Variabilidade espacial da comunidade vegetal e mecanismos de tolerância de espécies de plantas de cobertura do solo presentes em vinhedos com altos teores de cobre

Abstract

Pampa biome is one of the most diverse countryside ecosystems in the world. In Brazil, the conservation of this ecosystem has been neglected, and more than 54% of its original area has been eliminated. Nevertheless, the viticulture has been identified as sustainable activity to use Pampa areas, as it allows the maintenance of genetic resources of native species between the lines of culture. However, the cultivation of vines is frequently associated with the enrichment of the soil with copper (Cu), derived from frequent use of cupric fungicides. In this case, the soil Cu excess can cause environmental contamination and plants toxicity. Which, over the years, lead to the native plant species diversity degradation. On the other hand, the soil Cu excess can favor the establishment of plants, native or exotic, that develop adaptive tolerance mechanisms to the metal. These plants, adapted to environments contaminated with Cu, play an important role in the maintenance of vegetation and in the reduction of soil metal toxicity, including for the vines themselves. Thus, the goal of this thesis was to evaluate the composition of vegetal community that cohabits the vineyards and to identify tolerance mechanisms of plants to high soil Cu levels, expressed by vegetal species that predominate in these areas. As well as, evaluate the potential of these species to phytoremediation of Cu contaminated areas. To achieve the objective, four studies were carried out in this thesis, some of them in field and others in a greenhouse. In the first study (study I) three vineyards were selected, with increasing levels of soil available Cu, and an area of natural field, adjacent to the vineyards. In each of them, soil was sampled in the 0-20 cm topsoil layer to analyze Cu levels and the vegetation analyzed by: botanical composition, dry matter and shoot Cu concentration of the most frequent species in each area. To carried out the studies II and III, four species were selected from the first study (Cynodon dactylon, Axonopus affinis, Paspalum notatum and Paspalum plicatulum) and cultivated in nutrient solution with increasing concentrations of Cu (0.32, 15, 30 e 45 μM). In study II, the impacts of Cu levels on specific plant characteristics, such as photosynthetic activity, metal distribution in biomass, growth, root morphology and nutritional status were analyzed. In study III, the subcellular distribution and chemical forms of Cu in shoot part and in the root system of plants were evaluated; as well as the responses of your antioxidant system to Cu excess. In study IV, the same species were grown in soil contaminated with increasing rates of Cu (0, 35 and 70 mg of Cu kg-1). In this study, growth and gas exchange parameters, photosynthetic pigment concentration, activity of the antioxidant enzymes SOD and POD and the phytoremediation potential of the species were evaluated. As a result, in study I, the increase of available soil Cu concentrations of vineyards did not alter the biodiversity of the plant community that lives in these areas. However, the botanical composition has been modified. This may be the result of the increase in soil Cu levels or due to increase in soil fertility. In the study II, it was observed that the species P. plicatulum and A. conyzoides presented the highest values of Cu bioaccumulation factor in shoot and have potential for phytoremediation techniques. Other herbaceous plants that cohabit the vineyards, such as the native species P. notatum and the exotic C. dactylon, stood out for their wide distribution in the studied areas. In study III, the exposure of A. affinis, P. notatum, P. plicatulum and C. dactylon grasses to high concentrations of Cu in the growing environment increased the concentration of reactive oxygen species (ROS) and the lipid peroxidation of plant cell membranes. Moreover, excessive Cu absorption decreased the concentration of photosynthetic pigments, compromised photosynthetic activity, and altered the root morphology and nutritional status of plants (studies II and III). The Cu tolerance mechanisms expressed by plants consisted of metal accumulation in root system and the increase of antioxidant enzymes SOD and POD activity to combat the excessive production of ROS. Furthermore, in the root and leaves cells, most of the absorbed Cu was retained in the cell wall and vacuoles. Another strategy used by plants to minimize Cu toxicity was the complexation of the metal with pectates and proteins, phosphate and oxalates (study III). The study IV support that these grasses are potential phytostabilizers. Nevertheless, the use of native species as stabilizers in soil with moderate or high Cu contamination requires the adoption of strategies capable of reducing the soil Cu availability in order to minimize the phytotoxic effects of the metal and favor the development of native vegetation. Where the preservation of the Pampa native vegetation is not a priority, C. dactylon is the species with the greatest capacity to immobilize Cu in its biomass and can minimize the erosive processes and consequent dispersion of pollutants.