Comparative Paraquat Sensitivity of Newly Germinated and Mature Fronds of the Aquatic Macrophyte <i>Spirodela polyrhiza</i>

Here, we compared the intrinsic characteristics of 3-day-(newly germinated;“young”) and 8-week-old (“mature”) fronds of the aquatic plant Spirodela polyrhiza and their sensitivity to paraquat, a toxic herbicide. Endpoints measured were frond area and fresh weight, root length, chlorophyll a and b contents, and chlorophyll a fluorescence. Significant differences were detected in the intrinsic physiological traits between young and mature fronds. Young fronds showed higher root length, chlorophyll contents, maximum quantum yield (Fv/Fm), maximal relative electron transport rate (rETRmax) and saturating photon flux density (PFD), whereas mature fronds exhibited greater frond area and fresh weight. After a 72 h exposure to paraquat, root length and rETRmax were identified as the most sensitive endpoints of paraquat toxicity for both frond types, with EC50 values of 0.66 and 0.76 μg·L-1 for young fronds, respectively, and 5.53 and 2.28 μg·L-1 for mature fronds, respectively. Young fronds of S. polyrhiza showed significantly higher sensitivity to paraquat than mature fronds. A survey of other studies on paraquat toxicity to Lemna species revealed that EC50 values of paraquat-induced inhibition of root regrowth and rETRmax in both stages were the lowest, indicating that these two endpoints were the most sensitive to paraquat. In addition, EC50 values of both endpoints of mature fronds of S. polyrhiza appear to be similar to the current allowable concentrations in drinking water set by the World Health Organization (WHO), indicating that these values may have application for the assessment of toxicity risk of paraquat in aquatic ecosystems.