Metazachlor effect on poplar – Pioneer plant species for riparian buffers

Highlights

• Herbicide metazachlor is metabolized in Populus canescens cells/leaves/roots to 14 different metabolites.

• The ratio and amounts of metabolites was significantly different between the tissues.

• Lactate conjugate (deCl-MZCl-Lact) was accumulated in cultivation media.

• Metazachlor and its metabolites adversely affected the condition of the poplar plant at the molecular level.

Abstract

Metazachlor belongs to one of the most used herbicides throughout the world. In order to prevent the contamination of water bodies by such herbicides, the riparian buffers are constructed. The selection of appropriate plant species for this purpose is necessary. In our project, we studied the possibility of grey poplar to uptake and biotransform metazachlor, along with the phytotoxic effect of metazachlor and its metabolites. We used two different models – suspension cultures and poplar regenerants cultivated in vitro. Our results show that the herbicide metazachlor is readily metabolized by both suspension cultures and regenerants to 16 detectable metabolites. The detailed scheme of biotransformation pathway in poplar tissue is presented for the first time. The profile of detected metabolites was approximately the same in poplar cell cultures and regenerants, but the ratio and amounts of particular compounds was significantly different. Generally, the highest concentration (peak area/mg of DW) of all metabolites was present in the roots; the only exception was lactate conjugate (deCl-MZCl-Lact), which accumulated in the cultivation media. Although the plants were not visibly affected by metazachlor or its metabolites, they showed changes in activity of antioxidant enzymes and increased content of phenolic substances, the indicators of stress.

Introduction

Pollution of the aquatic ecosystems is a serious worldwide problem. Agricultural intensification and expansion of dairy farming have degraded the water quality and ecological health of many streams and rivers. The use of riparian buffers located between agricultural fields, where pesticides are applied, has become an important solution to contamination of water supplies (McKergow et al., 2016). The riparian buffers are formed by different species of grasses, phorbs, shrubs, trees and other vegetation. For the right function of these buffer zones, the selection of proper plant species with regard to its resistance to pesticides is an important factor. Passeport et al. (2014) studied the removal of pesticides through the forest buffer. They demonstrated that the reduction of the concentration of pesticides is not only a matter of pesticides’ physico-chemical properties, but it is also markedly effected by the organic layer on the top of the forest soil (Passeport et al., 2014).

Metazachlor (MZCl) is chloroacetanilide herbicide that inhibits cell division and is widely used in agriculture for the pre-emergence control of broad-leaved weeds and annual grasses. In the Czech Republic, it is largely applied in winter and spring for pre- and post-emergence weed control for Brassicaceae (rapeseed, cauliflowers and cabbage) fields (Hvezdova et al., 2018). Metazachlor is classified as H400 “Very toxic to aquatic life” and H410 “Very toxic to aquatic life with long lasting effects”. Because MZCl is a specific inhibitor of the biosynthesis of very-long-chain fatty acids with alkyl chains longer than C18, those higher plants, which possess high amounts of these very-long-chain fatty acids are the most prone organisms to damage.

The biological activity and fate of metazachlor were studied on the crop plants and vegetable for a long time. For example Cauderchet et al. compared metabolism of MZCl in cell suspension cultures and seedlings from rice and spinach (Couderchet et al., 1994). They confirmed the ability of both system to metabolized MZCl and concluded that cell suspension cultures were more tolerant to metazachlor than the corresponding plant systems. Metazachlor can cause the phytotoxicity directly, through its mode of action, or indirectly, through the induction of oxidative stress, which results in a temporary reduction of plant growth. It increases the activity of antioxidant enzymes, such as glutathione S-transferase (Vercampt et al., 2017).

During the registration procedure for plant protection products, assessments regarding the phytotoxic effects on non-target plants are often not taken into consideration. There is a lack of information on the effect of MZCl on plant species that usually form the riparian buffer zones. The grey poplar (Populus canescens) is rapidly growing pioneer tree, tolerant to high water levels, which quickly colonizes riparian woodlands. The goal of this study is to evaluate metazachlor toxicity to poplar cell suspension and poplar in vitro regenerants as models of poplar trees.