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Influence of salinity and temperature on the germination of Ruppia maritima L. from the North Atlantic and Gulf of Mexico
Aquatic Botany, 40 ( 1991 ) 387-391
387
Elsevier Science Publishers B.V., Amsterdam
Short Communication Influence of salinity and temperature on the germination of Ruppia maritima L. from the North Atlantic and Gulf of Mexico E.W. Koch ~ and C.J. Dawes Department of Biology, Universityof South Florida, Tampa, FL 33620, USA (Accepted for publication 27 November 1990 )
ABSTRACT Koch, E.W. and Dawes, C.J., 1991. Influence of salinity and temperature on the germination of Ruppia maritima L. from the North Atlantic and Gulf of Mexico. Aquat. Bot., 40: 387-391.
Seeds from the seagrass Ruppia maritima L. collected in North Carolina and Florida were tested for their germination at 17, 23 and 29°C and at 0, 15 and 30%o salinities. Percent germination was significantly higher for seeds from North Carolina and the seeds from Florida did not germinate in 30%o salinity. The northern population also had a more rapid rate of germination. These germination patterns suggest that ecotypic differentiation in germination occurred between the two populations in relation to salinity but not temperature.
INTRODUCTION
Ruppia maritima L. has an annual or perennial life cycle depending on the local environmental stresses (Koch and Seeliger, 1988). Thus the seeds of this species play an important role in regenerating annual populations (Brock, 1983 ), in contrast to other seagrasses of the Southeastern United States which propagate vegetatively (Moffier and Durako, 1987). It has been suggested that the germination of R. maritima seeds is controlled by environmental conditions (Orth and Moore, 1983). This study deals with the physiological ecology of two latitudinally isolated populations of R. maritima from the North Atlantic Ocean and the Gulf of Mexico germinated under common garden conditions. The results are discussed in terms of ecotypic differentiation and phenotypic plasticity in relation to salinity and temperature. ~Present address: Department of Marine Sciences, University of South Florida, St. Petersburg, FL 33701, USA.
0304-3770/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
388
E.W. KOCH AND C.J. DAWES
MATERIALS AND METHODS
Ruppia maritima was sampled in June 1986 in Pamlico Sound on Cedar Island, North Carolina (34 ° 58'N, 76 ° 2 2 ' W ) and in the estuary of the Weeki Wachee River on the west coast of Florida (28 ° 32'N, 82 ° 39'W). Temperature and salinity for the North Carolina population ranged from 10 to 36°C and 6 to 30%o salinity (M. Fonseca, personal communication, 1987) and for the Florida population from 20 to 27°C and 2 to 14%o salinity (Davis and Dawes, 1981; Koch, 1988 ). Seed collection at the two sites was carried out while some of the plants in each population were still flowering and had attached seeds. Seed densities in the sediments were determined for each population using 10 cores which were 5 cm deep and 10 cm in diameter (Verhoeven, 1979). The sediment was sieved with a 1 m m mesh. Intact, unblemished seeds were picked from the organic material and cleaned by shaking them alternately with 96% ethanol (pH 2 ) and autoclaved distilled water (Seeliger et al., 1984 ). Clean seeds were "stratified" by holding them in distilled water for 2 weeks at 6 °C as described by Seeliger et al. (1984). A preliminary experiment suggested that stratification did not inhibit the seed germination of either population. Germination was tested at different temperatures ( 17, 23, 29 °C using 0%o salinity) and salinities (0, 15, 30%o using diluted autoclaved seawater at 23 °C). All seawater was obtained on the west coast of Florida and diluted with distilled water to the desired salinities. Five replicates with 100 seeds were used for each treatment of the North Carolina population (i.e. 500 seeds per treatment) while four replicates with 20 seeds were used for the Florida population (i.e. 80 seeds per treatment). Germination was defined as the time when the cotyledon emerged. The number of germinated seeds was recorded weekly over a two m o n t h period as done with R. maritima from southern Brazil (Koch and Seeliger, 1988). ANOVAs were used to compare the number of seeds from the two sites. Results obtained from the germination experiments were analyzed as 3 × 2 factorial experiments using a SAS program (SAS Institute Inc., 1985 ). Stud e n t - N e w m a n - K e u l s (SNK) multiple range tests were carried out to test for significant differences (see Sokal and Rohlf, 1981 ). RESULTS
The numbers of seeds found in the sediment of the Florida site were significantly higher ( F = 2 4 . 4 0 , P<0.0001; data without normal distribution; 1006 +_613 seeds m - 2 ) than for the North Carolina site (38 +_86 seeds m - 2 ) . Most seeds from the northern population were unblemished while a large number from Florida had opened seed coats. The data for germination at three temperatures were not normal with no
S A L I N I T Y A N D T E M P E R A T U R E EFFECTS O N RUPPIA MARITIMA G E R M I N A T I O N
389
homogeneity of variances for both populations, but the strength of the test and the large F value should account for the lack of those prerequisites in the ANOVA performed. The northern seeds had a significantly higher germination rate ( F = 100.27, P < 0.0001; Fig. 1 ) at all temperatures. Although fewer seeds from each population germinated at the highest temperature (29 °C), there was no significant difference between germination at the three temperatures tested ( F = 3.20 for Florida, F = 0.95 for North Carolina) owing to the large standard error. North Carolina seeds germinated in the first 10 days while the Florida seeds continued to germinate over the entire 68 day period of the experiment. Ruppia maritima seeds from North Carolina had an earlier germination time (25 days) and a significantly higher germination rate at all salinities ( F = 37.01, P < 0.0001 ) compared with Florida seed germination (35 days; Fig. 2 ). However, germination of the northern seeds was significantly lower NORTH CAROLINA 60 ¸ ~) 50"
~z_4 o . i,i c, Lu
I,<
2 \T -o-2:c
50.
20. I0.
?.~ 2 0 ]
FLORIDA
101 L----
...... ~ " "E-- . . . . . .
-,~o
............ -- ................
~o
4o
~o
~
-~ -0
do
~o
-io
TIME (days)
Fig. 1. Germination of Ruppia maritima seeds from North Carolina and Florida exposed to 17 ° C ( s q u a r e s ) , 23 ° C ( t r i a n g l e s ) and 29 ° C (circles) at 0%o salinity. NORTH
CAROLINA
60o~ 5 0 z 0 40<[
z_N?.
i~/
•
O PPT
•
15PPT
•
30PPT
~ 20w I--
t(,.
•~§I,o z°
/r ...................................
_..___.i-----'~ -
,
I0
"FLO"'OA
AT, . . . . . . .
2'0
30
.
. . . . . . . . .
40 T I M E (days)
• .
. . . . . . .
50
.
. . . . .
60
~
,
70
Fig. 2. Germination ofRuppia maritima seeds from North Carolina and Florida exposed to 0%o ( s q u a r e s ) , 15%o ( t r i a n g l e s ) a n d 300/o0 salinity (circles) at 23 ° C.
390
E.W. KOCH AND C,J. DAWES
at 30%0 salinity ( F = 4 . 4 7 , P < 0 . 0 5 ) relative to rates in 0 and 15%o salinity which were not significantly different. In contrast, rates for seeds from the southern population were not significantly different between the three salinities tested ( F = 2.29) even though there was no germination in 30%o salinity (Fig. 2). DISCUSSION
The present study suggests ecotypic differentiation in seed germination patterns ofR. maritima populations from Florida and North Carolina to salinity but not to temperature. The continuous germination of Florida seeds over the duration of the experiments suggests that they are adapted to an environment where growth conditions are favorable year round. This capacity for continuous germination would also explain the large number of opened seed coats. In contrast, the North Carolina population has a short germination period of about 1 m o n t h which matches the shorter growing season. Environmental stresses can enhance the germination of Ruppia maritima seeds (Koch and Seeliger, 1988 ). A significantly higher germination rate was found for the northern population, under all temperatures and salinities tested. This high percent, as well as rapid rate of germination may be critical where water temperature shows a large seasonal fluctuation. The smaller seasonal fluctuation in the Gulf of Mexico may account for a lower and slower rate of germination during the entire year. In contrast to temperature, there were distinct germination patterns for the two populations in the three salinities tested suggesting ecotypic differentiation at each site. This difference is probably because the Weeki Wachee River population never experiences salinities as high as 30%0 (Dawes et al., 1978; Davis and Dawes, 1981; Koch, 1988) while the northern population experiences a broader salinity range (6-30%o). ACKNOWLEDGEMENTS
The first author would like to thank CNPq - Conselho Nacional de Desenvolvimento Cientifico e Tecnol6gico, Brazil - for financial support during the Masters program at the University of South Florida of which this study was part.
REFERENCES Brock, M.A., 1983. Reproductive allocation in annual and perennial species of the submerged aquatic halophyte Ruppia. J. Ecol., 71: 811-818. Davis, M. and Dawes, C.J., 1981. Seasonal photosynthetic and respiratory responses of the
SALINITY AND TEMPERATURE EFFECTS ON RUPPIA MARITIMA GERMINATION
391
intertidal red alga Bostrychia binderi Harvey (Rhodophyta, Ceramiales) from a mangrove swamp and a salt mash. Phycologia, 20: 165-173. Dawes, C.J., Moon, R.E. and Davis, M.A., 1978. The photosynthetic and respiratory rates and tolerances of benthic algae from a mangrove and salt marsh estuary: a comparative study. Estuarine Coastal Mar. Sci., 6:175-185. Koch, E.W., 1988. Physiological responses of North Carolina and Florida Ruppia maritima cultured from seeds. Masters Thesis. University of South Florida, Tampa, FL, 108 pp. Koch, E.W. and Seeliger, U., 1988. Germination ecology of two Ruppia maritima L. populations in Southern Brazil. Aquat. Bot., 31: 321-327. Moffler, M.D. and Durako, M.J., 1987. Reproductive biology of the tropical-subtropical seagrasses of the southeastern United States. In: M.J. Durako, R.C. Phillips and R.R. Lewis (Editors), Proceedings of the Symposium on Subtropical-Tropical Seagrasses of the Southeastern United States. Fla. Mar. Res. Publ., No. 42,209 pp. Orth, R.J. and Moore, K.A., 1983. Seed germination and seedling growth of Zostera marina L. (eelgrass) in Chesapeake Bay. Aquat. Bot., 15:117-131. Seeliger, U., Cordazzo, C. and Koch, E.W., 1984. Germination and algae-free laboratory culture of Widgeon Grass, Ruppia maritima. Estuaries, 7:176-178. Sokal, R.R. and Rohlf, F.J., 1981. Biometry. W.H. Freeman, San Francisco, CA, 2rid edn., 859 PP. Verhoeven, J.T.A., 1979. The ecology of Ruppia-dominated communities in western Europe. I. Distribution of Ruppia representatives in relation to their autoecology. Aquat. Bot., 6:197268.
387
Elsevier Science Publishers B.V., Amsterdam
Short Communication Influence of salinity and temperature on the germination of Ruppia maritima L. from the North Atlantic and Gulf of Mexico E.W. Koch ~ and C.J. Dawes Department of Biology, Universityof South Florida, Tampa, FL 33620, USA (Accepted for publication 27 November 1990 )
ABSTRACT Koch, E.W. and Dawes, C.J., 1991. Influence of salinity and temperature on the germination of Ruppia maritima L. from the North Atlantic and Gulf of Mexico. Aquat. Bot., 40: 387-391.
Seeds from the seagrass Ruppia maritima L. collected in North Carolina and Florida were tested for their germination at 17, 23 and 29°C and at 0, 15 and 30%o salinities. Percent germination was significantly higher for seeds from North Carolina and the seeds from Florida did not germinate in 30%o salinity. The northern population also had a more rapid rate of germination. These germination patterns suggest that ecotypic differentiation in germination occurred between the two populations in relation to salinity but not temperature.
INTRODUCTION
Ruppia maritima L. has an annual or perennial life cycle depending on the local environmental stresses (Koch and Seeliger, 1988). Thus the seeds of this species play an important role in regenerating annual populations (Brock, 1983 ), in contrast to other seagrasses of the Southeastern United States which propagate vegetatively (Moffier and Durako, 1987). It has been suggested that the germination of R. maritima seeds is controlled by environmental conditions (Orth and Moore, 1983). This study deals with the physiological ecology of two latitudinally isolated populations of R. maritima from the North Atlantic Ocean and the Gulf of Mexico germinated under common garden conditions. The results are discussed in terms of ecotypic differentiation and phenotypic plasticity in relation to salinity and temperature. ~Present address: Department of Marine Sciences, University of South Florida, St. Petersburg, FL 33701, USA.
0304-3770/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
388
E.W. KOCH AND C.J. DAWES
MATERIALS AND METHODS
Ruppia maritima was sampled in June 1986 in Pamlico Sound on Cedar Island, North Carolina (34 ° 58'N, 76 ° 2 2 ' W ) and in the estuary of the Weeki Wachee River on the west coast of Florida (28 ° 32'N, 82 ° 39'W). Temperature and salinity for the North Carolina population ranged from 10 to 36°C and 6 to 30%o salinity (M. Fonseca, personal communication, 1987) and for the Florida population from 20 to 27°C and 2 to 14%o salinity (Davis and Dawes, 1981; Koch, 1988 ). Seed collection at the two sites was carried out while some of the plants in each population were still flowering and had attached seeds. Seed densities in the sediments were determined for each population using 10 cores which were 5 cm deep and 10 cm in diameter (Verhoeven, 1979). The sediment was sieved with a 1 m m mesh. Intact, unblemished seeds were picked from the organic material and cleaned by shaking them alternately with 96% ethanol (pH 2 ) and autoclaved distilled water (Seeliger et al., 1984 ). Clean seeds were "stratified" by holding them in distilled water for 2 weeks at 6 °C as described by Seeliger et al. (1984). A preliminary experiment suggested that stratification did not inhibit the seed germination of either population. Germination was tested at different temperatures ( 17, 23, 29 °C using 0%o salinity) and salinities (0, 15, 30%o using diluted autoclaved seawater at 23 °C). All seawater was obtained on the west coast of Florida and diluted with distilled water to the desired salinities. Five replicates with 100 seeds were used for each treatment of the North Carolina population (i.e. 500 seeds per treatment) while four replicates with 20 seeds were used for the Florida population (i.e. 80 seeds per treatment). Germination was defined as the time when the cotyledon emerged. The number of germinated seeds was recorded weekly over a two m o n t h period as done with R. maritima from southern Brazil (Koch and Seeliger, 1988). ANOVAs were used to compare the number of seeds from the two sites. Results obtained from the germination experiments were analyzed as 3 × 2 factorial experiments using a SAS program (SAS Institute Inc., 1985 ). Stud e n t - N e w m a n - K e u l s (SNK) multiple range tests were carried out to test for significant differences (see Sokal and Rohlf, 1981 ). RESULTS
The numbers of seeds found in the sediment of the Florida site were significantly higher ( F = 2 4 . 4 0 , P<0.0001; data without normal distribution; 1006 +_613 seeds m - 2 ) than for the North Carolina site (38 +_86 seeds m - 2 ) . Most seeds from the northern population were unblemished while a large number from Florida had opened seed coats. The data for germination at three temperatures were not normal with no
S A L I N I T Y A N D T E M P E R A T U R E EFFECTS O N RUPPIA MARITIMA G E R M I N A T I O N
389
homogeneity of variances for both populations, but the strength of the test and the large F value should account for the lack of those prerequisites in the ANOVA performed. The northern seeds had a significantly higher germination rate ( F = 100.27, P < 0.0001; Fig. 1 ) at all temperatures. Although fewer seeds from each population germinated at the highest temperature (29 °C), there was no significant difference between germination at the three temperatures tested ( F = 3.20 for Florida, F = 0.95 for North Carolina) owing to the large standard error. North Carolina seeds germinated in the first 10 days while the Florida seeds continued to germinate over the entire 68 day period of the experiment. Ruppia maritima seeds from North Carolina had an earlier germination time (25 days) and a significantly higher germination rate at all salinities ( F = 37.01, P < 0.0001 ) compared with Florida seed germination (35 days; Fig. 2 ). However, germination of the northern seeds was significantly lower NORTH CAROLINA 60 ¸ ~) 50"
~z_4 o . i,i c, Lu
I,<
2 \T -o-2:c
50.
20. I0.
?.~ 2 0 ]
FLORIDA
101 L----
...... ~ " "E-- . . . . . .
-,~o
............ -- ................
~o
4o
~o
~
-~ -0
do
~o
-io
TIME (days)
Fig. 1. Germination of Ruppia maritima seeds from North Carolina and Florida exposed to 17 ° C ( s q u a r e s ) , 23 ° C ( t r i a n g l e s ) and 29 ° C (circles) at 0%o salinity. NORTH
CAROLINA
60o~ 5 0 z 0 40<[
z_N?.
i~/
•
O PPT
•
15PPT
•
30PPT
~ 20w I--
t(,.
•~§I,o z°
/r ...................................
_..___.i-----'~ -
,
I0
"FLO"'OA
AT, . . . . . . .
2'0
30
.
. . . . . . . . .
40 T I M E (days)
• .
. . . . . . .
50
.
. . . . .
60
~
,
70
Fig. 2. Germination ofRuppia maritima seeds from North Carolina and Florida exposed to 0%o ( s q u a r e s ) , 15%o ( t r i a n g l e s ) a n d 300/o0 salinity (circles) at 23 ° C.
390
E.W. KOCH AND C,J. DAWES
at 30%0 salinity ( F = 4 . 4 7 , P < 0 . 0 5 ) relative to rates in 0 and 15%o salinity which were not significantly different. In contrast, rates for seeds from the southern population were not significantly different between the three salinities tested ( F = 2.29) even though there was no germination in 30%o salinity (Fig. 2). DISCUSSION
The present study suggests ecotypic differentiation in seed germination patterns ofR. maritima populations from Florida and North Carolina to salinity but not to temperature. The continuous germination of Florida seeds over the duration of the experiments suggests that they are adapted to an environment where growth conditions are favorable year round. This capacity for continuous germination would also explain the large number of opened seed coats. In contrast, the North Carolina population has a short germination period of about 1 m o n t h which matches the shorter growing season. Environmental stresses can enhance the germination of Ruppia maritima seeds (Koch and Seeliger, 1988 ). A significantly higher germination rate was found for the northern population, under all temperatures and salinities tested. This high percent, as well as rapid rate of germination may be critical where water temperature shows a large seasonal fluctuation. The smaller seasonal fluctuation in the Gulf of Mexico may account for a lower and slower rate of germination during the entire year. In contrast to temperature, there were distinct germination patterns for the two populations in the three salinities tested suggesting ecotypic differentiation at each site. This difference is probably because the Weeki Wachee River population never experiences salinities as high as 30%0 (Dawes et al., 1978; Davis and Dawes, 1981; Koch, 1988) while the northern population experiences a broader salinity range (6-30%o). ACKNOWLEDGEMENTS
The first author would like to thank CNPq - Conselho Nacional de Desenvolvimento Cientifico e Tecnol6gico, Brazil - for financial support during the Masters program at the University of South Florida of which this study was part.
REFERENCES Brock, M.A., 1983. Reproductive allocation in annual and perennial species of the submerged aquatic halophyte Ruppia. J. Ecol., 71: 811-818. Davis, M. and Dawes, C.J., 1981. Seasonal photosynthetic and respiratory responses of the
SALINITY AND TEMPERATURE EFFECTS ON RUPPIA MARITIMA GERMINATION
391
intertidal red alga Bostrychia binderi Harvey (Rhodophyta, Ceramiales) from a mangrove swamp and a salt mash. Phycologia, 20: 165-173. Dawes, C.J., Moon, R.E. and Davis, M.A., 1978. The photosynthetic and respiratory rates and tolerances of benthic algae from a mangrove and salt marsh estuary: a comparative study. Estuarine Coastal Mar. Sci., 6:175-185. Koch, E.W., 1988. Physiological responses of North Carolina and Florida Ruppia maritima cultured from seeds. Masters Thesis. University of South Florida, Tampa, FL, 108 pp. Koch, E.W. and Seeliger, U., 1988. Germination ecology of two Ruppia maritima L. populations in Southern Brazil. Aquat. Bot., 31: 321-327. Moffler, M.D. and Durako, M.J., 1987. Reproductive biology of the tropical-subtropical seagrasses of the southeastern United States. In: M.J. Durako, R.C. Phillips and R.R. Lewis (Editors), Proceedings of the Symposium on Subtropical-Tropical Seagrasses of the Southeastern United States. Fla. Mar. Res. Publ., No. 42,209 pp. Orth, R.J. and Moore, K.A., 1983. Seed germination and seedling growth of Zostera marina L. (eelgrass) in Chesapeake Bay. Aquat. Bot., 15:117-131. Seeliger, U., Cordazzo, C. and Koch, E.W., 1984. Germination and algae-free laboratory culture of Widgeon Grass, Ruppia maritima. Estuaries, 7:176-178. Sokal, R.R. and Rohlf, F.J., 1981. Biometry. W.H. Freeman, San Francisco, CA, 2rid edn., 859 PP. Verhoeven, J.T.A., 1979. The ecology of Ruppia-dominated communities in western Europe. I. Distribution of Ruppia representatives in relation to their autoecology. Aquat. Bot., 6:197268.