Abstract
In the worldwide search for new strategies in sustainable weed management, the use of plant species able to produce and release phytotoxic compounds into the environment could be an effective alternative to synthetic herbicides. Eucalyptus globulus Labill. is known to be a source of biologically active compounds responsible for its phytotoxic and allelopathic properties. Our previous results demonstrated the bioherbicide potential of eucalyptus leaves incorporated into the soil as a green manure, probably through the release of phytotoxins into the soil solution. Thus, the aims of this study were to understand the phytotoxicity of the eucalyptus leaves aqueous extract applied in pre- and post-emergence, and to identify and quantify its potentially phytotoxic water-soluble compounds. The effects were tested on the germination and early growth of the model target species Lactuca sativa and Agrostis stolonifera, and on physiological parameters of L. sativa adult plants after watering or spraying application. Dose-response curves and ED50 and ED80 values for eucalyptus aqueous extracts revealed pre-emergence inhibitory effects on both target species, effects being comparable to the herbicide metolachlor. While spraying treatment reduced the aerial and root biomass and increased the dry weight/fresh weight ratio of lettuce adult plants, watering application reduced protein contents and chlorophyll concentrations with respect to control, reflecting different modes of action depending on the site of phytotoxin entry. Via HPLC analyses, a total of 8 phenolic compounds (chlorogenic, two ρ-coumaric derivatives, ellagic, hyperoside, rutin, quercitrin, and kaempferol 3-O-glucoside) and other 5 low weight organic acids (citric, malic, shikimic, succinic and fumaric acids) were obtained from aqueous extract, the latter being identified for the first time in E. globulus. Despite some phytotoxic effects were found on lettuce adult plants, the use of eucalyptus aqueous extract would be discarded in post-emergence, whereas it was promising as a pre-emergence bioherbicide.
Highlights
During the past years, an increasing demand for the environmental safety of pesticides has led to looking for new nonsynthetic management methods in agriculture
The first step to find possible candidates as bioherbicides consists in assessing the phytotoxic activity of plant material aqueous extracts on morpho-physiological parameters of model target species, by in vitro bioassays
Dose-response curves are useful for estimating parameters ED50 and ED80, which are important because they establish a frame of reference to which subsequent tests can be compared [28]
Summary
An increasing demand for the environmental safety of pesticides has led to looking for new nonsynthetic management methods in agriculture. In this context, sustainable weed management is one of the main challenges for both organic and conventional agriculture [1]. In the necessary search for new strategies for weed control in organic agriculture, the use of plant species able to produce and release phytotoxic compounds could be an effective tool. Together with other low molecular weight organic acids, are present in plant aqueous extracts because of their water-soluble nature, and several studies have demonstrated their phytotoxic potential against different target species [2,3]. Chemical identification procedures such as chromatographic techniques for analysis of phenolic compounds have been thoroughly improved in the last years [6]
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