The endangered seagrass Halophila johnsonii Eiseman, exhibits high-light adapted photophysiology consistent with its distribution in intertidal and shallow subtidal (0–3 m) coastal-lagoon habitats along 200 km of southeastern Florida. To examine the short-term responses of this seagrass to three controlled-irradiance treatments (PAR + UVA + UVB [full spectrum], PAR + UVA, and PAR only), greenhouse-acclimated plants were transferred to outdoor mesocosms during July–August 2002. Chlorophyll fluorescence, UV fluorescence, and samples for pigment extraction were collected in the greenhouse, prior to moving the plants outside and on days 1, 2, 3, 4, 6, 10, and 21 of the 24-day experiment. Typical of sun-adapted plants, effective quantum yields measured by pulse-amplitude modulated (PAM) fluorometry were relatively low in all treatments, ranging from 0.46 ± 0.09 (PAR only) to 0.58 ± 0.08 (PAR + UVA + UVB). In the PAR only treatments, there were strong effects on days 1 and 4, presumably because the irradiance in the greenhouse not only lacked all λ<400 nm, but also had low irradiance maxima (∼700 μmol photons m−2 s−1, compared with ∼1,500 μmol photons m−2 s−1 outside at midday). There were few treatment differences between PAR only and PAR + UVA treatments indicating little effect of UVA radiation on this species. Differences in effective quantum yields and relative electron transport rates between the PAR only and PAR + UVA + UVB treatments on day 4 indicated rapid acclimation to UVB radiation. Tissues of H. johnsonii contained compounds that absorbed strongly in the UV, with a λmax at ∼345 nm (depending on the extraction solvent). Absorption peak maxima and minima changed over the course of the experiment but there were no significant light-treatment differences in any pigment parameters. Percent UV shield values, measured using a newly developed UVA PAM fluorometer, were highest the day after plants were transferred from the greenhouse to the outdoor mesocosms and declined significantly to pretreatment levels in all treatments by day 21. Percent UV shield exhibited a significant positive relationship with UV-absorbing pigment (UVP) absorbance, however, the absence of treatment effects suggests that the wavelengths inducing pigment synthesis must lie between 400 and 700 nm (PAR). The results indicate that H. johnsonii rapidly acclimates to high UVB and PAR which may largely explain its distribution in intertidal and shallow subtidal areas.
Carotenoid Photosynthetic Photon Flux Density Irradiance Level Leaf Pair Relative Electron Transport Rate
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This project was supported by the National Oceanic and Atmospheric Administration, Recover Protected Species Program, and Center for Coastal Fisheries and Habitat Research, National Ocean Service, Beaufort, NC. Additional support was provided by the University of North Carolina’s Department of Biological Sciences and the Center for Marine Science. The authors thank Drs. Rolf Gademann and Peter Ralph for the loans of their UVA PAM fluorometers.
Beer S, Vilenkin B, Weil A, Veste M, Susel L, Eshel A (1998) Measuring photosynthetic rates in seagrasses by pulse amplitude modulated (PAM) fluorometry. Mar Ecol Prog Ser 174:293–300CrossRefGoogle Scholar
Beer S, Björk M, Gademan R, Ralph P (2001) Measurements of photosynthetic rates in seagrasses. In: Short FT, Coles RG (eds) Global seagrass research methods. Elsevier, Amsterdam, pp 183–198CrossRefGoogle Scholar
Bilger W, Veit M, Schreiber L, Schreiber U (1997) Measurement of leaf epidermal transmittance of UV radiation by chlorophyll fluorescence. Physiol Plantarum 101:754–763CrossRefGoogle Scholar
Bilger W, Johnsen T, Schreiber U (2001) UV-excited chlorophyll fluorescence as a tool for the assessment of UV-protection by the epidermis of plants. J Exp Bot 52(363):2007–2014CrossRefGoogle Scholar
Dawes CJ, Lobban CS, Tomasko DA (1989) A comparison of the physiological ecology of the seagrasses Halophila decipiens Ostenfeld and Halophila johnsonii Eiseman from Florida. Aquat Bot 33:194–154CrossRefGoogle Scholar
Dawson SP, Dennison WC (1996) Effects of ultraviolet and photosynthetically active radiation on five species of seagrasses. Mar Biol 125:629–638CrossRefGoogle Scholar
Detrés Y, Armstrong RA, Connelly XM (2001) Ultraviolet-induced responses in two species of climax tropical marine macrophytes. J Photochem Photobiol B 62:55–66CrossRefGoogle Scholar
Durako MJ, Kunzelman JI (2002) Photosynthetic characteristics of Thalassia testudinum measured in situ by pulse-amplitude modulated (PAM) fluorometry: methodological and scale-based considerations. Aquat Bot 73:173–185CrossRefGoogle Scholar
Durako MJ, Kunzelman JI, Kenworthy WJ, Hammerstrom KK (2003) Depth-related variability in the photobiology of two populations of Halophila johnsonii and Halophila decipiens. Mar Biol 142:1219–1228CrossRefGoogle Scholar
Eiseman NJ, McMillan C, (1980) A new species of seagrass, Halophila johnsonii, from the atlantic coast of Florida. Aquat Bot 9:15–19CrossRefGoogle Scholar
Federal Register (1998) Endangered and threatened species: threatened status for Johnson’s seagrass 63(177):49035–49041Google Scholar
Johnson GA, Day TA (2002) Enhancement of photosynthesis in Sorghum bicolor by ultraviolet radiation. Physiol Plant 116:554–562CrossRefGoogle Scholar
Kenworthy WJ (1993) The distribution, abundance, and ecology of Halophila johnsonii Eiseman in the lower Indian River, FL. Final report to the Office of Protected Resources, NMFS, Silver Springs, MarylandGoogle Scholar
Koch EM, Beer S (1996) Tides, light and the distribution of Zostera marina in Long Island Sound, USA. Aquat Bot 53:97–107CrossRefGoogle Scholar
Larkum AWD, Wood WF (1993) The effects of UV-B radiation on photosynthesis and respiration of phytoplankton, benthic microalgae, and seagrasses. Photosynth Res 36:17–23CrossRefGoogle Scholar
Runcie JW, Durako MJ (2004) Among-shoot variability and leaf-specific absorptance characteristics affect diel estimates of in situ electron transport of Posidonia australis. Aquat Bot 80:209–220CrossRefGoogle Scholar
Searles PS, Flint SD, Caldwell MM (2001) A meta-analysis of plant field studies simulating stratospheric ozone depletion. Oecologia 127:1–10CrossRefGoogle Scholar
Trocine RP, Rice JD, Wells GN (1981) Inhibition of seagrass photosynthesis by UV radiation. Plant Physiol 68:74–81CrossRefGoogle Scholar
Vincent WF, Roy S (1993) Solar ultraviolet-B radiation and aquatic primary production: damage, protection, and recovery. Environ Rev 1:1–12CrossRefGoogle Scholar
Virnstein RW, Morris LJ, Miller J, Miller-Myers R (1997) Distribution and abundance of Halophila johnsonii in the Indian River Lagoon. Technical Memorandum 24. St. Johns River Water Management District, Patalka, FloridaGoogle Scholar
Wellburn AR (1994) The spectral determination of chlorophylls a and b, as well as total carotenoids using various solvents with spectrophotometer of different resolution. J Plant Physiol 144:307–313CrossRefGoogle Scholar
Yakovleva IM, Titlyanov EA (2001) Effects of high visible and UV irradiance on subtidal Chondrus crispus: stress, photoinhibition and protective mechanisms. Aquat Bot 71:47–61CrossRefGoogle Scholar