Halophila Engelmannii Aschers Research Articles

Development of a medium and culture system for in vitro propagation of the seagrass Halophila engelmannii

An in vitro culture system was developed for axenic culture of the seagrass Halophila engelmannii Aschers. The culture system had a rhizome layer solidified with agar (0.8% w/v) and a liquid overlay layer. The rhizome layer contained artificial seawater (20%0, inorganic nutrients, plant growth regulators, sucrose (1% w/v), and activated charcoal (1% w/v). The overlay layer contained artificial seawater and inorganic nutrients. The medium f/4 provided the best source of inorganic nutrients of the five media compared. Axenic cultures of H. engelmannii died if NO3 was the only N source. They grew best when supplied with glutamic acid at a concentration of 3.4 mM as the N source. A combination of α-naphthaleneacetic acid at a concentration of 0.25 mg ∙ L−1 and 6-benzylaminopurine at a concentration of 10 mg ∙ L−1 provided the most rapid rates of propagation as measured by the numbers of pseudowhorls and branches produced. Key words: tissue culture, marine plants, media, plant growth regulators.

The longevity of seagrass seeds

The greatest longevity of seagrass seeds is reported for Syringodium filiforme Kütz. Seefs of Syringodium from Texas continued to germinate for 4 years and 1 month. Halodule wrightii Aschers. seeds from St. Croix, U.S. Virgin Islands, continued to germinate for 3 years and 10 months and Halodule uninervis (Forsk.) Aschers. seeds from Australia for 3 years and 5 months. Seeds of Halophila engelmannii Aschers. from Texas germinated after 2 years and storage under dark conditions.

Testing the influence of night length on the anthesis of staminate flowers of Halophila engelmannii aschers. (Hydrocharitaceae)

Staminate flowers of Halophila engelmannii Aschers. Studied under varied night lengths had the same pattern of bud opening. During nights of either 4 1/2 or 6 h duration, the staminate flowers opened and released pollen during a 9 1/2-h light period that extended from the commencement of light at 06:00 h until mid-afternoon, 15:30 h. Flowers that had not opened by 15:30 h and that were subsequently monitored in light from 15:30 h until the time of darkness at either 24:00 or 01:30 h did not reach anthesis until the following light period. Although a previous study using 9-h nights had indicated that night length was a possible determinant of the timing of anthesis, the present study suggests that bud opening is initiated by the onset of light and that subsequently buds continue to open during a 9 1/2-h period.

Germination and seedling development of Halophila engelmannii aschers. (hydrocharitaceae) under axenic conditions

Seeds of Halophila engelmannii Aschers. stored in seawater in the dark after collection in Redfish Bay and Laguna Madre, TX, were germinated in the light under axenic conditions following ultrasonic and chemical (EtOH, NaOCl, HgCl 2) cleaning. After germination, axenic seedlings were transplanted individually into sterile media and their development compared with seedlings under non-axenic soil culture. Seedling development in a medium composed of 10% coconut milk, 1% charcoal and 1.5% agar in artificial seawater (ASP 2) approximated the rhizome production and stem and leaf pseudowhorl formation in non-axenic sediment cultures. After development under axenic conditions, rhizomal ramets with attached stems and leaf pseudowhorls were established in clonal, axenic cultures during a 6-month period following germination. The sustained culture of clones of H. engelmannii in an axenic system represents a long-sought goal for seagrass investigations and should promote efforts to understand the role of cation exchange in seagrass biology and the nature of chemical interactions with other organisms.

Timing of anthesis for staminate flowers on Halophila engelmannii aschers. From Texas and Halophila decipiens ostenfeld from Panama

Seedlings of Halophila engelmannii Aschers. from Redfish Bay, Texas, and H. decipiens Otenfeld from Toro Point, Panama, opened statminate flowers in light. Among 12 H. engelmanii seedlings and 11 H. decipiens seedlings that had been germinated in the laboratory, flowers opened during a 9-h light period extending from the commencement of ligh at 05.30 until mid-afternoon, 14.30. Unopened flowers that were monitored in subsequent light from 14.30 until the time of darkness at 20.30 did not reach anthesis until the following day. The lenght of the light period for anthesis, 9 h, correlated with the length of the night.

The sex ratio and fruit production of laboratory-germinated seedlings of Halophila engelmannii Aschers. (Hydrocharitaceae) from Redfish Bay, Texas

Approximately equal numbers of staminate and pistillate plants were produced within a seedling population of Halophila engelmannii Aschers. Among 61 seedlings that produced flowers under laboratory conditions, 33 were male and 28 were female, indicating a probable 50:50 ratio. Under controlled conditions, staminate flowers released pollen during the daylight hours and the moniliform strands of pollen grains floated on the water surface. After artificial pollination, fruits and seeds developed in the laboratory cultures. Although flowering in the natural beds of Redfish Bay was not extensive in 1988, the ratio of staminate to pistillate shoots was recorded as 42:58.

The seed reserve of Halophila engelmannii (Hydrocharitaceae) in redfish bay, Texas

Seeds of Halophila engelmannii Aschers, were present in 36% of the sediment samples examined monthly from October 1986 to May 1987 in Redfish Bay, Texas. For samples with seed, the reserve varied from 100 to 800 m −2; for all samples, the mean seed reserve was 74 m −2. For seed that was collected in October, germination in light at 24–27°C began 27–33 days after collection, but for seed that was collected in November–May, germination began in light in the laboratory after 7–10 days. Seed that was collected in June and kept in the dark at 24–27°C for 18 weeks (late September) began to germinate after 27–38 days in light, but seed kept in the dark for 30 weeks began to germinate after 9–12 days. Seeds probably remain ungerminated if buried in the sediment and germinate upon exposure to light following sediment disturbance, allowing colonization of newly available substrates.

Photosynthetic Response of Seagrasses to Ultraviolet-A Radiation and the Influence of Visible Light Intensity

Inhibition of photosynthesis by ultraviolet-A radiation (UV-A, 315-380 nanometers) was examined in three marine angiosperms: Halophila engelmannii Aschers, Halodule wrightii Aschers, and Syringodium filiforme Kütz. Sensitivity to UV-A and photosensitization to UV-A by photosynthetically active radiation (PAR, 380-700 nanometers) were characterized.Net photosynthesis by Halodule and Syringodium was unaffected by UV-A irradiation in the absence of PAR. Irradiation of Syringodium by a combined beam of UV-A and PAR resulted in photosynthetic inhibition. The depression of net photosynthesis was found to be a function of PAR intensity at a fixed level of UV-A irradiation. Inhibition of photosynthesis in Halodule by the combined beam was minimal and suggests adaptation to environmental irradiation levels.Halophila was the only species examined, subject to photosynthetic inhibition by UV-A in the absence of PAR. Irradiation with PAR intensities characteristic to Halophila in the natural system as the combined beam, appeared to negate the inhibition. Increasing the PAR component of the combined beam above environmental norms resulted in photosynthetic inhibition greater than that observed for UV-A alone.

Phenolic acids in seagrasses

Six phenolic acids were found in leaves of over 50% of the seagrasses surveyed. p-Hydroxybenzoic acid was the most ubiquitous and was recorded in each of the 12 genera and in each of the 25 species that included both Gulf-Caribbean and Indo-Pacific seagrasses. p-Coumaric, caffeic, ferulic, vanillic and protocatechuic acids were also of frequent occurrence. The six phenolic acids were detected in rhizomes and roots as well as in leaves of four Texas seagrasses, Thalassia testudinum Banks ex König, Halodule wrightii Aschers., Syringodium filiforme Kütz., and Halophila engelmannii Aschers. The phenolic acids that predominate in the seagrasses also occur widely in land plants, but gallic acid was detected in a greater percentage of seagrasses than had been observed in a comparable number of land plants of Central Texas origin. Although trhe phenolic acids have been cited in the allelopathic literature of land plants as major water-borne inhibitors, their role in seagrass ecosystems is under investigation.

Chlorophyll composition under controlled light conditions as related to the distribution of seagrasses in Texas and the U.S. Virgin Islands

Seagrasses from various depths in St. Croix, U.S. Virgin Islands, and from shallow beds in Redfish Bay, Texas, were grown in the laboratory under three light conditions. For all collections, light absorption readings of extracted pigments showed that total chlorophyll content is inversely related to reduced light over the range from 35 to 200 μM m −2 s −1. The ratio of chlorophyll a to chlorophyll b decreases in response to reduced light for Caribbean collections of Halodule wrightii Aschers., Syringodium filiforme Kütz., and Halophila decipiens Ostenfeld but not for Thalassia testudinum Banks ex König or the Texas collections of Halodule, Syringodium, Thalassia, and Halophila engelmannii Aschers. There is a correlation of the maximum depth of the St. Croix seagrasses and the ratio of chlorophyll a to chlorophyll b: H. decipiens, with the greatest depth range, to −42 m, has the lowest ratio; T. testudinum, with the least depth range, to −12 m, has the highest ratio; H. wrightii and S. filiforme have intermediate depth ranges and rations. Although light quality and sea bottom characteristics may play roles in the ultimate depth to which a seagrass may occur, photon flux density is suggested as a primary environmental determinant.

Morphogeographic variation under controlled conditions in five seagrasses, Thalassia testudinum, Halodule wrightii, Syringodium filiforme, Halophila engelmannii, and Zostera marina

The morphogeographic evidence udner controlled conditions demonstrated variation in leaf width that correlated with the leaf patterns in the indigenous habitats. In Thalassia testudinum Banks ex König, Halodule wrightii Aschers., and Syringodium filiforme Kütz., the narrow-leaved variants from shallow bays in the northern Gulf of Mexico continued to produce narrow leaves in laboratory culture. Broader-leaved variants in the same three species that originated from seagrass beds associated with coral reefs in the southern Gulf of Mexico and the Carribbean continued to produce broad leaves. The turbid water and variable salinity and temperature of the shallow northern bays correlate with the presence of narrow-leaved plants, and the clear water and relatively constant salinity and temperature of the southern regions correlate with the broader-leaved populations. Although an ecotypic status for the narrow- and wide-leaved populations was suggested, the selective role of these habitat conditions needs further investigation. Clonal variation in leaf width was demonstrated within Halodule populations under uniform conditions. Plants of Halophila engelmannii Aschers. from deep seagrass beds in the Gulf of Mexico along the western coast of Florida continued to produce narrower leaves than those produced by plants from shallow Texas bays. Three collections of Zostera marina L. from Washington and Alaska produced leaves of significantly different widths under each of three temperature regimes. The experimental evidence suggests that the width of a seagrass leaf is dependent on its immediate environmental surroundings but that the limits of its ecoplasticity vary geographically depending on the genotype.