FLOWERING UNDER CONTROLLED CONDITIONS BY CYMODOCEA R O T U N D A T A FROM THE PALAU ISLANDS, MICRONESIA
CALVIN McMILLAN
Plant Ecology Research Laboratory and Department of Botany, The University of Texas at Austin, Austin, Texas 78712 (U.S.A.)
(Accepted 23 April 1979)
ABSTRACT
McMillan, C., 1979. Flowering under controlled conditions by Cymodocea rotundata from the Palau Islands, Micronesia. Aquat. Bot., 6: 397--401.
Flowering of Cymodoeea rotundata Ehrenb. & Hempr. ex Aschers. has been reported in natural seagrass beds only in 1871 and 1967. The production of staminate flowers on three separate occasions in 1977--78 in experimental cultures transplanted from Kayangel Atoll suggests primarily temperature control.
The flowering of experimental cultures of Cymodocea rotundata Ehrenb. & Hempr. had not seemed likely because of the rarity of its reported flowering in natural seagrass beds. Den Hartog (1970) had seen only one herbarium specimen with staminate flowers (collected in New Caledonia in 1871) before he observed both staminate and pistillate flowers at Thursday Island, Queens- land, Australia, in 1967. Den Hartog's collection is the only report of pistillate flowers but fruits were collected in 1872 in New Guinea and in 1929 in Bali (den Hartog, 1970). All the reports of flowering for C. rotundata are for material collected in the southern hemisphere.
Flowering in the other three species of Cymodocea has also rarely been reported. The staminate flowers of C. serrulata (R. Br.) Aschers. & Magnus were described for the first time by Kirkman (1975) for material collected in Queensland, Australia, during February of both 1973 and 1974. Pistillate flowers of C. serrulata were found in New Caledonia in 1869 (den Hartog, 1970) and in Kenya by Isaac (1968) and by Kay (1971), and fruits were found in Queensland, Australia, in 1967 (den Hartog, 1970). The flowering records of C. nodosa (Ucria) Aschers. are confined to 1852, 1861, and 1886, except for fruit collected in 1963 (den Hartog, 1970). Pistillate flowers and fruits were described for C. angustata Ostenfeld (Ostenfeld, 1916), but the staminate flowers are still undescribed for this species of northwestern Australia.
Vegetative axes of both C. rotundata and C. serrulata were washed free of most sediment at the time of collection in Palau. The plants of C. rotundata
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were collected at Kayangel Atoll, the northernmost of the Palau Islands, where they occurred together with Thalassia hemprichii (Ehrenb.) Aschers. and Halophila ovalis (R.Br.) Hook. f. The Cymodocea plants were in a relatively pure stand at the beachward-side of the seagrass bed. The collection of C. serrulata was in coral sand on the northeast side of the island of Urukthapel. The material collected in late March 1977 was transported moist in plastic bags inside polyfoam boxes to Texas and planted in standard seagrass culture (McMillan, 1979).
The standard soil cultures used 500 ml cups of fine sandy loam of Central Texas origin and synthetic seawater (Instant Ocean). Triplicate plantings of each of the Cymodocea species were placed in each of two Instant Ocean systems in one growth chamber. Other Western Pacific collections of T. hemp- richii, H. ovalis, H. minor (Zoll.) den Hartog, Enhalus acoroides (L.f.) Royle,
Fig. 1. S taminate f lower of Cymodocea rotundata in exper imenta l culture. View showing size and appearance of the anthers c rowned by a subulate process and two acute pro- tuberances.
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S yringod ium isoetifolium (Aschers.) Dandy, Halodule uninervis (Forsk.) Aschers., and Thalassodendron ciliatum (Forsk.) den Hartog were cultured in the same aquaria. The day lengths and the temperatures were varied during the 18-months study, but the salinity remained relatively constant, 30--35%o.
The first flowering of C. rotundata was by plants that were under a program of diurnal temperature variation. A single staminate flower appeared 7 months after transplanting to the culture conditions: temperature of 27--32°C (night-- day}; day length sequence of 2 months at 14 h, 4 months at 12.5 h and 1 month at 13 h. Only one of the triplicates produced flowers under this treat- ment.
The second and the third flower were produced by plants that had been kept under a sequence of constant temperatures. The flowers appeared 13 months and 17 months, respectively, after transplanting to culture conditions. During the initial 7 months, the plants were under the same day length se- quence indicated above but were at temperatures that varied from 21 to 25°C at intervals. During the 10 months in which flowers appeared, the temperature was held constantly at 27°C and the day lengths were 13 h for 4 months and then 12.5 h for the remaining portion of the study. A single staminate flower was produced by each of two of the replicates under the 27°C.
The three flowers were similar in size and in general appearance. As shown in Figs. 1--3, the first flower had anthers crowned by a subulate process and two acute protuberances, yellow, 15 mm long. The pollen released in Figs. 2--3. Was filiform. The stalk bearing the flower was 2--3 cm long.
Fig. 2. Flower with pollen being released.
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Fig. 3. Flowering plant in 500 ml culture cup.
The cultures of C. serrulata were kept under the two sets of environmental conditions with those of C. rotundata. No flowering was observed in C. serrulata during the 18-months study.
Although there was insufficient flowering to determine inductive conditions with any exactness, the three flowers were produced under day lengths of 12.5 or 13 h and temperatures of 27°C or above. It seems likely that these conditions may represent the inductive ones, but a nutrient effect of the water column and/or sediments may also be involved.
Flowering in other seagrass species has primarily been a consequence of temperature. In Halophila engelmannii Aschers., flowers were induced at 22--24°C under day lengths ranging from 14 to 24 h (McMillan, 1976). Recent studies of Syringodium filiforme Kfitz. (McMillan, in press) show floral induction at temperatures from 20 to 24°C under day lengths ranging from 12 to 24 h. The studies of Syringodium show inhibition of flowering under 11 h day lengths at an inductive temperature. In the studies of Syringodium the non:flowering of material under inductive temperature-- photoper iod conditions suggested that nutrient conditions may also play a role in flowering. Because flowering in laboratory cultures has involved synthetic seawater and non-marine planting medium, it may be possible to determine the role of nutrient conditions with more exactness.
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ACKNOWLEDGEMENTS
This is a con t r i bu t i on o f the Seagrass Ecosys t em S tudy sponsored by the In te rna t iona l Decade of Ocean Explora t ion , NSF Grants OCE 74-24357 , OCE 76-84298 and OCE 77-26399.
REFERENCES
Den Hartog, C., 1970. The Sea-Grasses of the World. North Holland, Amsterdam, 275 pp. Isaac, F.M., 1968. Marine botany of the Kenya coast. 4. Angiosperms. J. East Aft. Nat.
Hist. Soc. Nat. Mus., 27: 29--47. Kay, Q.O.N., 1971. Floral structure in the marine angiosperms Cymodocea serrulata and
Thalassodendron ciliatum (Cymodocea ciliata). Bot. J. Linn. Soc., 64: 423--429. Kirkman, H., 1975. Male floral structure in the marine angiosperm Cymodocea serrulata
(R.Br.) Ascherson & Magnus (Zannichelliaceae). Bot. J. Linn. Soc., 70: 267--268. McMillan, C., 1976. Experimental studies on flowering and reproduction in seagrasses.
Aquat. Bot., 2: 87--92. McMillan, C., 1979. Culture methods. In: R.C. Phillips and C.P. McRoy (Editors), A Hand-
book of Seagrass Biology: An Ecosystem Perspective. Garland, New York, in press. McMillan, C., 1979. Reproductive physiology in the seagrass, Syringodium filiforme, from
the Gulf of Mexico and Carribean. Am. J. Bot., in press. Ostenfeld, C.H., 1916. Contributions to West Australian botany. I. The sea-grasses of
