Leaf chlorophyll fluorescence, reflectance, and physiological response to freshwater and saltwater flooding in the evergreen shrub, Myrica cerifera

Photosynthesis, water relations, chlorophyll fluorescence, and leaf reflectance were used to evaluate stress due to freshwater and saltwater flooding in the evergreen coastal shrub, Myrica cerifera, under controlled conditions. M. cerifera forms large monospecific thickets that facilitate scaling up from leaf-level measurements to the landscape. Based on physiological responses, stress began by day 3 in flooded plants treated with 5, 10, and 15 g L−1 salinity, as seen by significant decreases in stomatal conductance and net photosynthesis relative to control plants. Decreases in physiological measurements occurred by day 9 in freshwater flooded plants. Visible signs of stress occurred by day 5 for plants treated with 15 g L−1, day 8 for flooded plants exposed to 10 g L−1, and day 10 for those treated with 5 g L−1 salinity. Significant differences in light-adapted fluorescence yield (ΔF/F′m) were observed by day 3 in plants flooded with 5, 10, and 15 g L−1 salinity and day 6 in freshwater flooded plants. Non-photochemical quenching (ΦNPQ) increased with decreasing ΔF/F′m. In comparison, statistical differences in dark-adapted fluorescence yield (Fv/Fm) were observed by day 12 in plants flooded with 5, 10, and 15 g L−1 salinity, well after visible signs of stress were apparent. Fluorescence parameters were successful at detecting and distinguishing both freshwater and saltwater flooding stress. A positive, linear correlation (r2 = 0.80) was observed between ΔF/F′m and the physiological reflectance index (PRI). Xanthophyll-cycle dependent energy dissipation appears to be the underlying mechanism in protecting photosystem II from excess energy in saltwater flooded plants. ΔF/F′m was useful in detecting stress-induced changes in the photosystem before any visible signs of damage were evident at the leaf-level. This parameter may be linked to hyperspectral reflectance data for rapid detection of stress at the canopy-level.