects could be covered partly by the repetition of some experiments with seeds matured in the two successive years. 2.1. Seed collection Ripe seeds of O. aquatica and O. conioides were collected in autumn in 1999 and 2000 along the river Elbe and its surroundings between Schnackenburg and Glu¨ckstadt (river kilometres 585– 675). The area is located between 98 and 128 eastern longitude and 538 and 548 northern latitude. While a sufficient amount of seeds could be collected in 1999, the harvest of 2000 was so small that the experiments performed in 1999 could only be replicated partly with seeds harvested in 2000, and replication number in each experiment was reduced from 8 to 4. Additionally, seeds of 2000 were harvested from only few mother plants (Table 1). Some populations collected here as O. conioides were shown after our study to be intermediate populations by means of genetic analysis (Kadereit and Kadereit, 2005). These genetic studies revealed that the only ‘‘pure’’ populations of O. conioides found nowadays are those of ‘‘Heuckenlock’’ and ‘‘Schweenssand’’. Genetically they form a ‘‘coherent genetic lineage originating from one ancestor’’ which arose from O. aquatica, thus they can be regarded as a progenitor-derivative taxon pair (Kadereit and Kadereit, 2005). Herbarium material proved that in former times the morphological separation was much clearer than for the
Table 1 Origin of seeds of both Oenanthe species (if the origins are at or near the river Elbe, river kilometres (Elbe km) are given) Taxon
Date of collection
Number of seeds
Origin
Number of mother plants
Oenanthe conioides
25.8.99 27.8.99 29.8.99
3000 ca. 500 3000
31.8.99
3000
Heuckenlock, nature reserve Elbe km 612/613 Schweenssand, nature reserve, Elbe km 612/613 Glu¨ckstadt, confined disposal area for dredged material of the Elbe, near Elbe km 675 Neßsand, nature reserve, Elbe km 638
6.–26.9.00
1200
Heuckenlock, nature reserve, Elbe km 612/613
11 1 Not collected separately, counted as if from six mother plants Not collected separately, counted as if from four mother plants 2
7.9.99 19.9.99 21.9.99 27.9.99 5.10.99 12.10.99 24.10.00 24.10.00
3000 >2000 ca. 200 ca. 400 ca. 500 ca. 300 1020 390
Altengamme, Hamburg Buxtehude, Niedersachsen Grippel, Niedersachsen, Elbe km 497 Duvenstedter Brook, nature reserve, Hamburg Ahrensburg, Schleswig-Holstein Walmsburger Werder, Niedersachsen, Elbe km 539 Altengamme, Hamburg Gut Moor, Hamburg
5 4 2 1 2 5 5 5
Oenanthe aquatica
D. Jensch, P. Poschlod / Aquatic Botany 89 (2008) 345–351
material found nowadays at the river Elbe estuary (Poppendieck, in press). An introgression of O. aquatica into the habitat of O. conioides in the wake of habitat changes by harbour and river engineering is suspected (Poppendieck, in press). Poppendieck (in press) could show that hybrids of the taxa exist with intermediate morphology. This means that the seed material from ‘‘Glu¨ckstadt’’ and ‘‘Neßsand’’ consisted not only of Oenanthe conioides seeds but partly of intermediate populations nearer to Oenanthe aquatica (Neßsand). Therefore, seed experiments on O. conioides performed with the pooled seed collection in 1999, contained a large proportion (45%) of seeds belonging to these intermediate populations. However, since from a morphological point of view they could be clearly assigned to O. conioides and were additionally collected from specific O. conioides habitats germination behaviour of these populations has been assumed here to be similar as that of the ‘‘pure’’ populations. We find some support for this in the reportedly rapid adaptation of the germination niche of short living species to specific habitat conditions (temperature, seasonal germination niche, etc.) have shown (Van der Vegte, 1978; Gutterman, 1992; Otte, 1994; Donohue, 2005). First germination experiments started a few days after seed collection. The remaining seeds were stored at 5 8C in moist sand separated according to their mother plants. For each germination test, the same number of seeds per mother plant was mixed for each taxon and taken arbitrarily. Seeds sampled in Glu¨ckstadt and Neßsand in 1999, which were not collected separately per mother plant, were treated as if they came from six and four mother plants, respectively, with the following exception: the experiments on the tidal hydroregime were carried out for the origins ‘‘Heuckenlock’’ and ‘‘Glu¨ckstadt’’ (O. conioides), ‘‘Buxtehude’’ and ‘‘Altengamme’’ (O. aquatica) separately. 2.2. Germination tests In each germination test, 25 mericarps (in the following named ‘‘seeds’’ for convenience) were placed on two layers of filter paper in a plastic Petri dish (exception: experiments on temperature, see below). One Petri dish was considered as our unit of replication. Radicle emergence was taken as proof of germination. Experiments lasted 28 days, except the experiments on temperature (24 days or 26 days, see Table 2) and tidal hydroregime (21 days because of beginning infestation with algae). Non-germinated seeds were tested for viability after the experiment by means of the
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tetrazolium test (ISTA, 1993). All tests were planned to be conducted with eight replicates. Because of shortage of seeds some experiments had to be reduced to three or four replicates (see Table 2 and Section 2.1). All germination experiments except the experiments on temperature and tidal hydroregime were conducted in a climate chamber with a diurnally fluctuating temperature of 22/14 8C and light supply of 14 h by Osram-Lumilux daylight fluorescent tubes (ca. 10,000 lx at Petri dish) during the higher temperature period. Except for the experiments on the hydroregime (hypoxia, tide) and salinity/osmolarity seeds were supplied with 8 ml of deionized water at the beginning of the experiment to saturate the two layers of filter paper with water. The Petri dish was sealed afterwards. 2.2.1. Light/dark To test germination under dark conditions Petri dishes were placed in a black (light-tight) box and the course of germination in the 1999 experiments was recorded under ‘‘safe green light’’. In 2000, the seeds were already placed into the dishes and watered under ‘‘safe green light’’, then placed into the carton which was opened only after the end of the experiment to exclude any possible green light effects this time (Grime and Jarvis, 1975; Blom, 1978). 2.2.2. Temperature For the temperature experiments a thermogradient bar apparatus was used containing 12 small long chambers along the temperature gradient. They were provided with long strips of filter paper and watered as required. Water loss was prevented by a cover of transparent plastic. Light supply was similar to that in the climate chamber used for the experiments on light, salinity/ osmolarity and hypoxia. Because of the small space available at each position in the chambers these experiments were conducted with only 20 seeds per replicate (see Table 2). There were only three replicates due to a reduced number of seeds available for the experiment in 2000 (see above). Constant temperatures used were 5, 10, 15, 20, 25 and 30 8C. Temperature fluctuation regimes had a temperature during the 14 h day period of 22 8C and night temperatures of 3, 6, 9, 12, 15 and 18 8C. 2.2.3. Flooding Flooding conditions reducing oxygen available to the seeds were simulated by shallow flooding with deionized water (water
Table 2 Germination tests, origin of seed and year of seed collection Year of seed collection
Factor tested
Seed origin
Number of replicates
Number of seeds/ replicate
Duration of test (days)
1999
Light at 14/22 8C Dark at 14/22 8C Hypoxia (1 cm flooding, covered with filter paper) at light and 14/22 8C Control of hypoxia (as ‘‘light’’, but with cover of filter paper) Tidal hydroregime with light and 14/22 8C
Mixture of all 1999 origins per taxon Mixture of all 1999 origins per taxon Mixture of all 1999 origins per taxon
8 8 8
25 25 25
28 28 28
Mixture of all 1999 origins per taxon
4
25
28
Conioides: Heuckenlock + Schweenssand, Glu¨ckstadt; aquatica: Buxtehude, Altengamme Mixture of all 1999 origins per taxon
4 of each origin 3
25
21
20
24
Mixture of all 1999 origins per taxon
3
20
26
Mixture Mixture Mixture Mixture
4 4 4 4
25 25 25 25
28 28 28 28
Constant temperatures of 5, 10, 15, 20, 25, 30 8C with light Temperature fluctuations: day 14 h of 22 8C, night 10 h of 3, 6, 9, 12, 15, 18 8C with light 2000
Light at 14/22 8C Dark at 14/22 8C Salinity (33, 10, 3, 1, 0,3 und 0% NaCl) with light Osmolarity (503, 258, 136, 63, 28 und 0 g PEG 6000 per liter) with light
of of of of
all all all all
2000 2000 2000 2000
origins origins origins origins
per per per per
taxon taxon taxon taxon
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Table 3 Germination rates (% mean and standard deviation) in Oenanthe conioides (Oc) and O. aquatica (Oa) at the environmental variables light (L), darkness (dark, D) and flooding (F; C, control reference) Treatment
Year of collection
Light
Dark
p (L–D)
Light
Dark
p (L–D)
L
D
Light–dark
1999 2000
92 6.8 100 0.0
7 6.0 4 5.4
0.001 0.001
98 2.3 98 2.5
14 6.9 0 0.0
0.001 0.001
0.05 0.13
0.08 0.20
Treatment
Year of collection
Oenanthe conioides
C
F
0.82
0.05
Flooding
1999
Oenanthe conioides
Oenanthe aquatica
p (Oc–Oa)
Oenanthe aquatica
Control
Flooding
p (C–F)
Control
Flooding
p (C–F)
92 5.7
73 7.7
0.05
97 4.3
88 7.5
0.17
A posteriori t-test with Sidak correction, p > 0.05, 0.01, 0.001.
level of 1 cm) in the Petri dish. An additional filter paper above the seeds prevented them from floating. In a corresponding control seeds were covered with filter paper and supplied with the normal amount of 8 ml water. 2.2.4. Tidal hydroregime Four aquaria were placed in a climate chamber and pairwise provided with pumps and hoses that water could be exchanged between them. In each of them four Petri dishes were fixed onto clay flowerpots and provided with draining holes. A filter paper fixed with a locking ring prevented the seeds from floating. The pumps were controlled by time switches. Electricity wire was laid through the optional gas supply of the climate chamber. This experiment was not done with a mixture of seeds from all origins within a taxon, but with O. conioides from ‘‘Heuckenlock’’ and ‘‘Glu¨ckstadt’’ origin and with O. aquatica from ‘‘Buxtehude’’ and ‘‘Altengamme’’ origin separately. For each origin there were four replicates, one in each aquarium. In this experiment, light supply was not equal for each position within an aquarium. Therefore, Petri dishes with seeds of the same origin occupied a different position in each aquarium. Because of lack of space it was not possible to place additional water tanks into the climate chamber, so water had to be exchanged pairwise between the tanks with the result that the tidal regime of two aquaria was opposed to that of the other two. The tidal regime was 12/12 h, so that every tank had half time day light and night during high tide as well as low tide.
calculated for single comparisons. We chose an overall level of significance of 5%. Due to the larger variation of the percentage of germination in the experiment on the tidal hydroregime, we applied here a non-parametric Kruskal–Wallis H-test to assess differences among the different origins of the seed/species and tidal regimes followed by Mann–Whitney U-tests for pairwise comparisons between the different origins. Data are presented as Box–Whisker plots. 3. Results 3.1. Light/dark Seeds from both years germinated between 80% and 100% in light and only from 0% to 14% in dark in both Oenanthe taxa (Table 3). Pairwise comparisons of the species showed a significant
2.2.5. Salinity/osmolarity In the experiments on the influence of salinity and osmolarity watering was done with deionized water in the controls and with NaCl-solutions of 33% (approximate concentration of sea water at the Elbe mouth), 10%, 3.3%, 1% (different degrees of brackish water) and 0.3% in the salinity treatments. The experiments on osmolarity were done with solutions of the same osmotic potential as the salt solutions but this time made with polyethylenglycol (PEG) 6000 (503, 258, 136, 63.5 and 28 g PEG 6000 per liter, values converted corresponding to the formulae given by Michel and Kaufmann, 1973). Results are given as ratio of germinated to viable seeds. In O. conioides average viability of seeds was 85% in both 1999 and 2000, in O. aquatica the viability was 88% and 96% in 1999 and 2000, respectively. 2.3. Data analysis Germination percentages were arcsine transformed to reach homogeneity of variance. They were analysed by means of an analysis of variance (ANOVA). If the comparisons were not primarily pairwise, a-posteriori t-tests with Sidak correction were
Fig. 2. Germination rates of Oenanthe conioides and O. aquatica at constant (a) and fluctuating temperatures (b; day temperature 22 8C, night temperature = lower temperature). Differences between both species are significant at 15 8C constant temperature (p < 0.01) and at 3/22 and 6/22 8C fluctuating temperatures (p < 0.001 and p < 0.01, respectively).
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Fig. 3. Germination rates of Oenanthe conioides origin ‘‘Heuckenlock’’, an intermediate population origin ‘‘Glu¨ckstadt’’ and of O. aquatica origins ‘‘Buxtehude’’ and ‘‘Altengamme’’ under tidal hydroregime (H-test, p < 0.05; U-test, no significant differences in the tidal hydroregime between the different origins).
difference (p < 0.05) between the germination in light in 1999, O. conioides had a significantly lower percentage than O. aquatica. 3.2. Temperature At constant temperature, germination rates were relatively low (Fig. 2) and germination was delayed in both taxa. The reduction compared to the 14/22 8C regime was more pronounced in O. conioides than in O. aquatica, the taxa reacting significantly different at 15 8C. The optimum of germination under constant temperature was found at 20 8C. At low temperatures (5 and 10 8C), germination was much more delayed than at high temperatures. In all temperature fluctuation regimes germination percentage of O. conioides was about 90%. While showing the same reaction at 9, 12, 15 and 18 8C night temperature, O. aquatica had a significantly lower final germination percentage at 3 and 6 8C night temperature (Fig. 2) than O. conioides. Germination in O. aquatica was delayed for night temperatures of 12 8C and lower. 3.3. Tidal hydroregime and flooding Under 12 h flooding and 12 h of aeration O. conioides seeds of the origin ‘‘Heuckenlock’’ germinated slightly better than those of O. aquatica and those of O. conioides of Glu¨ckstadt origin (Fig. 3). In contrast to all other results the variance in this experiment was very high. 1 cm of flooding reduced germination in O. conioides significantly while O. aquatica was not significantly affected (Table 3). 3.4. Salinity/osmolarity Influence of salinity and the corresponding osmolarity were very similar for both Oenanthe taxa (Fig. 4). A toxic effect of salt (higher germination in PEG solution than in NaCl solution) could not be observed. Both Oenanthe species tolerate lightly brackish water. Only at 10% salinity (corresponding to 258 g PEG l 1) an inhibition of germination rate was found. The influence on further growth of the seedlings might be stronger since germination at 10% salinity was delayed and fungi occupied many seedlings. No germination was observed at 33% salinity (equivalent to sea water). However, even a 4 weeks’ incubation in 33% salinity did not necessarily damage the seeds. When rinsed with deionized
Fig. 4. Germination rates of Oenanthe conioides and O. aquatica at different levels of salinity (a) and osmolarity (b). Differences between both species in the total germination rate are significant at 3.3% and 63.3 g PEG l 1. There are no significant differences in the relative germination rates between both species.
water, 80% of the seeds germinated afterwards in non-saline water, although with higher susceptibility for fungi. 4. Discussion Both Oenanthe species could be characterized as species with a germination strategy adapted to rapid colonization of disturbed sites on emerged river banks and shores of ponds and lakes characterized either by long-term flooding and subsequent drop of water level (habitats of O. aquatica) or by storm tides and ice scouring (habitats of O. conioides). Both taxa have non-dormant seeds. In both species, germination is strongly light dependent and enhanced by fluctuating than under constant temperature regimes. These germination features permit detection of gaps and non-flooded or shallow water conditions (Schu¨tz, 2000). While germination triggered by light is a more general feature of aquatic plant species (Baskin and Baskin, 1998; Poschlod et al., 1999), the lack of dormancy is not. Of the emerged aquatic plant species listed in Baskin and Baskin (1998), only few show no dormancy, including Nasturtium officinale L. In this respect, the two Oenanthe taxa are not similar to the majority of emerged aquatic plants but of mud-flat plants (Baskin and Baskin, 1988). Like mudflat species and also Nasturtium, the most serious competitor of O. conioides, the lack of dormancy permits both Oenanthe taxa to react rapidly to favourable conditions, necessary on these very nutrient rich sites where vegetation-free space is quickly occupied. However, dormancy may not necessarily be absent in O. aquatica: Hroudova´ et al. (1992) found a profound percentage of dormancy (about 90%) in seeds collected in August. These were probably not yet ripe, since fresh seeds collected in September germinated to about 80%. Thus, both Oenanthe species do not exhibit the usual dormancy pattern within the family Apiaceae which is a linear embryo’s primary dormancy (Grime et al., 1981; Baskin and Baskin, 1998).
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The germination patterns in both Oenanthe taxa related to the temperature regimes is also very widespread in semiaquatic or amphibious plants (Thompson, 1974; Baskin and Baskin, 1998). Differences in the germination niches of both Oenanthe taxa were found in the flooding and tidal hydroregime treatments. O. conioides showed reduced germination rates at flooding while O. aquatica did not. Both reactions on flooding were already described for aquatic and amphibious plant species (Morinaga, 1926a,b; Arts and Van der Heijden, 1990; Voesenek et al., 1992; Leck, 1996; Baskin and Baskin, 1998). The reaction of O. conioides prevents part of the seeds from germinating under permanently flooded conditions. Together with the ability to tolerate tidal conditions better than O. aquatica this enables the taxon to occupy its specific spatial niche around and little below average high-water level. O. conioides can germinate under temperature fluctuations with lower ‘‘night’’ temperatures which already inhibit germination of O. aquatica. This may permit the taxon to germinate later in autumn and earlier in spring than O. aquatica and, therefore, provides an advantage in a tidal habitat. Phenological observations in natural populations of O. conioides showed that mortality was high for seedlings having germinated in autumn (Below, in press). On the other hand, O. conioides suffers from competition by other species like N. officinale (Kurz and Below, in press). In most cases, the species is successful which germinates first. From N. officinale we only know that seeds are non-dormant and germinate at 19 8C day and 15 8C night temperature (Muenscher, 1936). A large germination niche concerning temperature might represent an advantage for O. conioides in comparison to its competitor. Both Oenanthe taxa can tolerate an environment of brackish water and/or a high load of dissolved matter during germination which is comparable to other coastal species where germination is inhibited by 3.5–13% salinity (Ungar, 1978; Ignaciuk and Lee, 1980; Bakker et al., 1985; Ishikawa and Kachi, 2000; Noe and Zedler, 2000). All in all, the germination behaviour of the two studied closely related taxa shows many similarities but there are differences in the effect of hydroregime on germination rates concerning daily (tidal) and permanent flooding. Together with the differences found in the reaction on light and temperature regime we can speak of a fine-tuning for the specific habitat of O. conioides. However, these factors do not exclusively explain the occurrence of both species in different habitats. Further experiments could therefore concentrate on the study of survival of seedlings at respective hydroregimes. Acknowledgements We gratefully acknowledge the help of H. Below, H. Bertram, I. Fiebig, the late H.-W. Kallen and J. Neubecker who collected the seeds. M. Fett, H. Flores, B. Heitmann and A. Staaf helped with the field and laboratory work. The climate chamber in which the experiments on tidal hydroregime were done was provided by Firma Damm, Reiskirchen. We thank H. Below, H. Kurz and H.-H. Poppendieck for helpful discussions in the course of our work and Kai Jensen for comments on an earlier draft of this paper. Finally, the comments and help of an anonymous referee are acknowledged. The project was financially supported by the Federal Agency of Nature Conservation (Bundesamt fu¨r Naturschutz, BfN) and the Agency of Environment and Health (Beho¨rde fu¨r Umwelt und Gesundheit) Hamburg from May 2000 to April 2001. It was
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Keimungso¨kologie und Samenbank des Schierlings-Wasserfenchels. Naturschutz und Biologische Vielfalt, in press. Kadereit, G., Kadereit, J.W., 2005. Phylogenetic relationships, evolutionary origin, taxonomic status and genetic structure of the endangered local Lower Elbe river (Germany) endemic Oenanthe conioides (Nolte ex Rchb. f.) Lange (Apiaceae) ITS and AFLP evidence. Flora 200, 15–29. Kurz, H., Below, H. Konkurrenzbiologie des Schierlings-Wasserfenchels. Naturschutz und Biologische Vielfalt, in press. Leck, M.A., 1996. Germination of macrophytes from a Delaware River tidal freshwater wetland. Bull. Torr. Bot. Club. 123, 48–67. Michel, B.E., Kaufmann, M.R., 1973. The osmotic potential of polyethylene glycol 6000. Plant Physiol. 51, 914–916. Morinaga, T., 1926a. The favourable effect of reduced oxygen supply upon the germination of certain seeds. Am. J. Bot. 13, 159–166. Morinaga, T., 1926b. Germination of seeds under water. Am. J. Bot. 13, 126–140. Muenscher, W.C., 1936. Storage and germination of seeds of aquatic plants. Cornell University Agric. Exp. Station Bull. 22, 23–32. Neubecker, J. Pha¨notypische Plastizita¨t von Oenanthe conioides und Oenanthe aquatica. Naturschutz und Biologische Vielfalt, in press. Noe, G.B., Zedler, J.B., 2000. Differential effects of four abiotic factors on the germination of salt marsh annuals. Am. J. Bot. 87, 1679–1692. Otte, A., 1994. The temperature requirements of crop field herbs at the time of germination—another cause for changes in the plant association of crop fields. In: Song, Y., Dierschke, H., Wang, X. (Eds.), Applied Vegetation Ecology. East China Normal University Press, Shanghai, pp. 280–288. Poppendieck, H.-H. Taxonomie und Systematik. Naturschutz und Biologische Vielfalt, in press. Poschlod, P., Bo¨hringer, J., Fennel, S., Prume, C., Tieko¨tter, A., 1999. Aspekte der ¨ kologie von Arten der Zwergbinsenfluren. Mitt. bad. Landesver. Biologie und O Naturkunde Naturschutz 17, 219–260. Redondo, S., Rubio-Casal, A.E., Castillo, J.M., Luque, C.J., Alvarez, A.A., Luque, T., Figueroa, M.E., 2004. Influences of salinity and light on germination of three Sarcocornia taxa with contrasted habitats. Aquat. Bot. 78, 255–264. Schu¨tz, W., 1998. Seed dormancy cycles and germination phenologies in sedges (Carex) from various habitats. Wetlands 18, 288–297.
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Germination ecology of two closely related taxa in the genus Oenanthe: Fine tuning for the habitat? Doris Jensch a,b, Peter Poschlod a,* a b
Institute of Botany, Faculty of Biology and Preclinical Medicine, University of Regensburg, D-93040 Regensburg, Germany Wilhelm-Leuschner-Straße 39, D-61169 Friedberg, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 24 July 2007 Received in revised form 26 March 2008 Accepted 28 March 2008 Available online 18 April 2008
For two closely related amphibious plant taxa of the genus Oenanthe (Apiaceae) germination traits were examined. Habitats of the two taxa differ in hydroregime. The endemic Oenanthe conioides of the river Elbe estuary experiences daily tidal fluctuations whereas the widespread Oenanthe aquatica grows at the edge of ponds and in freshwater wetlands with rare and unpredictable fluctuations of water table. Seeds of both taxa could be characterized as non-dormant and light dependent. Under temperature fluctuations, germination percentage was higher than under constant temperatures. Salinity was tolerated to up to 3.3%. At 10% there was a strong decrease in germination percentage, which did not result from ionic toxicity, as experiments with a gradient in osmolarity showed. Differences between the taxa were found concerning hydrological and temperature fluctuations. While O. aquatica showed no reduction in germination percentage under permanent 1 cm flooding, O. conioides did. On the other hand, germination under an artificial tidal hydroregime was better in O. conioides than in O. aquatica. During fluctuating temperatures of 3/22 and 6/22 8C O. conioides germinated much quicker and had a higher final germination percentage. Differences between the taxa could be correlated with differences of the hydrological regime in the specific habitat. Taking into account that O. conioides is phylogenetically a relatively young taxon, it may be hypothesized that a quick adaptation to the tidal conditions might have taken place. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Keywords: Germination ecology Amphibious habitat Tidal water regime Oenanthe aquatica Oenanthe conioides
1. Introduction An important life-history trait of plant species in flood plains is the timing of germination. Strong selection for gap detection mechanisms and for the detection of a suitable position in relation to the average water-level (hence to frequency of flooding) can lead to different germination characteristics and to different realized niches of closely related species (e.g. Rumex spp.: Voesenek and Blom, 1992; Voesenek et al., 1992). As selection on germination in space and time is important in flood plains we suspect ‘‘fine-tuned’’ differences between closely related taxa or ecotypes which are restricted to specific hydro-regimes. We analysed the germination characteristics of Oenanthe aquatica (L.) Poiret and Oenanthe conioides (Nolte) Lange. O. conioides has probably arisen sympatrically from O. aquatica through ecological divergence (Kadereit and Kadereit, 2005). Both taxa are annual or biennial, germination hence being an important
* Corresponding author. Tel.: +49 941 943 3108; fax: +49 941 943 3106. E-mail address: [email protected] (P. Poschlod).
life stage. O. aquatica is wide-spread, mostly found in freshwater wetlands, edges and mud flats of ponds and ditches and in backwaters. Its habitat is characterized by low-frequency (erratic or regular) flooding events. The taxon is able to endure more or less long-time flooding. A clear morphological adaptation are specific submerged leaves to which already the primary leaves can develop under flooded conditions (Fig. 1). O. conioides is restricted to the tidal freshwater flood plains of the Elbe estuary (Germany). This is a habitat with regular tidal floodings (hence of high frequency) and erratic high (long-time) floodings by storm tides or high water of the river (several times per year). O. conioides is morphologically adapted to these conditions by the absence of specific submerged leaves and by less pinnated leaves, thus being probably more desiccation-resistant and less vulnerable towards currents than O. aquatica. Leaf differences between the taxa are most distinct in the primary leaves (Fig. 1). The shape of the adult leaves shows high plasticity, although both taxa might be distinguished by the relation of circumference to leaf area (Neubecker, in press; Poppendieck, in press). Therefore, hydroregimes of habitats of both Oenanthe taxa differ mostly in the predictable (tidal) component of frequent floodings
0304-3770/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2008.03.013
D. Jensch, P. Poschlod / Aquatic Botany 89 (2008) 345–351
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lakes). Such an alteration must be tolerated because successfully reproducing individuals of the taxon can be found at 1.0 m below to 0.1 m above average high-tide water level. Since the river Elbe has a high load of suspended and dissolved matter and the distribution of O. conioides reaches to the zone of light brackish water (psu up to >3%), the influence of osmolarity and salinity could be another factor which might significantly affect the germination (e.g. Ignaciuk and Lee, 1980; Redondo et al., 2004). Therefore, we analysed in a series of laboratory experiments the germination responses of these two related taxa to a range of conditions reflecting habitat variability which were light, constant temperatures as well as temperature fluctuations, flooding, tidal hydroregime, osmolarity and salinity. 2. Methods
Fig. 1. Typical seedlings of Oenanthe conioides and Oenanthe aquatica from the Elbe estuary and its surroundings under flooding of 1 cm (above) and without flooding (below).
while long-term erratic floodings are more or less frequent. Until recently, O. aquatica was not found in the Elbe estuary (Poppendieck, in press). In the most recent studies, some populations of O. aquatica were found in this habitat (Below et al., 1996) and three of these were confirmed as genetically different from O. conioides or intermediate (Kadereit and Kadereit, 2005). Both Oenanthe species set fruit in summer to early autumn (Hroudova´ et al., 1992; Below, in press) and probably form a seed bank of several years (Hroudova´ et al., 1992; Jensch and Poschlod, in press). As regulation mechanisms of germination stratification, light (red/far-red ratio), temperature, temperature fluctuations, and the tolerance to flooding are widely described for plant species in flood-plains (Morinaga, 1926a,b; Shamsi and Whitehead, 1974; Washitani and Masuda, 1990; Voesenek et al., 1992; Clevering, 1995; Leck, 1996; Baskin and Baskin, 1998; Schu¨tz, 1998; Poschlod et al., 1999). In the case of O. conioides, the alteration of flooding (often correlated to anoxic conditions) and terrestrial conditions may also be suitable to detect the position of a seed on the river bank (see Arts and Van der Heijden, 1990: Littorella uniflora in
All experiments were performed with seeds originating from the river Elbe estuary and its surroundings, collected in two successive years within 2 weeks in August and September 1999 and September and October 2000, respectively; hence, excluding differences in seed ripening due to interannual differences in habitat conditions. Differences caused by maternal effects could be covered partly by the repetition of some experiments with seeds matured in the two successive years. 2.1. Seed collection Ripe seeds of O. aquatica and O. conioides were collected in autumn in 1999 and 2000 along the river Elbe and its surroundings between Schnackenburg and Glu¨ckstadt (river kilometres 585– 675). The area is located between 98 and 128 eastern longitude and 538 and 548 northern latitude. While a sufficient amount of seeds could be collected in 1999, the harvest of 2000 was so small that the experiments performed in 1999 could only be replicated partly with seeds harvested in 2000, and replication number in each experiment was reduced from 8 to 4. Additionally, seeds of 2000 were harvested from only few mother plants (Table 1). Some populations collected here as O. conioides were shown after our study to be intermediate populations by means of genetic analysis (Kadereit and Kadereit, 2005). These genetic studies revealed that the only ‘‘pure’’ populations of O. conioides found nowadays are those of ‘‘Heuckenlock’’ and ‘‘Schweenssand’’. Genetically they form a ‘‘coherent genetic lineage originating from one ancestor’’ which arose from O. aquatica, thus they can be regarded as a progenitor-derivative taxon pair (Kadereit and Kadereit, 2005). Herbarium material proved that in former times the morphological separation was much clearer than for the
Table 1 Origin of seeds of both Oenanthe species (if the origins are at or near the river Elbe, river kilometres (Elbe km) are given) Taxon
Date of collection
Number of seeds
Origin
Number of mother plants
Oenanthe conioides
25.8.99 27.8.99 29.8.99
3000 ca. 500 3000
31.8.99
3000
Heuckenlock, nature reserve Elbe km 612/613 Schweenssand, nature reserve, Elbe km 612/613 Glu¨ckstadt, confined disposal area for dredged material of the Elbe, near Elbe km 675 Neßsand, nature reserve, Elbe km 638
6.–26.9.00
1200
Heuckenlock, nature reserve, Elbe km 612/613
11 1 Not collected separately, counted as if from six mother plants Not collected separately, counted as if from four mother plants 2
7.9.99 19.9.99 21.9.99 27.9.99 5.10.99 12.10.99 24.10.00 24.10.00
3000 >2000 ca. 200 ca. 400 ca. 500 ca. 300 1020 390
Altengamme, Hamburg Buxtehude, Niedersachsen Grippel, Niedersachsen, Elbe km 497 Duvenstedter Brook, nature reserve, Hamburg Ahrensburg, Schleswig-Holstein Walmsburger Werder, Niedersachsen, Elbe km 539 Altengamme, Hamburg Gut Moor, Hamburg
5 4 2 1 2 5 5 5
Oenanthe aquatica
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material found nowadays at the river Elbe estuary (Poppendieck, in press). An introgression of O. aquatica into the habitat of O. conioides in the wake of habitat changes by harbour and river engineering is suspected (Poppendieck, in press). Poppendieck (in press) could show that hybrids of the taxa exist with intermediate morphology. This means that the seed material from ‘‘Glu¨ckstadt’’ and ‘‘Neßsand’’ consisted not only of Oenanthe conioides seeds but partly of intermediate populations nearer to Oenanthe aquatica (Neßsand). Therefore, seed experiments on O. conioides performed with the pooled seed collection in 1999, contained a large proportion (45%) of seeds belonging to these intermediate populations. However, since from a morphological point of view they could be clearly assigned to O. conioides and were additionally collected from specific O. conioides habitats germination behaviour of these populations has been assumed here to be similar as that of the ‘‘pure’’ populations. We find some support for this in the reportedly rapid adaptation of the germination niche of short living species to specific habitat conditions (temperature, seasonal germination niche, etc.) have shown (Van der Vegte, 1978; Gutterman, 1992; Otte, 1994; Donohue, 2005). First germination experiments started a few days after seed collection. The remaining seeds were stored at 5 8C in moist sand separated according to their mother plants. For each germination test, the same number of seeds per mother plant was mixed for each taxon and taken arbitrarily. Seeds sampled in Glu¨ckstadt and Neßsand in 1999, which were not collected separately per mother plant, were treated as if they came from six and four mother plants, respectively, with the following exception: the experiments on the tidal hydroregime were carried out for the origins ‘‘Heuckenlock’’ and ‘‘Glu¨ckstadt’’ (O. conioides), ‘‘Buxtehude’’ and ‘‘Altengamme’’ (O. aquatica) separately. 2.2. Germination tests In each germination test, 25 mericarps (in the following named ‘‘seeds’’ for convenience) were placed on two layers of filter paper in a plastic Petri dish (exception: experiments on temperature, see below). One Petri dish was considered as our unit of replication. Radicle emergence was taken as proof of germination. Experiments lasted 28 days, except the experiments on temperature (24 days or 26 days, see Table 2) and tidal hydroregime (21 days because of beginning infestation with algae). Non-germinated seeds were tested for viability after the experiment by means of the
347
tetrazolium test (ISTA, 1993). All tests were planned to be conducted with eight replicates. Because of shortage of seeds some experiments had to be reduced to three or four replicates (see Table 2 and Section 2.1). All germination experiments except the experiments on temperature and tidal hydroregime were conducted in a climate chamber with a diurnally fluctuating temperature of 22/14 8C and light supply of 14 h by Osram-Lumilux daylight fluorescent tubes (ca. 10,000 lx at Petri dish) during the higher temperature period. Except for the experiments on the hydroregime (hypoxia, tide) and salinity/osmolarity seeds were supplied with 8 ml of deionized water at the beginning of the experiment to saturate the two layers of filter paper with water. The Petri dish was sealed afterwards. 2.2.1. Light/dark To test germination under dark conditions Petri dishes were placed in a black (light-tight) box and the course of germination in the 1999 experiments was recorded under ‘‘safe green light’’. In 2000, the seeds were already placed into the dishes and watered under ‘‘safe green light’’, then placed into the carton which was opened only after the end of the experiment to exclude any possible green light effects this time (Grime and Jarvis, 1975; Blom, 1978). 2.2.2. Temperature For the temperature experiments a thermogradient bar apparatus was used containing 12 small long chambers along the temperature gradient. They were provided with long strips of filter paper and watered as required. Water loss was prevented by a cover of transparent plastic. Light supply was similar to that in the climate chamber used for the experiments on light, salinity/ osmolarity and hypoxia. Because of the small space available at each position in the chambers these experiments were conducted with only 20 seeds per replicate (see Table 2). There were only three replicates due to a reduced number of seeds available for the experiment in 2000 (see above). Constant temperatures used were 5, 10, 15, 20, 25 and 30 8C. Temperature fluctuation regimes had a temperature during the 14 h day period of 22 8C and night temperatures of 3, 6, 9, 12, 15 and 18 8C. 2.2.3. Flooding Flooding conditions reducing oxygen available to the seeds were simulated by shallow flooding with deionized water (water
Table 2 Germination tests, origin of seed and year of seed collection Year of seed collection
Factor tested
Seed origin
Number of replicates
Number of seeds/ replicate
Duration of test (days)
1999
Light at 14/22 8C Dark at 14/22 8C Hypoxia (1 cm flooding, covered with filter paper) at light and 14/22 8C Control of hypoxia (as ‘‘light’’, but with cover of filter paper) Tidal hydroregime with light and 14/22 8C
Mixture of all 1999 origins per taxon Mixture of all 1999 origins per taxon Mixture of all 1999 origins per taxon
8 8 8
25 25 25
28 28 28
Mixture of all 1999 origins per taxon
4
25
28
Conioides: Heuckenlock + Schweenssand, Glu¨ckstadt; aquatica: Buxtehude, Altengamme Mixture of all 1999 origins per taxon
4 of each origin 3
25
21
20
24
Mixture of all 1999 origins per taxon
3
20
26
Mixture Mixture Mixture Mixture
4 4 4 4
25 25 25 25
28 28 28 28
Constant temperatures of 5, 10, 15, 20, 25, 30 8C with light Temperature fluctuations: day 14 h of 22 8C, night 10 h of 3, 6, 9, 12, 15, 18 8C with light 2000
Light at 14/22 8C Dark at 14/22 8C Salinity (33, 10, 3, 1, 0,3 und 0% NaCl) with light Osmolarity (503, 258, 136, 63, 28 und 0 g PEG 6000 per liter) with light
of of of of
all all all all
2000 2000 2000 2000
origins origins origins origins
per per per per
taxon taxon taxon taxon
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Table 3 Germination rates (% mean and standard deviation) in Oenanthe conioides (Oc) and O. aquatica (Oa) at the environmental variables light (L), darkness (dark, D) and flooding (F; C, control reference) Treatment
Year of collection
Light
Dark
p (L–D)
Light
Dark
p (L–D)
L
D
Light–dark
1999 2000
92 6.8 100 0.0
7 6.0 4 5.4
0.001 0.001
98 2.3 98 2.5
14 6.9 0 0.0
0.001 0.001
0.05 0.13
0.08 0.20
Treatment
Year of collection
Oenanthe conioides
C
F
0.82
0.05
Flooding
1999
Oenanthe conioides
Oenanthe aquatica
p (Oc–Oa)
Oenanthe aquatica
Control
Flooding
p (C–F)
Control
Flooding
p (C–F)
92 5.7
73 7.7
0.05
97 4.3
88 7.5
0.17
A posteriori t-test with Sidak correction, p > 0.05, 0.01, 0.001.
level of 1 cm) in the Petri dish. An additional filter paper above the seeds prevented them from floating. In a corresponding control seeds were covered with filter paper and supplied with the normal amount of 8 ml water. 2.2.4. Tidal hydroregime Four aquaria were placed in a climate chamber and pairwise provided with pumps and hoses that water could be exchanged between them. In each of them four Petri dishes were fixed onto clay flowerpots and provided with draining holes. A filter paper fixed with a locking ring prevented the seeds from floating. The pumps were controlled by time switches. Electricity wire was laid through the optional gas supply of the climate chamber. This experiment was not done with a mixture of seeds from all origins within a taxon, but with O. conioides from ‘‘Heuckenlock’’ and ‘‘Glu¨ckstadt’’ origin and with O. aquatica from ‘‘Buxtehude’’ and ‘‘Altengamme’’ origin separately. For each origin there were four replicates, one in each aquarium. In this experiment, light supply was not equal for each position within an aquarium. Therefore, Petri dishes with seeds of the same origin occupied a different position in each aquarium. Because of lack of space it was not possible to place additional water tanks into the climate chamber, so water had to be exchanged pairwise between the tanks with the result that the tidal regime of two aquaria was opposed to that of the other two. The tidal regime was 12/12 h, so that every tank had half time day light and night during high tide as well as low tide.
calculated for single comparisons. We chose an overall level of significance of 5%. Due to the larger variation of the percentage of germination in the experiment on the tidal hydroregime, we applied here a non-parametric Kruskal–Wallis H-test to assess differences among the different origins of the seed/species and tidal regimes followed by Mann–Whitney U-tests for pairwise comparisons between the different origins. Data are presented as Box–Whisker plots. 3. Results 3.1. Light/dark Seeds from both years germinated between 80% and 100% in light and only from 0% to 14% in dark in both Oenanthe taxa (Table 3). Pairwise comparisons of the species showed a significant
2.2.5. Salinity/osmolarity In the experiments on the influence of salinity and osmolarity watering was done with deionized water in the controls and with NaCl-solutions of 33% (approximate concentration of sea water at the Elbe mouth), 10%, 3.3%, 1% (different degrees of brackish water) and 0.3% in the salinity treatments. The experiments on osmolarity were done with solutions of the same osmotic potential as the salt solutions but this time made with polyethylenglycol (PEG) 6000 (503, 258, 136, 63.5 and 28 g PEG 6000 per liter, values converted corresponding to the formulae given by Michel and Kaufmann, 1973). Results are given as ratio of germinated to viable seeds. In O. conioides average viability of seeds was 85% in both 1999 and 2000, in O. aquatica the viability was 88% and 96% in 1999 and 2000, respectively. 2.3. Data analysis Germination percentages were arcsine transformed to reach homogeneity of variance. They were analysed by means of an analysis of variance (ANOVA). If the comparisons were not primarily pairwise, a-posteriori t-tests with Sidak correction were
Fig. 2. Germination rates of Oenanthe conioides and O. aquatica at constant (a) and fluctuating temperatures (b; day temperature 22 8C, night temperature = lower temperature). Differences between both species are significant at 15 8C constant temperature (p < 0.01) and at 3/22 and 6/22 8C fluctuating temperatures (p < 0.001 and p < 0.01, respectively).
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Fig. 3. Germination rates of Oenanthe conioides origin ‘‘Heuckenlock’’, an intermediate population origin ‘‘Glu¨ckstadt’’ and of O. aquatica origins ‘‘Buxtehude’’ and ‘‘Altengamme’’ under tidal hydroregime (H-test, p < 0.05; U-test, no significant differences in the tidal hydroregime between the different origins).
difference (p < 0.05) between the germination in light in 1999, O. conioides had a significantly lower percentage than O. aquatica. 3.2. Temperature At constant temperature, germination rates were relatively low (Fig. 2) and germination was delayed in both taxa. The reduction compared to the 14/22 8C regime was more pronounced in O. conioides than in O. aquatica, the taxa reacting significantly different at 15 8C. The optimum of germination under constant temperature was found at 20 8C. At low temperatures (5 and 10 8C), germination was much more delayed than at high temperatures. In all temperature fluctuation regimes germination percentage of O. conioides was about 90%. While showing the same reaction at 9, 12, 15 and 18 8C night temperature, O. aquatica had a significantly lower final germination percentage at 3 and 6 8C night temperature (Fig. 2) than O. conioides. Germination in O. aquatica was delayed for night temperatures of 12 8C and lower. 3.3. Tidal hydroregime and flooding Under 12 h flooding and 12 h of aeration O. conioides seeds of the origin ‘‘Heuckenlock’’ germinated slightly better than those of O. aquatica and those of O. conioides of Glu¨ckstadt origin (Fig. 3). In contrast to all other results the variance in this experiment was very high. 1 cm of flooding reduced germination in O. conioides significantly while O. aquatica was not significantly affected (Table 3). 3.4. Salinity/osmolarity Influence of salinity and the corresponding osmolarity were very similar for both Oenanthe taxa (Fig. 4). A toxic effect of salt (higher germination in PEG solution than in NaCl solution) could not be observed. Both Oenanthe species tolerate lightly brackish water. Only at 10% salinity (corresponding to 258 g PEG l 1) an inhibition of germination rate was found. The influence on further growth of the seedlings might be stronger since germination at 10% salinity was delayed and fungi occupied many seedlings. No germination was observed at 33% salinity (equivalent to sea water). However, even a 4 weeks’ incubation in 33% salinity did not necessarily damage the seeds. When rinsed with deionized
Fig. 4. Germination rates of Oenanthe conioides and O. aquatica at different levels of salinity (a) and osmolarity (b). Differences between both species in the total germination rate are significant at 3.3% and 63.3 g PEG l 1. There are no significant differences in the relative germination rates between both species.
water, 80% of the seeds germinated afterwards in non-saline water, although with higher susceptibility for fungi. 4. Discussion Both Oenanthe species could be characterized as species with a germination strategy adapted to rapid colonization of disturbed sites on emerged river banks and shores of ponds and lakes characterized either by long-term flooding and subsequent drop of water level (habitats of O. aquatica) or by storm tides and ice scouring (habitats of O. conioides). Both taxa have non-dormant seeds. In both species, germination is strongly light dependent and enhanced by fluctuating than under constant temperature regimes. These germination features permit detection of gaps and non-flooded or shallow water conditions (Schu¨tz, 2000). While germination triggered by light is a more general feature of aquatic plant species (Baskin and Baskin, 1998; Poschlod et al., 1999), the lack of dormancy is not. Of the emerged aquatic plant species listed in Baskin and Baskin (1998), only few show no dormancy, including Nasturtium officinale L. In this respect, the two Oenanthe taxa are not similar to the majority of emerged aquatic plants but of mud-flat plants (Baskin and Baskin, 1988). Like mudflat species and also Nasturtium, the most serious competitor of O. conioides, the lack of dormancy permits both Oenanthe taxa to react rapidly to favourable conditions, necessary on these very nutrient rich sites where vegetation-free space is quickly occupied. However, dormancy may not necessarily be absent in O. aquatica: Hroudova´ et al. (1992) found a profound percentage of dormancy (about 90%) in seeds collected in August. These were probably not yet ripe, since fresh seeds collected in September germinated to about 80%. Thus, both Oenanthe species do not exhibit the usual dormancy pattern within the family Apiaceae which is a linear embryo’s primary dormancy (Grime et al., 1981; Baskin and Baskin, 1998).
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The germination patterns in both Oenanthe taxa related to the temperature regimes is also very widespread in semiaquatic or amphibious plants (Thompson, 1974; Baskin and Baskin, 1998). Differences in the germination niches of both Oenanthe taxa were found in the flooding and tidal hydroregime treatments. O. conioides showed reduced germination rates at flooding while O. aquatica did not. Both reactions on flooding were already described for aquatic and amphibious plant species (Morinaga, 1926a,b; Arts and Van der Heijden, 1990; Voesenek et al., 1992; Leck, 1996; Baskin and Baskin, 1998). The reaction of O. conioides prevents part of the seeds from germinating under permanently flooded conditions. Together with the ability to tolerate tidal conditions better than O. aquatica this enables the taxon to occupy its specific spatial niche around and little below average high-water level. O. conioides can germinate under temperature fluctuations with lower ‘‘night’’ temperatures which already inhibit germination of O. aquatica. This may permit the taxon to germinate later in autumn and earlier in spring than O. aquatica and, therefore, provides an advantage in a tidal habitat. Phenological observations in natural populations of O. conioides showed that mortality was high for seedlings having germinated in autumn (Below, in press). On the other hand, O. conioides suffers from competition by other species like N. officinale (Kurz and Below, in press). In most cases, the species is successful which germinates first. From N. officinale we only know that seeds are non-dormant and germinate at 19 8C day and 15 8C night temperature (Muenscher, 1936). A large germination niche concerning temperature might represent an advantage for O. conioides in comparison to its competitor. Both Oenanthe taxa can tolerate an environment of brackish water and/or a high load of dissolved matter during germination which is comparable to other coastal species where germination is inhibited by 3.5–13% salinity (Ungar, 1978; Ignaciuk and Lee, 1980; Bakker et al., 1985; Ishikawa and Kachi, 2000; Noe and Zedler, 2000). All in all, the germination behaviour of the two studied closely related taxa shows many similarities but there are differences in the effect of hydroregime on germination rates concerning daily (tidal) and permanent flooding. Together with the differences found in the reaction on light and temperature regime we can speak of a fine-tuning for the specific habitat of O. conioides. However, these factors do not exclusively explain the occurrence of both species in different habitats. Further experiments could therefore concentrate on the study of survival of seedlings at respective hydroregimes. Acknowledgements We gratefully acknowledge the help of H. Below, H. Bertram, I. Fiebig, the late H.-W. Kallen and J. Neubecker who collected the seeds. M. Fett, H. Flores, B. Heitmann and A. Staaf helped with the field and laboratory work. The climate chamber in which the experiments on tidal hydroregime were done was provided by Firma Damm, Reiskirchen. We thank H. Below, H. Kurz and H.-H. Poppendieck for helpful discussions in the course of our work and Kai Jensen for comments on an earlier draft of this paper. Finally, the comments and help of an anonymous referee are acknowledged. The project was financially supported by the Federal Agency of Nature Conservation (Bundesamt fu¨r Naturschutz, BfN) and the Agency of Environment and Health (Beho¨rde fu¨r Umwelt und Gesundheit) Hamburg from May 2000 to April 2001. It was
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