عنوان مقاله [English]
Introduction: From the floating aquatic plant species, duckweed (i.e., Lemna sp.) is perhaps the most commonly used in toxicity testing. As its name implies, it is a staple in the summer diet of ducks as well as other aquatic organisms. Actually, “duckweed” can refer to both Lemna sp. and Spirodela sp. (greater duckweed), although, it is usually associated with Lemna minor. The Lemnaceae (duckweeds) are cosmopolitan free-floating aquatic monocotyledonous angiosperms that commonly occur in fresh and brackish stagnant ponds and sluggish streams, mostly in tropical regions but also as far north as 60° and as far south as 40° latitude. Duckweeds are vascular floating non-rooted aquatic plants with a reduced root system and lack stems and true leaves. It has been speculated that the roots serve as anchors to keep the fronds right side up and to form the tangled masses, which are of some importance in dispersal and protection from water movement. They can reproduce both sexually and asexually, however, asexual vegetative reproduction is the more commonly occurring mode and, under laboratory conditions, a doubling in frond number and surface area covered can occur every 1.5–2 days. Duckweed can rapidly colonize open surface waters and develop a large biomass. These characteristics, plus its ability to accumulate inorganic ions favor it for use in wastewater treatment systems and makes duckweed an ideal model for studies on the metabolism of pollutants by aquatic macrophytes as well as for toxicity studies. They are very sensitive to many substances and are already used as convenient test organisms for toxicity evaluation of a number of pollutants including industrial and wastewater effluents, herbicides, heavy metals, surfactants and other common chemicals. The typical test end points are changes in the growth rate (expressed as frond production, fresh and dry weight and frond area) and changes in pigment content. For a few years biotests with lemna were used in some cases to supplement or replace the algal growth inhibition test. Green algae tolerate only a relative narrow pH-range, whereas lemnaceas are able to grow in a wide range from pH 3.5 to 10.5. This allows testing of samples such as sewage waters, which often show unfavorable pH-values, without previous adjustment of the pH. Triazines like terbuthylazine, a PS II inhibitor, and sulfunilurea like metsulfuron-methyl, an ALS inhibitor, and 3,5-dichlorophenole (DCP) which is mostly used in the production of the herbicide 2, 4 dichlorophenoxyacetic acid, are amongst the most widely used herbicides. Since they can be found in many environmental compartments, their fate in ecosystems and the characterization of their toxicity are to be determined. The objective of the study is the comparison of toxic effects of three xenobiotics with different mode of actions and different metabolic pathways by duckweed (lemna minor L).
Materials and Methods: The plants (Lemna minor), which had been purchased from the university of waterloo, Canada, in 2012, and surface sterilized with hypochlorite (0.1 molar), were used in the experiment. Plants moved to the media and growth conditions used for experiment at least two weeks before the start of the experiment. The nutrient solutions described by Steinberg were used. At the start of the experiment, 1 litter Steinberg medium was prepared. Then eight dilutions of the xenobiotics in nutrient media were made. A factor of 10 higher than EC50 values for the highest concentration and dilute to half the concentration 7 times has been used. The dose ranges for metsulfuron-methyl (EC50 = 1 µg l-1) were (0.08 – 10 µg l-1), for terbuthylazine (EC50 = 150 µg l-1) were (10-1500 µg l-1) and for dichlorophenole (EC50 = 3000 µg l-1) were (230-30000 µg l-1). The pictures of the plants were imported into an image-processing program as Photoshop, and the numbers of pixels of the plants were related to the standard surface area. The area specific relative growth rates of the plants were calculated. The relative growth rate (RGR) of Lemna minor as a function of the xenobiotics concentration was described with a sigmoid dose- response curve (log-logistic dose-response curve) and toxicity parameters as EC50 were determined.
Results and Discussion: The EC50 values which derived from the log-logistic fitted curves, showed that the metsulfuron-methy is the most toxic compounds than terbuthylazine and dichlorophenole and made the significant decrease in relative growth rates (RGR) of lemna at much lower concentrations than two other xenobiotics. Dichlorophenole was made less toxicity effects on lemna than others. The high toxicity probably is referred to the mode of action and detoxification pathways of xenobiotics by duckweed, more than xenobiotics concentration. Terbuthylazine is amongst the most widely used herbicides which is used as selective pre-emergence herbicide, applied generally in aqueous solutions directly to the soil, in many cases together with the spring fertilizers. The high toxicity of Metsulfuron-methyl can be referred to the fact that most plants cannot metabolize and detoxify this herbicide. Also slightly higher sensitivity of macrophytes when they exposed to metsulfuron-methyl compare to the terbuthylazine has been reported. It is necessary to more experiments be established on biodegradation pathways and also to determine if the physicochemical properties of xenobiotics play an important role in phytotoxicity.
Conclusions: Metsulfuron-methyl was more toxic than terbuthylazine, and terbuthylazine itself was more toxic than dichlorophenole as expected (see the EC50 values). This high toxicity probably is referred to the mode of action of theses xenobiotics and the fate (biodegradation) of these toxic compounds in duckweed more than their concentrations.