Germination and growth of Ruppia polycarpa and Lepilaena cylindrocarpa in ephemeral saltmarsh pools, Westernport bay, Victoria

Aquatic Botany, 26 (1986) 165-179

165

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

G E R M I N A T I O N A N D G R O W T H OF R U P P I A P O L Y C A R P A A N D L E P I L A E N A C Y L I N D R O C A R P A IN E P H E M E R A L S A L T M A R S H P O O L S , W E S T E R N P O R T BAY, V I C T O R I A

P.J. VOLLEBERGH 1 and R.A. CONGDON ~

Botany Department, Monash University, Clayton, V ic. 3168 (Australia) (Accepted for publication 17 July 1986)

ABSTRACT Vollebergh, P.J. and Congdon, R.A., 1986. Germination and growth of Ruppia polycarpa and Lepilaena cylindrocarpa in ephemeral saltmarsh pools, Westernport Bay, Victoria. A quat. Bot., 26: 165-179. Changes in standing crop of the euryhaline macrophytes Ruppia polycarpa R. Mason and Lepilaena cylindrocarpa ( Koernicke ex Walp. ) Benth. were monitored from March to November 1983 in saltmarsh pools at Westernport Bay, Victoria. The salinities of these pools varied from 370%~ total dissolved salts (TDS) in February to 14%o TDS in September. In March the bottom sediments contained approximately 10:*seeds m 2 and 400-3500 turions m 2 of Ruppia and 10~ seeds

m '~ofLepilaena. GerminationcommencedinApril, atsalinitiesofaround40%cTDS. Countsof emerging seedlings suggest that less than 30% of Ruppia and 1% of Lepilaena propagules germinated. Maximum mean standing crops were 52 g AFDW m 2 for Ruppia and 60 g AFDW m ~ for Lepilaena. In laboratory experiments, Lepilaena and Ruppia seeds showed maximum germination (85 and 45 %, respectively) in freshwater at 20 ° C. Ruppia turions germinated equally well (60%) in 50 and 100% seawater at 20°C, and after 25 days the germination percentage was higher in 225 % seawater than in freshwater. Some propagules of both species germinated in 225% seawater, but none germinated in 450% seawater. The germination potential of seeds soaked in 225 and 450% seawater for 14 days and then transferred to freshwater was generally unaffected. Some propagules germinated in freshwater after soaking in 1000% seawater for six days. Shoot and root growth of seedlings of both species was better in 50 and 100% seawater than in freshwater. The life-histories of these species show them to be opportunists well adapted to survive in this ephemeral habitat of variable salinity.

INTRODUCTION

Ruppia ( Potamogetonaceae ) and Lepilaena ( Zannichelliaceae ) are two prominent genera of the euryhaline angiosperm flora of Australia and New 1Present address: 2 Cooinda Court, Mt. Waverley, Vic. 3149, Australia. 2Present address: Botany Department, James Cook University of North Queensland, Townsville, Qld. 4811, Australia.

0304-3770/86/$03.50

© 1986 Elsevier Science Publishers B.V.

166 Zealand and dominate many estuarine and inland saline habitats. They are the only Australian angiosperm genera which tolerate conditions ranging from freshwater to hypersalinity. There are five species of Lepilaena. They are endemic to Australia and New Zealand, and have generally been neglected by aquatic botanists. L. cylindrocarpa {Koernicke ex. Walp.) Benth. is a delicate dioecious plant which is restricted to Australia (den Hartog, 1981 ). There are no published studies of the ecology of this species. There are four Australian species of Ruppia: R. megacarpa R. Mason; R. polycarpa R. Mason; R. maritima L.; and R. tuberosa Davis and Tomlinson (Jacobs and Brock, 1982). Several recent investigations have examined the ecology of Ruppia species growing in Australian aquatic habitats (Higginson, 1966; Congdon and McComb, 1979; Brock, 1981a, 1982a,b, 1983). No published studies exist for Victorian communities, and little work has been done on R. polycarpa in particular. R. polycarpa produces vegetative perennation organs called turions. Brock (1981a) recognised two types of turions on R. tuberosa. Turion type I is a swell: ing at the leaf base, whilst turion type II is a swelling at the rhizome tip. Only turion type I was found on R. polycarpa. This study examines the germination and growth of R. polycarpa and L. cylindrocarpa in ephemeral pools of a Victorian saltmarsh. MATERIALSAND METHODS

Study site The study site was a saltmarsh at Yaringa, on the north-western shore of Westernport Bay. Westernport Bay lies some 70 km south-east of Melbourne and to the east of Port Phillip Bay (Fig. 1 ). It is a large tidal embayment of 1450 km 2 and contains 45% of Victoria's total intertidal zone, and consequently the most significant areas of seagrasses, saltmarshes and mangroves in the state {Bird, 1976). The pools develop at the rear of the marshes, in communities dominated by the chenopods Sarcocornia quinqueflora A.J. Scott and Sclerostegia arbuscula ( R.Br. ) P.G. Wilson. The macro-algae Lamprothamniumpapulosum ( Wallr. ) J. Gr., Enteromorpha intestinalis ( L. ) Link and Rhizoclonium riparium {Roth) Harvey also inhabit the pools. The pools are present in aerial photographs taken in 1939, when the area was not very accessible, but they have increased considerably in area since then. This is largely due to the erosion of the saltmarsh by trampling and recreational vehicles. Two pools, separated by some 250 m, were chosen for this study.

167

Hos iogs Fig. 1. Map showing the location of the study site at Yaringa, and the locations of the nearest Bureau of Meteorologyrecording stations for rainfall (Hastings), evaporation (Melbourne) and temperature {Cape Schanck). Rainfall occurs largely in May-October, with high evaporation rates in summer (Fig. 2 ). Evaporation exceeds rainfall in all months but M a y - S e p t e m b e r . Mean air temperatures are highest over summer and lowest in winter, and the temperatures recorded with m a x i m u m / m i n i m u m thermometers in salt marsh pools between sampling occasions reflect this ( Fig. 3 ). There were large differences between m a x i m u m and minimum temperatures, suggesting large diurnal temperature fluctuations within the pools.

Depth and salinity Depth measurements and water samples for salinity determinations were taken at intervals of one to two weeks from February to September. Water samples were stored at constant temperature ( 20-25 ° C ) for no more than 10 days. Conductivity was measured with a Radiometer type CDM2e conductivity meter (Copenhagen, D e n m a r k ) and a Radiometer type CDC 104 electrode. The conductivity of a standard seawater sample was measured each time to check the calibration of the meter, which was also calibrated regularly against a standard silver nitrate solution. The salinities of the field samples were determined from a calibration graph, obtained by measuring the conductivities of a range of standards of known salinities.

168 120

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Fig. 2. Average monthly rainfall for Hastings and evaporation for Melbourne (data supplied by

Bureau of Meteorology, Melbourne). Fig. 3. Mean monthly maximum and minimum air temperatures for Cape Schanck, and maximum

and minimum water temperatures recorded at Site 1 between sampling occasions.

Biomass determination A modified stratified random sampling method was employed (Yamane, 1967). Each pool was subdivided into five segments. On each sampling occasion one sample was taken from each segment, from a patch of vegetation judged to be homogeneous. The sites were sampled at 4-6-week intervals. Samples were harvested with a square tube ( 22.7 × 22.7 cm ), which could be pushed into the sediment to facilitate the collection of propagules and belowground material. These samples were sieved to remove sediment, and stored at 4°C in the dark until sorted. Material was dried (105°C, 24 h) to obtain dry weight, and then ashed ( 550 ° C, 3 h ) to obtain ash-free dry weight (AFDW). Propagules were counted in samples taken in early March, prior to germination. Subsamples of approximately 10% were taken from each sample to estimate the numbers of Lepilaena seeds present. Complete samples were sorted for Ruppia propagules. Propagule biomass was determined by drying and ashing three replicates of 100 Lepilaena seeds and 50 of each type of Ruppia propagule.

169

Laboratory studies of propagule germination and seedling growth Propagules were sorted from the sediment taken from dry pools in late Febmary and early March, and stored dry in the dark at 20-25 ° C for 3-5 months prior to use. Germination was tested in water of five different salinities (0, 17.5, 35.5, 81.5 and 158.0%o TDS (parts per thousand total dissolved solutes)), corresponding approximately to 0, 50, 100, 225 and 450% seawater. Between 20 and 50 propagules were placed in 100-200 ml of water in 9-cm glass dishes: and incubated in growth cabinets at a constant temperature (11, 15, 20, 25 or 30 °C ). Since there were fewer propagules of R. polycarpa available, the salinity trial for this species was carried out only at 20 oC. The salinity of each dish was maintained by the addition of distilled water. Illumination in the growth cabinets was provided by Sylvania Grolux ( U.S.A. ) 40W fluorescent tubes. Light intensities were some 5-8% of full sunlight (59-88/IE m -2 s - l ) . Daylength was set at 10 h day/14 h night. Controls were also run in the dark. The number of germinated propagules was scored each day, except for experiments with propagules in the dark when only total germination at the termination of the experiment was scored, after 14 days. Germination of the seeds was taken as the splitting of the seed coat and emergence of the cotyledon, and germination of turions was taken to be when either new shoot or root growth became noticeable.

Laboratory studies of photosynthesis and respiration of mature shoots at different salinities Measurement of photosynthesis and respiration was done by measuring net oxygen production and oxygen uptake, respectively. A Clark-type oxygen electrode (Hansatech, D.W., Norfolk, England) based on a design by Delieu and Walker (1972) was used. The reaction chamber was water-jacketed to maintain a temperature of 18-19 ° C, and the rate of oxygen production/uptake calculated from the slope of a 5-10 min chart record. Measurements were made at salinities corresponding to 0, 50, 100, 225 and 450% seawater. Illumination was provided by three 250W globes (1 X Osram incandescent, 2 X Philips Comptalux R40 Flood), providing 1150 ttE m -2 s-1 at the top of the reaction chamber. For each species and salinity, 10-12 shoots from several plants (ca. 10-50 mg fresh weight) were placed in a petri dish in the dark for 15-30 min prior to the experiment to ensure that possible oxygen reserves in the leaf lacunae were exhausted. They were then transferred to the reaction chamber and 0.2 ml of 0.01 M NaHCO3 added to prevent depletion of bicarbonate during photosynthesis. Respiration rates were initially measured by covering the sample chamber

170 Site 1 o

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Fig. 4. Changes in depth and salinity at (a) Site 1 and (b) Site 2. The broken line at 35%o corresponds to 100% seawater concentration.

with a black cloth for 10 min. The cloth was then removed, the lights switched on, and the rate of photosynthesis measured for the next 5-10 min. Experiments were usually replicated at least three times. After each measurement, the dry weight of the plant material was determined (105 ° C, 24 h). A one-way analysis of variance with a Student-Newman-Keuls (SNK) multiple range test was carried out to test for significant differences. RESULTS

Depth and

salinity

The depth and salinity of the pools respond directly to the seasonal variation in rainfall and evaporation (Fig. 4). Depth was significantly negatively correlated with salinity in three different pools, with a probability of more than

171 TABLE I The number and biomass (g AFDW m -2) of propagules of L. cylindrocarpaand R. polycarpa found in each pool in March 1983. Data are given as mean ± S.E.M. Site

1 2

Number Biomass Number Biomass

Lepilaena

Ruppia

Seeds

Seeds

Turions (I)

Turions (II)

100594 ± 31795 13.28 _+4.20 98239 ± 18301 10.76 ± 2.00

959 _+211 0.37 ± 0.08 818 ± 126 0.52 _ 0.08

1225 ___494 1.29 ± 0.52 429 ± 113 0.60 _+0.16

2273 ± 839 1.07 +_0.40 0 0

99.9%. Mean high water spring tides do not penetrate to these pools, and they are only rarely reached by exceptionally high tides. The pools were dry over summer and began to fill in February. As water levels increased, salinities decreased to around 30%0 TDS in June. The lowest salinity recorded was 13.7 ?i,~, TDS in September, after which salinities began to increase as depth fell. Salinities as high as 370%~ TDS were recorded in depressions near the study sites, when they were dry, in mid-February.

Propagule and seedling numbers In March, the sediments of the two pools contained approximately 100 000 seeds m -2 of Lepilaena and 1 000 seeds m -2 of Ruppia (Table I). At Site 1 there were approximately 1200 turions of Type I and 2300 of Type II per square metre. At Site 2 only the first type were found; approximately 400 m 2. Ruppia seeds had a greater mass t h a n Lepilaena seeds and Ruppia Type I turions were larger t h a n Type II turions ( Table I ). Seedlings were first observed at Site 1 in late-March when heavy rain caused the salinity to decrease from 100 to 40%o TDS. T h e y were first observed at Site 2 in mid-May after heavy rain caused a similar salinity decrease. Germinating seedlings were counted in May. In this period only 0.6-0.7% of Lepilaena seeds germinated. Some 27% of Ruppia propagules germinated at Site 1 and 10% at Site 2 (Table II).

Changes in biomass In terms of biomass, Ruppia was d o m i n a n t at Site I and Lepilaena at Site 2. Biomass increased until it reached a m a x i m u m in the spring. The m a x i m u m recorded mean standing crop of Ruppia was 52 g A F D W m -2, at Site 1 in September (Fig. 5), and 60 g A F D W m -2 recorded for Lepilaena at Site 2 in November (Fig. 6).

172 T A B L E II Number of seedlings ofL. cylindrocarpa and are mean _+S.E.M. per square metre

R. polycarpa recorded at Yaringa in May 1983. Data

Lepilaena

Site

1 2

Ruppia

Seedling no.

% germination

Seedling no.

% germination

736 + 316 612 + 215

0.7 0.6

1192 _ 177 120 + 29

26.7 9.6

Shoots comprised about 50% of the biomass in each species (Fig. 7 ). Roots also comprised a large proportion of the Lepilaena biomass, while rhizome biomass was proportionally greater for Ruppia. Turions constituted a large pro60

Ruppia polycarpo

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cylindrocarpa. Bars represent the standard errors of the means.

173 1110

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Fig. 7. Changes in the proportion of biomass allocated to each plant part at Site 1 for (a)

R. polycarpa (F=flowers and fruits) and (b) L. cylindrocarpa. Fig. 8. Percentage germination of seeds of L. cylindrocarpa after 14 days, in relation to (a) temperature ( O - 11, {}=15, A = 2 0 , & = 2 5 , V1=30 °C) and (b) salinity (O =0, {}=50, [3 = 100%

seawater).

portion of Ruppia biomass early and late in the growing season. Both species flower, fruit and set seed within a few months. Lepilaena first flowered in late-July, and Ruppia in mid-August. The first fruits appeared in early-September.

Propagulegermination Lepilaena seeds exhibited maximum germination (ca. 85% ) in freshwater at 20 ° C, and in general, germination decreased as salinity increased (Fig. 8). However, some 60% germination was achieved in 100% seawater at some temperatures. Although best germination occurred at 20 ° C, similar germination percentages were obtained at 11 and 15 °C for some treatments. After 65 days three Lepilaena seeds had germinated in 225% seawater. Ruppia seeds germinated best (35-45%) in freshwater and 100% seawater at 20 °C (Fig. 9). One Ruppia seed germinated in 225% seawater after 20 days, and three after 32 days. The initial germination rate of Ruppia seeds was highest at 25 ° C, although germination percentages were similar at all temperatures after 24 days.

174

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Fig. 9. Percentage germination againsttime of seeds ofR. polycarpa (a) at 20°C and three different salinities( O = 0, • = 50, A = 100% seawater) and (b) in 100% seawater at 5 differenttemperatures ( O = 11, • = 15, A =20, • =25, [] =30°C). Fig. 10. Percentage germination (a) against time at 20°C and four differentsalinities,and (b) after 14 days in 50 and 100% seawater at 5 differenttemperatures, of turions (1) of R. polycarpa

( O =0, • =50, A = 100, • = 2 2 5 % seawater).

No propagules of either species germinated in 450% seawater, even after 10 weeks. The germinationpotential of Ruppia and Lepilaenaseeds soaked in 225 and 450% seawater for 14 days, and then transferred to freshwater, was generally unaffected. Some Lepilaenaseeds and Ruppiapropagules germinated in freshwater after soaking in 1000% seawater for 6 days. No type II turions of Ruppia germinated under any conditions. They decomposed after a few days in water. Type I turions exhibited best germination in 50 and 100% seawater at 20°C (Fig. 10). After 25 days the germination percentage was higher in 225% seawater than in freshwater, resulting in 40% germination after 30 days. Turions germinated best at temperatures of 15-25 ° C. It was found that turions could "germinate" more than once during several wetting and drying cycles. Only half as many seeds of both species germinated in the dark as in the light, but no reduction in the germination percentage of RuppiaType I turions occurred (Table III).

175 TABLE III Germination percentages after 14 days of L. cylindrocarpa and R. polycarpa propagules kept in the dark and in light Propagule type

Temperature (°C)

Lepilaena seed Ruppia seed Ruppia turion (I) Ruppia turion (I)

20 20 20 25

Salinity (% seawater)

Percentage germination

0 100 50 50

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Light

40 15 50 60

85 35 60 65

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Sotinity(% seawater)

Fig. 11. Shoot lengths of seedlings after 14 days for (a) L. cylindrocarpa seeds, (b) R. polycarpa seeds and (c) R. polycarpa turions (I), relative to salinity and temperature ( O = 0 , @=50, A = 100% seawater). Fig. 12. Rates of (a) photosynthesis (as 02 evolution) and (b) respiration (as 02 uptake), for

L. cylindrocarpa (• ) and R. polycarpa (0 ) at different salinities.

176

Seedling growth at different salinities The growth of seedlings was best for both species in 50 and 100% seawater at 20 and 25 °C (Fig. 11 ). Shoot growth in freshwater was always limited and no roots were produced by either species, whereas root growth was often extensive in 50 and 100% seawater. Ruppia turions (Type I) which germinated in 225% seawater at 20°C still showed apparently healthy shoot growth after 40 days. The few seeds of Lepilaena and Ruppia which germinated in 225% seawater subsequently died. Shoot growth of both species was best at 20 and 25 ° C. Ruppia turions produced much longer shoots than seeds under the same conditions, and many commenced rhizome production less than 14 days after germination.

Photosynthesis and respiration of mature shoots at different salinities Lepilaena showed the highest rate of photosynthesis in 100% seawater and Ruppia in freshwater. Rates of photosynthesis decreased with increasing salinity above 100% seawater (Fig. 12). There was no significant difference between respiration rates of Lepilaena at all salinities. There was also no significant difference between respiration rates of Ruppia at salinities from 0 to 100% seawater, but rates decreased at higher salinities. DISCUSSIONAND CONCLUSIONS The two species display a "winter annual" reproductive strategy, as an adaptation to the environmental stresses associated with the seasonal desiccation and fluctuating salinity of the ephemeral pool habitat at Yaringa. The propagules are able to withstand the extreme hypersaline conditions as the pools dry out in summer, and then tolerate the summer dryness and heat. They then germinate in hypersaline waters the following autumn. Propagules of both species were able to germinate under a wide range of environmental conditions. Only a small percentage of the large number of propagules of L. cylindrocarpa germinated when the pools first began to fill. Seeds of both species were still germinating at Yaringa in September. This prolonged germination ensures the maintenance of the population in this variable environment. If all seeds germinated following the first rains, a sudden dry spell would decimate the population. L. cylindrocarpa produces many more seeds than R. polycarpa, but the Ruppia propagules are larger and a larger percentage germinate. Ruppia turions are particularly efficient perennation organs as they are able to "germinate" more than once after wetting, drying and subsequent rewetting. Shoot growth from turions is more rapid than for seedlings. Seeds of many halophyte species germinate best in freshwater (Ungar, 1982;

177 Phillips et al., 1983), although they may still be able to germinate at higher salinities, indicating the possession of salt tolerance rather than a salt requirement. Also these seeds may withstand high salinities, and then germinate when the salinity is reduced ( Rozema, 1975). Lepilaena and Ruppia propagules were subjected to salinities equivalent to 1000% seawater, both in the field at Yaringa and in laboratory experiments, and then were able to germinate at lower salinities. This ability has been reported for seeds of Zannichellia pedunculata Rchb. (van Vierssen, 1982) and R. maritima var. maritima (van Vierssen et al., 1984). This is an important adaptation for such plants growing in a variable ephemeral habitat, where salinities are regularly hypersaline. R. polycarpa seeds from Yaringa germinated equally well in freshwater and 100% seawater. Brock {1982a) found that seeds of the perennial R. megacarpa germinated best in freshwater, while those of the annual R. tuberosa did better in saline waters. No doubt the ability of seeds of R. tuberosa and R. polycarpa to germinate in saltwater reflects their adaptation to ephemeral habitats in which conditions are hypersaline at the start of the growing season. The same is true of Lepilaena. Setchell (1924), Verhoeven (1979) and Richardson (1980) all reported germination maxima in the temperature range 11-20 ° C for Ruppia. Propagules of Ruppia and Lepilaena from Yaringa showed a wide optimal temperature range for germination. This suggests that a proportion of propagules is able to germinate at any given temperature. This allows germination under a broad range of field conditions, but ensures that not all propagules germinate at once. The seedlings must establish themselves under conditions of high salinity and continue to grow so that within two or three winter months, new propagules are produced. Seedlings of Lepilaena and Ruppia grew best in 50 and 100% seawater; those Ruppia seedlings which grew in 225% seawater were also healthy. Van Vierssen et al. (1984) also found that seedlings of R. maritima var. maritima grew better at raised salinities than in freshwater. The potential of the seedling to survive high salinities is an important adaptation as salinities of 50-100% seawater are most prevalent during the growing season at Yaringa. Rates of photosynthesis for both species were quite high at salinities of 0-100% seawater, with a marked decrease at 225 and 450%. As salinities in the field were less than 100% seawater for much of the growing season at Yaringa, this suggests that salinity was not limiting productivity. Brock (1981b) showed that proline contents of the three major Australian Ruppia species increased with increasing salinities, and that the amount of proline fluctuated during the season. This suggests that an individual plant can tolerate a wide range of salinities provided changes are relatively gradual. This is supported by the findings of McMillan and Moseley (1967) who concluded that Ruppia may have a higher salinity tolerance than that of its usual habitat. In the saltmarsh pools at Yaringa Ruppia polycarpa and Lepilaena cylindro-

178

carpa have the characteristics of opportunists in that they are short-lived, have a high seed production and are adapted to physically disturbed habitats. They are well adapted to the seasonally changing environment of these ephemeral pools. In particular, the Type I turions of Ruppia polycarpa are highly efficient in terms of germination and growth. ACKNOWLEDGEMENTS

This work formed part of the research for a B.Sc. Honours degree performed by P.J.V. and supported by the Botany Department, Monash University. The authors wish to thank Dr. M.A. Brock for her comments on the manuscript. The Bureau of Meteorology (Melbourne) kindly supplied data on evaporation, rainfall and temperature.

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