Laboratory Study to Investigate the Response of Cucumis sativus L. to Roundup and Basta Applied to the Rooting Medium

Abstract

Roundup and Basta are commonly used herbicides in both agricultural and non-agricultural systems. They are non-selective broad spectrum post-emergence herbicides, relatively nontoxic to environment. Their mode of action is connected with inhibition of amino acids biosynthesis (Cobb, 1992; Wakabayashi & Boger, 2004). Herbicide Basta is also know as glufosinate or phosphinothricin. Glufosinate is a structural analogue of glutamic acid and it irreversibly inhibits glutamine synthetase (GS) which synthesises glutamine from glutamate and ammonium. Consequently, ammonium concentration in plant tissues strongly increases and causes metabolic disruption and plants’ death. Roundup’s active ingredient is glyphosate [N-(phosphonomethyl)glycine] that inhibits 5-enolpyruvyl shikimate-3phosphate synthase (EPSPS). As a consequence, the biosynthesis of aromatic amino acids is inhibited. Besides, glyphosate is known to have secondary mode of action connected with the inhibition of chlorophyll biosynthesis resulting from a glyphosate-mediated inhibition of -aminolevulinic acid synthesis (Pline et al., 1999). Both herbicides alter the cellular pools of amino acids that results in disruption of nitrogen metabolism. Indirectly they affect protein synthesis, nitrate assimilation and nitrogen fixation (Bellaloui et al., 2006; Sacala et al., 2008). Nitrogen is an essential element that strongly influence and regulate plants’ growth. For most plants in moderate climate regions, the main source of nitrogen is nitrate obtained from the soil. Nitrate is utilized in a linear pathway that involves its uptake and transport within the plant and then its assimilation, ammonium assimilation and amino acid and protein biosynthesis (Sitt et al., 2002). The first step in nitrate assimilation pathway, the reduction of nitrate to nitrite, is catalyzed by nitrate reductase (NR, EC 1.6.6.1–2). This enzyme plays a central role in nitrogen assimilation and is considered as a key point of metabolic regulation, as well as a rate-limiting enzyme in this pathway. Nitrate reductase is known to modulate rapidly to environmental stress conditions and is responsive to metabolic and physiological status of plants (Kaiser & Huber, 1994). Hence, sometime it is used as a biomarker of plant stress. Nitrate reduction occurs in both shoots and roots of plants and the relative contribution of each is dependent on the particular species, plant age and growth conditions.