ORIGINAL ABSTRACT

This study investigated the herbicidal effects of glyphosate on the invasive perennial weed Chromolaena odorata. Natural weed populations of selected 1.2 m in height plants were sprayed with 1% glyphosate. Measurements of gas exchange, chlorophyll fluorescence and glyphosate concentrations were taken five consecutive days after spraying in late summer (February 2023). Glyphosate was determined by high-performance liquid chromatography (HPLC). After herbicide application, CO2 uptake decreased from 9.5 µmol m− 2s− 1 on day 1 to 0.95 µmol m− 2s− 1 on day 5. Leaf conductance and transpiration followed trends similar to those of CO2 uptake. Saturation of CO2 uptake occurred at a Photosynthetic Photon Flux Density (PPFD) of 1750 µmol m− 2s− 1 and at a leaf conductance of 0.09 mol m− 2s− 1 on day one. Trends in photosynthetic quantum yield (ΦPSII), electron transport rate (ETR) through Photosystem II (PSII) and maximum quantum yield (Fv/Fm) were similar to those of CO2 uptake. There was a linear relationship between CO2 uptake and leaf conductance, CO2 uptake and transpiration and CO2 uptake and Fv/Fm over all days. Initially, glyphosate concentrations were high in leaves and stems but declined with time. After day three, most glyphosate was located in roots, indicating transport from above to below-ground organs. Significant decreases in CO2 uptake, ΦPSII, and ETR through PSII and Fv/Fm indicate the inhibition of the shikimate pathway by glyphosate. The responses suggests that glyphosate was easily absorbed and translocated throughout the plant, resulting in wilting, abscission and 100% mortality within six days. This work provides novel information on the ecophysiological mechanism of glyphosate action in controlling C. odorata.