The chlorophyll fluorescence (CF) assay is recognized as an important tool for monitoring the electron transport system of photosynthesis. The increase or decrease of fluorescence, compared to control, can indicate a lesion in the chain of events in photosynthesis or damage to the chloroplasts. Inhibition of photosynthesis in leaf segments of Tradescantia by herbicides (glyphosate, picloram & 2,4-D, triclopyr and hexazinone), heavy metals (Zn, Cu, Cd), a surfactant (sodium lauryl sulfate), and sodium fluoride (NaF) has been demonstrated using this method. The strongest changes of fluorescence induction curves were caused by hexazinone. Photosynthetic inhibition was observed as an increase in initial fluorescence (Fo) and a decrease in variable fluorescence (Fv) and electron pool size (EP). A CF assay predicted herbicidal injury in Tradescantia leaves at least 24 h before leaf necrosis appeared.
Three heavy metals (Zn, Cu, Cd) are known as strong respiration and photosynthetic inhibitors in plant cells. In this study cadmium chloride, at the highest concentration (1000 ppm) tested, caused the strongest changes in Fv and EP size, which are characteristics of Photosystem II (PS II) photochemistry. Phytotoxicity of NaF caused an increase in Fv and EP compared to the control.
Bioassays utilizing the green alga Selenastrum capricornutum were performed on filtered eluates from soil treated with six commonly used forestry herbicides applied at labelrecommended rates. The bioassays were conducted at three time periods after herbicide application—one hour, five days, and ten days. The 96-h EC50 values indicated growth inhibition (relative to control sample) for all treatments when assayed 1 h after herbicide application. Algal EC50 values of +100 (Control), +27.3 (Roundup™), -20.4 (Arsenal™ [2 lb Acid Equivalent] [AE]/gal), -22.4 (Garlon™ 4), -49.4 (Tordon 101M™), -100 (Velpar L™), and -100 (Velpar ULW™) were obtained. Assays conducted ten days after herbicide application to soil revealed substantially reduced toxicity of two herbicides. The 96-h EC50 values for Roundup and Arsenal were both +100. There was a significant enhancement effect observed with Roundup. A slight reduction in toxicity was noted for Garlon 4 (-15.9) and Tordon 101M (+9.9). No change in toxicity occurred for Velpar L or Velpar ULW. The herbicides were also applied to water and the following 96-h EC50 values in μg/ml were obtained: 5500 (Arsenal [2 lb AE]), 5300 (Arsenal [4 lb AE]), 5000 (Tordon 101M), 5000 (Garlon 4), 2600 (Roundup), 2.5 (Velpar L), and 1.2 (Velpar ULW).
A recently developed bioassay protocol was used to characterize seven different soil types and Promix BX. The green alga Selenastrum capricornutum was used as the test organism. A ruggedness test which altered seven variables was conducted to identify procedural stipulations that should be carefully controlled in the Selenastrum capricornutum bioassay. Stimulatory or inhibitory effects as measured by population growth were compared to a control using two different nutritional methods termed proportional amendment (PA) and complete amendment (CA). Population growth, calculated effect (CE) values, and EC50 values are given for each soil. A wide range of effects were documented indicating a substantial difference among soils with respect to their ability to stimulate or inhibit algal population growth.
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