Comparative Paraquat Sensitivity of Newly Germinated and Mature Fronds of the Aquatic Macrophyte &amp;lt;i&amp;gt;Spirodela polyrhiza&amp;lt;/i&amp;gt;

Jihae Park,Hojun Lee,Taejun Han

doi:10.4236/ajps.2020.117072

Abstract

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.

Highlights

Herbicides are a class of chemical pesticides that are widely used to remove or control nuisance plants in agriculture and horticulture [1]
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
Determination of the optimal conditions for turion germination is important since several environmental factors can simultaneously influence germination of the turions of S. polyrhiza [29], but standardization of optimal germination for toxicity testing has not yet been established [17]

Summary

Introduction

Herbicides are a class of chemical pesticides that are widely used to remove or control nuisance plants in agriculture and horticulture [1]. N,N-dimethyl-4,4-bipyridinium dichloride, is one of the most widely used ionic herbicides for controlling the growth and spread of broadleaved weeds, grasses and aquatic weeds. It is a systemic herbicide that is highly toxic to plants, as it diverts electrons away from the donor side of the photosystem I (PSI) complex by accepting electrons from iron-sulphur (Fe-S) centres and/or ferredoxin, preventing electron transfer to NADP [4]. There is growing concern over the exposure to small quantities of paraquat in the aquatic environment, as it may have negative impacts on the endocrine systems of humans and wildlife. A recent report suggests that paraquat be closely related to a less immediately apparent effect—Parkinson’s disease [10]

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Conclusion