Comparação do potencial herbicida do glifosato e seus complexos metálicos através de estudo enzimático e de metabólitos em Glycine max e Brachiaria decumbens

Abstract

The glyphosate, herbicide used in different countries, inhibits the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), blocking the synthesis of aromatic amino acids. With this, it still affects the activity of phenylalanine ammonia lyase (PAL) and the concentration of metabolites in plants. Its activity was first reported as a metal chelator. Therefore, to make use of this herbicide, may be formation of complexes with metals from soil, occurring change in the effectiveness of the product and loss of minerals. Thus, to avoid such problems,this study aimed to compare the effectiveness of three forms of glyphosate: purified, previously complexed with metals, and a commercial form (Roundup WG®). The effects of glyphosate complexes with copper, cobalt and nickel with different stoichiometries and different pH values on the in vitro activity of EPSPs were evaluated. Selected complex for the first experiment in vivo were Cu421 and Co821 (first number for pH and the latter two to stoichiometry glyphosate: metal). These complexes were used in the in vivo experiment I, as well as distilled water (control), purified Roundup and glyphosate on transgenic soybean and B. decumbens. In the following experiments (II and III), mineral oil was used in all treatments to reduce leaching of the compounds. The complex selected for continuation of this study was Cu421 due to its emphasis on the results obtained in the first experiment in vivo. When comparing the effect of the compounds in Falker Chlorophyll Index (ICF) of B. decumbens compared to control, there was fall of this index when using Roundup, Cu421 and purified glyphosate (the latter only in the second experiment). Later it was analyzed fresh mass accumulation and collected fresh material for determination of enzyme activities and N-NO3- levels. The complex Cu421 caused a more significant fresh mass reduction and still allowed the increase in the content of soluble nitrate in Brachiaria decumbens, probably due to the presence of nitrate in this complex. In the case of transgenic soybean there was no significant change in ICF. The use of Roundup and the complex did not change significantly the fresh mass accumul ation in this species. The application of the purified glyphosate, however, caused increase in mass accumulation and all treatments altered soybeans in nitrate content. In experiments with mineral oil in B. decumbens, the Cu421 complex inhibited significantly the EPSPs activity, as well as purified Roundup and glyphosate (the latter only in the experiment III). No inhibition occurred in transgenic soybean. All treatments in vivo stimulated PAL activity only on weed, which suggests that the increase in this activity is a consequence of the inhibition of EPSPs and other deleterious effects of compounds on susceptible plants. This study showed that especially Cu421 complex might be used effectively as a herbicide since it inhibits EPSPs, activates the PA,L reduces fresh mass accumulation and influences photosynthesis in the tested weed. Besides, the transgenic soybean showed little sensitive to this compound, which further increases the prospect of using the same as herbicide.