Marine Biology

, Volume 160, Issue 3, pp 591–605 | Cite as

Diurnal variation in chlorophyll fluorescence of Thalassia testudinum seedlings in response to controlled salinity and light conditions

  • Jacqueline F. Howarth
  • Michael J. DurakoEmail author
Original Paper


Diurnal variability in chlorophyll fluorescence caused by dynamic irradiance conditions is an important issue when using pulse amplitude modulation fluorometry to measure physiological conditions of plants at the landscape scale. We examined the use of slopes and y-intercepts of diurnal effective photochemical efficiency of photosystem II (PSII) (ΔF/F m′) versus photosynthetically active radiation (PAR) regressions in addition to direct measurements of maximum photochemical efficiencies of PSII (F v/F m) values to assess physiological status of Thalassia testudinum seedlings in a controlled mesocosm study. Seedlings were exposed to two light treatments (full sun and 50–70 % light reduction) and three salinity treatments (20, 35, and 50). Measurements were taken at 0600, 0900, 1200, 1500, 1800, and 2100 hours in order to assess the diurnal variation in photochemical efficiency of PSII and PAR, with measurements at 2100 providing F v/F m. Results indicated significant effects of light and salinity on regression y-intercepts and measured F v/F m values. Shaded seedlings had higher values for both parameters, suggesting low-light acclimation. The highest salinity treatment resulted in significant reductions for both parameters, suggesting stress. Stress was also indicated by significant reductions in both seedling leaf growth and mean differences between seedling leaves and media osmolalities in the hypersaline treatments (152.0 ± 26.4 vs. 630 ± 40.2 mmol kg−1 for the control treatments). Slopes of ΔF/F m′ versus PAR significantly differed with varying light treatments, with full sun seedlings exhibiting shallower slopes than shaded seedlings, indicating higher efficiency of dissipation of excess energy (photoprotection). These experimental results confirm field data suggesting that diurnal ΔF/F m′ versus PAR regressions are responsive to changes in the physiological status of T. testudinum and that the y-intercepts of diurnal regressions may be used as a proxy for F v/F m.


Photosynthetically Active Radiation Salinity Treatment Shade Treatment Photochemical Efficiency Pulse Amplitude Modulate 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.



This research was funded by a grant from the Florida Fish and Wildlife Conservation Commission (Grant Nos. 509620 and 56990) supported by a cooperative agreement with the South Florida Water Management District (SFWMD #4600001348). The authors would like to thank Justin Campbell of Florida International University, Miami Florida for collecting and shipping the T. testudinum seedlings.


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Copyright information

© Springer-Verlag Berlin Heidelberg 2012

Authors and Affiliations

  1. 1.Department of Biology and Marine Biology, Center for Marine ScienceThe University of North Carolina WilmingtonWilmingtonUSA

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