Estudo alelopático e fitoquímico de Eragrostis lugens Nees. (POACEAE)

Abstract

Interactions between organisms mediated by the release of chemical compounds into the environment is named allelopathy. This concept describes the influence of one organism on the other, which may be harmful or beneficial, and suggests that this effect is due to the release of allelochemicals. Many commercial crops suffer interference from several factors during the biological cycle, with negative impacts on productivity, the main one being the presence of weeds, known for their allelopathic potential. In the state of Rio Grande do Sul, in recent years, invasive grass species have been found in the areas where irrigated rice is cultivated, causing losses in the growth, development and productivity of this crop. Among them, are the species of genus Eragrostis, which includes Eragrostis lugens. Aqueous and hydroethanolic extracts of aerial parts of E. lugens from two collections (spring and summer) were prepared at the concentrations 4,0, 8,0 e 16 % (p/v) with the objective of evaluating the allelopathic potential in de germination and initial growth of Oryza sativa. For the germination and initial growth bioassays of Lactuca sativa, only the summer extracts were used in the same concentrations. Germination and growth bioassays were carried out on 150 mm diameter Petri dishes with two layers of germitest paper and every 24 h after inoculation of the diaspores the number of germinated seeds were checked and the percentage of germination (%G) an d the germination rate index (IVG) were obtained. Under the same conditions of the germination of the previous study, for the initial growth evaluation, the rice and lettuce diaspores were previously placed in distilled water for seed germination and when the radicle was 1.0 mm were transferred to the plates containing the extracts. The cultures were maintained in a growth chamber with photoperiod of 16 h and temperature of 25 ºC ± 1 for seven days. After these, the length (cm) of the radicle and shoot was measured, for rice and lettuce seedlings. To evaluate the residual effect of the treatments on the rice, after the first evaluation the seedlings were transferred to plastic trays containing commercial substrate/vermiculite mixture and maintained for 15 days in a growth chamber for acclimatization, being watered daily with 100 mL of water. At the end of these, data of shoot length (cm) and number of leaves were collected, an d the trays were transferred to a greenhouse, where they received a water depth of 2.0 cm, kept daily for a period of 15 days. After this period, the parameters of leaf length (cm), number of leaves, number of tillers, leaf area (cm³) and dry mass (g) were evaluated. The content of polyphenols and flavonoids of the extracts was obtained by spectrophotometry. For polyphenols, the absorbances were read at the wavelength of 730 nm using a standard curve developed for gallic acid and the results expressed in milligrams equivalent of gallic acid per gram of sample. For total flavonoids the absorbances were read at 420 nm and calculated using the quercetin curve. The results were expressed in milligrams of quercetin per gram of extract. The aqueous and hydroethanolic extracts of aerial parts of E. lugens from the two collections presented allelopathic potential, having a negative effect on seed germination and on the parameters of vegetative growth of rice. About lettuce, summer extracts showed negative effects on both germination and root growth, mainly root necrosis and deformities. The aerial parts of E. lugens accumulated metabolites with this allelopathic potential, such as phenolic and / or flavonoid compounds, possibly responsible for the negative e ffects on the studied variables, with a greater emphasis on the spring collections.