Aquatic Botany, 26 (1986) 181--188 181 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

Short Communication

GERMINATION AND SEEDLING DEVELOPMENT OF CYMODOCEA NODOSA (UCRIA) ASCHERSON UNDER LABORATORY CONDITIONS AND "IN SITU"

H. PIRC' , M.C. BUIA 2 and L. MAZZELLA 2

' Institut flir Pflanzenphysiologie, Biozentrum, A-1091 Vienna (Austria) 2 Lab. di Ecologia del Benthos della Stazione Zoologiea di Napoli, Ischia (Italy)

(Accepted for publication 26 June 1986)

ABSTRACT

Pirc, H., Buia, M.C. and Mazzella, L., 1986. Germination and seedling development of Cymodocea nodosa (Ucria) Aseherson under laboratory conditions and "in situ". Aquat. Bot., 26: 181--188.

Seeds of Cymodocea nodosa (Ucria) Aschers., collected in the field in November, ex- hibited a distinct dormancy period of about 7--8 months and started to germinate in May.

The germination rate in the laboratory at ambient water temperatures reached 100% in July. When seeds were exposed to a constant temperature of 15 and 20°C during dor- mancy, germination rates were 59.5 and 30.9%, respectively. Maximum "in situ" germina- tion was 54.0%.

Sandy mud from the collection site of the seeds was the most suitable substratum for germination; peat pots and quartz sand lowered germination rates. Seeds kept without any substratum also germinated.

INTRODUCTION

Cymodocea nodosa (Ucria) Aschers. is widely distributed in the Mediter- ranean Sea and extends along the Atlantic coast of Africa as fax south as Senegal (Den Haxtog, 1970). Although only a few cases of flowering and fruiting were reported by Den Hartog (1970), recent investigations have shown that flowering and fruiting in Cymodocea nodosa occur very regular- ly in various places (Simonetti, 1973; Lipkin, 1977; Pirc et al., 1983; Caye and Meinesz, 1985; Mazella et al., in press).

For different seagrass species, germination was studied "in situ" as well as under laboratory conditions (Lewis and Phillips, 1980; Birch, 1981; Mc- Millan, 1981, 1983; McMillan et al., 1982). Germination without dormancy was reported for some species (Den Hartog, 1970), while in others, including Cymodocea, seed dormancy is suspected to occur (Mc- Mfllan, 1981). McMillan et al. {1982) stated that seedlings of the tropical

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species Cymodocea rotundata Ehrenb. & Hempr. ex Aschers. may be pro- duced from the seed-reserves almost constantly throughout the year. In his work on Phucagrostis major Cavol. (= Cymodocea nodosa), Bornet (1864) mentioned that seeds did not germinate during winter; he provided a precise description of the germination. We studied the germination pattern and the seedling development in the laboratory under various conditions and com- pared the results with those of seedlings from the field.

MATERIAL AND METHODS

Germination experiment 1984

Seeds and seedlings were collected from a well-established community of Cymodocea nodosa at 3 m depth near Lacco Ameno, Ischia (Gulf of Naples) on 17 November 1983. At the time of collection most seeds were still direct- ly attached to the rhizomes. All seeds used for the germination experiments in the laboratory were produced in the summer of 1983. The experimental design is shown in Table IA.

TABLEI

Experimental design and germination rates of the germination experiments in 1984 (A) and 1985 (B). (A) Germination experiment 1984. Seeds were collected in September 1983 at Ischia, Lacco Ameno. Experiments: I Lab. Ischia at ambient water temperature; II Lab. Vienna at 15°C, temperature was increased to 20°C after 12 June; III Lab. Vienna at 20°C; IV "in situ" at Lacco Ameno

Experiment No. of seeds Substratum Temperature % Germination (°C)

I 120 Sandy mud 12--25 100.0 II 126 Quartz sand 15 59.5 III 126 Quartz sand 20 30.9 IV 100 Sandy mud 12--25 54.0

(B) Germination experiment 1985. Seeds were collected in September 1984 (left) and in November 1984 (right) at Ischia. The experiment was carried out at the laboratory in Vienna; 50 seeds each were used for the experiments (jiffy = peat pot). Water tempera- ture was kept at 20°C until 15 January, decreased to 15°C until 15 February, kept at 12°C until 15 April and was maintained again at 22°C throughout the remainder of the experiment

Substratum % Germination Substratum % Germination

Quartz sand 29.6 Quartz sand 20.4 Jiffy 38.0 Jiffy 55.6 No substratum 30.0 No substratum 19.3

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Germination at ambient seawater temperature (I) Seeds were planted in pots (9 per pot) in the laboratory at Ischia. Sedi-

ment (sandy mud) was taken from the collection site. The culture tanks were placed in flowing sea water at ambient temperature. In June, when the first leaves appeared, additional light was supplied by fluorescent tubes (GRO- LUX F 36 W); a photoperiod of 14 h day/10 h night was chosen.

Germination at constant temperature (H and III) Seeds were transported in aerated sea water to Vienna. Seven seeds were

planted in each pot and kept either at 15 (II) or 20 ° C (III) in greenhouses. Quartz sand was used as a culture medium; artificial sea water (360/00) was used in the aquarium. Evaporated sea water was replaced with tap water when necessary. After 12 June 1984, the experimental conditions were changed. Temperature was adjusted to 20°C for all pots; artificial light was supplied by two HPI-lamps at an intensity of about 160 t~E m -2 s -1 ; a photoperiod of a 14/10 hours day--night was chosen.

Germination "in s i tu" (IV) Determination of " in si tu" germination was obtained by taking sediment

cylinders (30 cm diameter) including roots and rhizomes from the meadow. In the laboratory, sediment was washed off and seeds and seedlings separ- ated from the remaining plant material. The germination rate was deter- mined on 100 seeds biweekly from May to July 1984.

Germination experiment 1985 (Table IB)

The influence of the substratum on germination was tested. Seeds were collected on 28 September and 22 November 1984. Fif ty seeds were put in quartz sand and Jiffy (peat pots), another 50 were kept wi thout substrate. Artificial sea water was used. The temperature varied between 12 and 22°C as described in Table IB. Additional light was supplied as described above.

In all experiments, germination rates, number of both leaves and roots, as well as individual leaf length (seedlings) were recorded.

RESULTS

Germination "in situ " ( IV) and at ambient water temperatures in the labora- tory (1)

In March, water temperature dropped to a minimum of 12°C; none of the seeds germinated at this time, neither in the field nor in any of the labora- tory experiments. Germination in tanks occurred in May 1984 and 1985, reaching 100% at the end of July (Table II). " In situ", the first seeds also germinated in May, but the germination rate was much lower compared to that of the laboratory experiments. Water temperature at this time was

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TABLE III

Mean h y p o c o t y l (H) and co ty l edon (C) length of Cymodocea seedlings f rom the labora- tory expe r imen t s at d i f fe ren t t empe ra tu r e s and f rom the field (cm) ~

15 June '84 17 July '84 28 July '84 20 Aug. '84 3 Sept . '84

I H 0.70 + 0.27 0.92 ± 0.42 0.92 ± 0.43 1.07 ± 0.41 0.68 ± 0.23 C 3.17 + 1.36 4.15 + 0.96 4.11 ± 0.91 3.70 n.d.

II H 0.45 ± 0.30 n.d. 0.49 ± 0.10 n.d. 0.54 ± 0.20 C 1.39 + 1.14 n.d. 1.99 ± 0.66 n.d. 1.92 _+ 0.60

III H 0.50 ± 0.25 n.d. 0.51 ± 0.10 n.d. 0.54 ± 0.20 C 1.10 ± 0.44 n.d. 1.15 ± 0.40 n.d. 1.67 ± 0.81

IV H 1.57 ± 1.08 0.89 ± 0.44 0.97 ± 0.43 n.d. n.d. C 3.56 ± 1.04 3.82 ± 1.20 3.35 n.d. n.d.

In.d. = no data

Fig. 1. Different stages of seedling development of Cymodocea nodosa.

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186

18°C. A maximum rate of germination of 54% was found in July. By 20 August, 50% of germinated seedlings had been lost by mortality. The growth of seedlings "in situ" was sunilar to that in the laboratory.

After the appearance of the first ]eaves the hypocotyl and cotyledons reached their maximum length as shown in Table III.

The number of leaves per seedling increased continuously, reaching 3.8 leaves in July (Fig. 1, Table II). This was followed by a decrease, and in Sep- tember one leaf plus a mean of 3.5 lea ~ scars were found per seedling in the laboratory experiment (I). The numL~, of roots increased continuously throughout the observation period "in situ" (IV), but in the laboratory (I) some of the roots died after July, resulting in a decrease in root number (Table II). However, in the laboratory the number of leaves was higher and leaves grew longer compared with those of seedlings collected in the field

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187

(Fig. 2). In September in the laboratory (I), leaf length as well as the number of leaves and roots decreased rapidly and the seedlings died.

Germination at constant temperature conditions (II and III)

Seeds started to germinate at the end of April at both experimental tem- peratures (15 and 20°C). By June, 16.8% of the seeds kept at 15°C had germinated. Continued germination during summer meant that 59.5% of the seeds had produced seedlings by September. At 20°C (III), the germination rate was 20.6% in June. This percentage increased only slightly to 31% in September (Table II). The number of roots and leaves as well as the mean leaf lengths did not greatly differ at different temperatures (II and III).

First germination at both constant temperatures was observed about one month earlier than at ambient temperatures or in the field. Cotyledons and hypocotyls were significantly shorter at both constant temperatures.

Germination experiments 1985

Germination rates were similar in quartz sand and without any substrate, but those collected in September had a higher germination rate compared with those collected in November. Seeds planted in peat pots (Jiffy) showed highest germination, the rates being higher for seeds collected in November.

DISCUSSION

"In situ" observations and experiments under laboratory conditions demonstrated a high germination rate for Cymodoeea nodosa seeds. The seeds of Cymodocea nodom have a distinct period of dormancy. None of the seeds, even when collected in August (H. Pirc, unpublished data, 1983), germinated earlier than at the end of April. Constant temperature conditions (1984) as well as changing temperatures (1985) during dormancy resulted in lower germination rates when compared with ambient water temperature conditions.

In September, seeds were already mature when collected; this may explain higher germination rates in the following season. Although higher germina- tion rates occurred in sandy mud from the collection site, other substrates also yielded good results and seeds even germinated without a substrate. Seeds are packed with starch. These high carbohydrate reserves make the seeds to a certain extent independent of the substrate when germinating. In the field most of the seeds germinate in June, although some were still germinating in late-summer.

In the Mediterranean, Cymodocea plants may be uprooted by storms during winter, but some of the seeds remain buried in the sediment. They germinate in spring when increasing temperatures allow rapid growth of the seedlings. Some of the seeds are washed out and exported from the system.

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This dispersal a l lows a c o n t i n u o u s p r o p a g a t i o n o f the species. McMillan e t al. (1982) sugges ted a t ime ,capsu le t y p e o f g e r m i n a t i o n fo r the t rop ica l species C y m o d o c e a rotundata. Due to the absence o f seasons, ge rmina t i on o f C. ro tundata occurs c o n t i n u o u s l y t h r o u g h o u t the y e a r f r o m the seeds s to red in t he s ed imen t . In the Med i t e r r anean a d o r m a n c y o f C. nodosa seeds cou ld be seen as an a d a p t a t i o n to the e n v i r o n m e n t a l cond i t i ons in win te r , which are u n f a v o u r a b l e f o r ge rmina t ion .

ACKNOWLEDGEMENTS

This w o r k was s u p p o r t e d f inancial ly b y the Fonds zur FSrde rung de r wissenschaf t l i chen F o r s c h u n g in Oster re ich , Proj. No. 5135 , and b y the Ci ty Counci l o f Ischia , I s land o f I sch ia (Naples) . M a n y t h a n k s are due to Prof . C. McMillan f o r rev iewing the m a n u s c r i p t .

REFERENCES

Birch, W.R., 1981. Morphology of germinating seeds of the seagrass Halophila spinulosa (R.Br.) Aschers. Aquat. Bot., 11: 79--90.

Bornet, E., 1864. Recherches sur le Phucagrostis major Cavol. Ann. Sci. Nat. 5e Ser. Bot. Fr., 1: 5--51.

Caye, G. and Meinesz, A., 1985. Observations on the vegetative development, flowering and seeding of Cymodocea nodosa (Ucria) Ascherson, on the Mediterranean coasts of France. Aquat. Bot., 22: 277--289.

Den Hartog, C., 1970. The Seagrasses of the World. North-Holland, Amsterdam, 275 pp. Lewis II'I, R.R. and Phillips, R.C., 1980. Occurrence of the seeds and seedlings of Tha-

lassia testudinum Banks ex K~nig in the Florida Keys (U.S.A.). Aquat. Bot., 9: 377-- 380.

Lipkin, Y., 1977. Seagrass vegetation of Sinai and Israel. In: C.P. McRoy and C. Helf- ferich (Editors), Seagrass Ecosystems: A Scientific Perspective. Marcel Dekker, New York, pp. 264--293.

Mazzella, L., Buia, M.C. and Russo, F.G., 1984. Osservazioni "in situ" sul ciclo ripro- duttivo della Cymodocea nodosa (Ucria) Aschers. XV Congresso S.I.B.M., Trieste, September 1983, Nova Thalassia, 6: Suppl. 719.

McMillan, C., 1981. Seed reserves and seed germination for two seagrasses Halodule wrightii and Syringodium filiforme, from the Western Atlantic. Aquat. Bot., 11: 279--296.

McMillan, C., 1983. Seed germination in Halodule wrightii and Syringodium filiforme from Texas and the U.S. Virgin Islands. Aquat. Bot., 15: 217--220.

McMfllan, C., Bridges, K.W., Kock, R.L. and Falanruw, M., 1982. Fruit and seedlings of Cymodocea rotundata in Yap, Micronesia. Aquat. Bot., 14: 99--105.

Pirc, H., MazzeUa, L. and Russo, G.F., 1983. Record of Cymodocea nodosa (Ucria) Archers. fruiting in a prairie of the Island of Ischia (Gulf of Naples). Rapp. Comm. Int. Met. M~dit., 28 (3): 121--122.

Simonetti, G., 1973. I consorzi a fanerogame marine nel Golfo di Trieste. Atti Ist. Ven. Sci. Lett. Arti, 181: 459--502.