Aquatic Botany, 18 (1984) 413--416 413 Elsevier Science Publishers B.V., Amsterdam - -Pr in ted in The Netherlands

Short Communication

MALE FLOWERS OF HALOPHILA ENGELMANNI: DESCRIPTION AND FLOWERING ECOLOGY

FREDERICK T. SHORT 1

Harbor Branch Foundation, Inc., R.R. 1, Box 196, Ft. Pierce, FL 33450 (U.S.A.)

MARION L. CAMBRIDGE

Department of Plant Physiology, Biological Centre, Box 14, 9750 AA Haren (The Netherlands)

(Accepted for publication 18 January 1984)

ABSTRACT

Short, F.T. and Cambridge, M.L., 1984. Male flowers of Halophila engelmanni: descrip- tion and flowering ecology. Aquat. Bot., 18: 413--416.

The male flowers of Halophila engelmanni Ascherson are described and illustrated • from near the northern end of the species range. Flowering is discussed with emphasis on environmental factors influencing flower initiation.

INTRODUCTION

Halophila engelmanni Aschers. grows in the shallow euryhaline water of the Indian River, Florida, U.S.A., a 180-kin long coastal lagoon separated from the ocean by a sand barrier broken by channels at three points along its length. Den Hartog (1970) reported that the male flower of this species was unknown. McMillan (1976) briefly described the flower produced during the manipulation of conditions in aquarium culture of plants from Redfish Bay, Texas.

Several stands of H. engelmanni with male flowers were found near Fort Pierce, Florida, in May 1982, growing in coarse sand at a depth range of 0.3--1.8 m at the edge of dredge spoil islands. H. engelmanni was growing either in small pure stands, heterogeneously mixed with Halodule wrightii Aschers., or mixed with patches of Halophila decipiens Ostenfeld.

DESCRIPTION OF MALE FLOWERS

Halophila engelmanni Aschers. The male flower (Fig. 1) is borne singly subtended by lanceolate acuminate spathal bracts 6--7 mm long, 2--3 mm

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0304-3770/84/$03.00 © 1984 Elsevier Science Publishers B.V.

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Fig. 1. Flowering shoot of Halophila engelmanni from Indian River Lagoon, FL. Left: with a male flower initiated at the center of the leaf pseudowhorl. Upper right: male flower after pollen release. Lower right: Floral spathe with pollen intact, l-leaves, b- spathal bracts, p-pedicel, t-tepals, a-anthers. Scale 5 ram.

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wide, apex acute, margins serrate, sessile in the axils of 2--4 pairs of leaves arranged in a pseudowhorl. Pedicel whitish, 4--10 mm long; tepals broadly elliptic, obtuse, keeled, convex, reflexed when mature with mid-rib pro- truding on upper surface, apex apiculate, color translucent brown with red- dish margins; anthers 4 mm long, bilocular, beige; pollen grains in fine fila- ments within the anthers, yellow.

DISCUSSION

The male flowers of Halophila stipulacea (Forssk.) Aschers. (Lipkin, 1975) and Hatophila ovalis (R.Br.) Hook.f. (Masamune, 1964) are similar in form to those of H. engelmanni, although the flower pedicel is attached basally in these species. The infrequent occurrence of male flowers in H. stipulacea, resulting from the short life-period of these delicate flowers (Lipkin, 1975), is also evident in H. engelmanni.

The period of flowering (May--June) corresponds to the end of the winter dry season and the start of the summer rainfall. Fresh water influx from numer- ous drainage canals to the Indian River decreases salinity from 33--35°/oo in April to 20--290/00 in May and June (Von Zwek and Adragna, 1976) in the shallow area where seagrass is present. Water temperatures in the lagoon in- crease from a winter low of 15°C in mid-February to a summer maximum of 32°C at the end of July. Day length also increases from the December mini- mum of 10 h to a maximum of 14 h in June.

Flower buds were found on all, or nearly all, shoots attached to a contig- uous rhizome. These flower buds were observed even within leaf whorls that had not begun to unfold. This indicates that the occurrence of flowering in individual plants is predetermined in H. engelmanni. The onset of flowering was reported to be chiefly controlled by temperature (McMillan, 1976), al- though McMillan (1976) also indicated that continuous flowering occurred at 14 h of light in laboratory cultures.

McMillan found that flowering in the laboratory could be maintained when H. englemanni was grown in water at 22--24°C. In contrast, flowering occurred in the Indian River lagoon at somewhat higher water temperatures (24--29°C). The salinity values in the Indian River were low during the flowering period, averaging 230]o0, but did not appear to inhibit flowering. However, the extreme summer conditions in the Indian River may cause the discontinuation of H. engelmanni flowering, since temperatures exceed 32°C and salinity drops to 18%0.

These environmental controls on H. engelmanni flowering were indirectly tested by the effect of an unusually warm and rainy winter and spring in 1983 in east-central Florida. Water temperatures were 3--5°C warmer than previous years, and heavy precipitation reduced salinities in the Indian River lagoon from the usual 30%0 to 16%0. Such a change in climatic conditions could be expected to alter the onset of flowering if salinity and temperature were truly controlling factors. The first visible evidence of flowering (Fig. 1)

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was again observed during the first week of May. Thus, it is suggested that the higher temperature did not stimulate early flowering and the low salinity did not inhibit H. engelmanni flowering. Photoperiod is independent of year- to-year climatic changes, implicating it as the probable factor influencing flowering.

ACKNOWLEDGEMENTS

We wish to thank Mark Fonseca, Denise Heller and Catherine Short for assisting in collection of Halophila flowers. Drawings by Carol Henry. Harbor Branch Foundat ion Contribution No. 360.

REFERENCES

Den Hartog, C., 1970. The Seagrasses of the World. North Holland, Amsterdam, 275 pp. Lipkin, Y., 1975. On the male flower of Halophila stipulacea. Israel J. Bot., 24: 198--200. Masamune, G., 1964. Information on the sea plants of the Noto Peninsula, Japan. I.

Halophila ovalis. In: Annual Report of the Noto Marine Laboratory, Vol. 4, pp. I--2. McMillan, C., 1976. Experimental studies on flowering and reproduction in seagrasses.

Aquat. Bot., 2: 87--92. Von Zwek, O. and Adragna, G., 1976. Physical Oceanographic Studies of the Indian

River Region. In: Indian River Coastal Study, Annual Report, Harbor Branch Con- sortium, 187 pp.