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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

The threatened seagrass Halophila johnsonii Eiseman coexists subtidally with H. decipiens Ostenfeld in southeastern Florida, but only H. johnsonii also oc- curs intertidally. Pulse amplitude modulated fluorome- try and fiber-optic spectrometry were used to investigate the photobiology of two populations of H. johnsonii and H. decipiens in an attempt to explain these distribution patterns. Maximum photosynthetic quantum yields (Fv/ Fm) were measured in situ as a function of depth dis- tribution within, and between, these two species at two sites (Jupiter Sound, 26� 57¢N; 80� 04¢W, and northern Biscayne Bay, 25� 55¢N; 80� 07¢W) along the east coast of Florida, USA, during 6-10 March 2001. Reciprocal transplants at the northern site were used to evaluate the plasticity of photosynthetic patterns and pigment absorption spectra and to gain insights into the mecha- nisms responsible for variations in the observed depth- distribution patterns. Subtidal-population Fv/Fm values were generally higher for H. johnsonii than for H. deci- piens, at both sites. At the northern site, intertidal H. johnsonii had significantly lower Fv/Fm (0.494±0.138) than both subtidal H. johnsonii (0.696±0.045) and subtidal H. decipiens (0.668±0.048). In contrast, at the southern site intertidal H. johnsonii had the highest Fv/ Fm (0.663±0.047) and were the largest plants. Fv/Fm values of subtidal plants of both species decreased when they were transplanted into shallow, intertidal beds. Correspondingly, Fv/Fm increased for intertidal H. johnsonii transplanted into the subtidal, 2 m deep beds. Rapid light curves indicated that H. decipiens had lower maximum relative electron transport rates (RE- TRmax) than did H. johnsonii. In addition, the onset of photoinhibition occurred at lower irradiances for H. decipiens (537-820 lmol photons m )2 s )1 ) compared to H. johnsonii (1141-2670 lmol photons m )2 s )1 ). RETRmax values decreased for intertidal H. johnsonii transplanted into subtidal beds, but they increased for both species when transplanted from subtidal to inter- tidal beds. Absorption spectra for the acetone-soluble leaf pigments of intertidal H. johnsonii exhibited a dominant peak near 345 nm; this UV peak was 30% lower for subtidal plants. Pigment absorption spectra for H. decipiens lacked the 345 nm peak and absorbances, normalized to leaf pairs, were lower across the spectrum. Our results indicate that photosynthetic tolerance to higher irradiances and presence of UV-absorbing pig- ments (UVP) in H. johnsonii may allow this species to exploit the shallowest waters without competition from the closely related, but UVP-lacking H. decipiens.

Shoot-to-landscape scale sources of variation in photosynthetic characteristics of Thalassia testudinum, as measured in situ using a submersible pulse-amplitude modulated fluorometer (diving-PAM), were investigated. Shoot-scale variation was assessed to develop a standard-methods protocol for this species. Significant within-shoot and among-shoot scale variation was observed for several PAM-fluorescence parameters. The fraction of incident photosynthetically active radiation (PAR) absorbed by rank 1 (youngest) leaves, 0.67±0.03, was significantly lower than the PAR absorbed by rank 2 and 3 leaves (0.78±0.04 and 0.77±0.04, respectively). Quantum yields (Y) and photosynthetic efficiency (Fv/Fm) exhibited greater variability and generally decreased with increasing leaf age (Y=0.78±0.02, 0.78±0.03, and 0.76±0.06 and Fv/Fm=0.79±0.02, 0.77±0.5, and 0.75±0.06 for rank 1, 2, and 3 leaves, respectively). Maximum fluorescence of light-acclimated leaves (F′m) significantly decreased from the base to the tip of leaves (1032±203, 1059±139, and 793±107 for the base, middle and tip of the leaf) and Y was significantly reduced from 0.68–0.76 to 0.27–0.62 when measured where leaf lesions were present. However, paired comparisons (with versus without lesions at the tip of the leaf) for the base, middle, and tip areas of rank 2 leaves indicated that lesions effects on photosynthesis were localized only to the area of the lesion (tip). The Y exhibited relatively high variability for short-shoots within seagrass die-off patches and were significantly reduced for short-shoots exhibiting die-off symptoms (rotten leaf base grayish-green leaf color, but no lesions). Negative response slopes for Y and Fv/Fm, measured as part of a landscape-scale sampling program, revealed the presence of significant diurnal (time-of-day) and photosynthetic photon flux density (PPFD) related variation, with a more negative slope for Y. These results indicate that the selection of leaf tissue, short-shoot location, and time of measurement need to be considered when determining photosynthetic rates for seagrasses in situ.

Halophila johnsonii Eiseman is a shallow-water marine angiosperm which contains UV-absorbing metabolites. Studies on methanol extracts of H. johnsonii by means of HPLC–UV, NMR, HPLC–MS resulted in isolation and identification of seven previously unknown flavone glycosides: 5,6,7,3′,4′,5′-hexahydroxyflavone-7- O -β-glucopyranoside ( 1 ), 5,6,7,3′,4′,5′-hexahydroxyflavone-7- O -(6″- O -acetyl)-β-glucopyranoside ( 2 ), 6-hydroxyluteolin-7- O -(6″- O -acetyl)-β-glucopyranoside ( 3 ), 6-hydroxyapigenin-7- O -(6″- O -acetyl)-β-glucopyranoside ( 4 ), 6-hydroxyapigenin-7- O -(6″- O -[E]-coumaroyl)-β-glucopyranoside ( 5 ), 6-hydroxyapigenin-7- O -(6″- O -[E]-caffeoyl)-β-glucopyranoside ( 6 ) and 6-hydroxyluteolin-7- O -(6″- O -[E]-coumaroyl)-β-glucopyranoside ( 7 ). Also isolated were three known flavone glycosides, 6-hydroxyluteolin 7- O -β-glucopyranoside ( 8 ), scutellarein-7- O -β-glucopyranoside ( 9 ), and spicoside ( 10 ), and five known flavones, pedalitin ( 11 ), ladanetin ( 12 ), luteolin ( 13 ), apegenin ( 14 ) and myricetin ( 15 ). Qualitative comparison of the flavonoid distribution in the leaf and rhizome-root portions of the plant was also investigated, with the aim of establishing the UV-protecting roles that flavonoids played in the sea grass.