Diurnal variation in chlorophyll fluorescence of Thalassia testudinum seedlings in response to controlled salinity and light conditions
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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/Fm′) versus photosynthetically active radiation (PAR) regressions in addition to direct measurements of maximum photochemical efficiencies of PSII (Fv/Fm) 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 Fv/Fm. Results indicated significant effects of light and salinity on regression y-intercepts and measured Fv/Fm 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/Fm′ 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/Fm′ 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 Fv/Fm.