Alterações fisiológicas e bioquímicas em plantas cultivadas em solos com acúmulo de cobre e zinco

Abstract

Soil pollution by heavy metals such as copper (Cu) and zinc (Zn) has deserved special attention as it may represent risks to the quality of the environment. In Brazil particularly in the state of Rio Grande do Sul, the application of high doses of pig slurry in soil as nutrient source to plants and the continued application of copper-based fungicides in vineyards are two activities that are alarming due to the addition of high amounts of Cu and Zn in the soil. For this reason continued application of copper-based fungicides and pig slurry along the years has caused great increment in Cu and Zn contents, mainly in the superficial layers, thus increasing bioavailability of these metals which can cause phytotoxicity to the plants. The objectives of this paper were to infer the possibility of phytotoxicity of Cu and Zn to plants grown in soils submitted to successive applications of pig slurry and in soils collected from vineyards with a long history of copper-based fungicide applications of the Serra Gaúcha and Campanha Gaúcha regions, as well as to evaluate bioavailability of accumulated Cu in soils collected from vineyards of the aforementioned regions. To develop this work was carried out three studies. In first study was grown corn until 25 days after plant emergence in a greenhouse using deformed and undeformed soil samples collected in 0 - 20 cm depth was setup where 19 successive pig slurry applications of 0, 20, 40 and 80 m3 ha-1 were done for the period of seven years. A second study which composes this work was done with soils collected from vineyards of the Serra Gaúcha and Campanha Gaúcha regions. In March 2009, soil samples were collected in soil depths of 0 - 20 and 0 - 5, 5 - 10, 10 - 15, 15 - 20, 20 - 25, 25 - 30, 30 - 40 cm in three vineyards and native forest from the Serra Gaúcha region and in two vineyards and native land of the Campanha Gaúcha region. Soil samples of the 0 20 cm depth were dried, grounded and later used for the cultivation of black oat in a greenhouse for period of 40 days after plant emergence the plants. In both studies, enzymatic biochemical parameters linked to oxidative stress in the aerial part of corn and black oat plants such as lipid peroxidation, H2O2 concentration, superoxide dismutase enzyme (SOD) activity, catalase (CAT) and ascorbate peroxidase (APX) were evaluated. Further, non-enzymatic parameters linked to oxidative stress in the aerial part of corn and black oat plants such as ascorbic acid and non-protein thiol group concentrations were also evaluated. The third study was conducted with vineyard soils, which assessed the bioavailability of Cu in soils. The total content of Cu, pseudo-total content (extraction using the EPA 3050B method), available content (extraction with HCl 0.1 mol L-1 and EDTA), as well the chemical fractionation of Cu through sequential extraction were determined. Overall, the biochemical parameters were not intensely altered in the corn plants, thus indicating that the 19 pig slurry applications did not cause excessive accumulation of copper and zinc in soil. On the other hand, Haplumbrept soil from vineyards of the Serra Gaúcha region and the Typic Hapludalf soil from the vineyards of the Campanha Gaúcha region with a long history of cupric fungicide application caused great oxidative stress and the antioxidant system of the black oat plants was not effective enough to reverse the stress condition, indicating that these soils present toxic levels of copper to the black oat plants. Lastly, it was observed that a high concentration of Cu in black oat roots may be an indicator of bioavailability of Cu in soil, but it is not an indicator to evaluate phytotoxicity to the plants. In addition, copper added to the soil via foliar application in grapevines accumulates mainly in the superficial layers of soil predominately in bioavailable forms, especially those linked to the mineral fraction of the soil.