The application of the acetolactate synthase (ALS)–inhibiting herbicide metsulfuron on greenhouse- and field-grown black nightshade and greenhouse-grown ladysthumb resulted in progressive inhibition of the level of carbon dioxide (CO2) fixation, the relative quantum efficiency of electron transport through photosystem I (ΦPSI) and II (ΦPSII), and the leaf chlorophyll content. Photosynthetic-related measurements, measured 2 to 4 d after treatment (DAT) at photon flux densities of 400 to 500 μmol m−2 s−1, provided valuable information before the visual symptoms that first appeared at 7 to 10 DAT with the herbicide. Measurements of the quantum efficiency for electron transport by photosystem II and the loss in leaf chlorophyll content appeared to be two of the most practical parameters to use when designing an early detection method to assess the toxicity of metsulfuron. The use of chlorophyll fluorescence would require a comparison of steady-state ΦPSII measurements for control and treated plants, which could be realized by either measuring in time (before/after application) or space (treated/untreated patch).
Nomenclature: Metsulfuron; black nightshade, Solanum nigrum L. SOLNI; ladysthumb, Polygonum persicaria L. POLPE.