Aspectos fisiológicos, metabólicos e alterações no ciclo celular de Lactuca sativa L. (Asteraceae) em resposta ao cobre, peróxido de hidrogênio e óxido nítrico

Abstract

Understanding the mechanisms that make the presence of copper a toxic substance, as well as a regulation of the plants’ response to signals and environmental cues has became a fundamental aspect of Plant Biology. In this context, the goal of this work was to characterize the effect of different concentrations of copper (Cu), hydrogen peroxide (H2O2) and nitric oxide (NO) on the germination, initial growth, cellular cycle and the metabolism of Lactuca sativa L., as well as a possible protector action of NO on the phytotoxicity of the copper. Three experiments were carried out in completely randomized experimental design. To characterize the effect of the copper and H2O2 on the physiological and cytological aspects on L. sativa, seeds were exposed to 50; 100; 250 e 500 µM of CuSO4 and 1.0; 2.5; 5.0; 7.5 e 10 mM of H2O2, being the water used as negative control. The copper did not influence the germination (G%) and the dry matter (BS), however, root length (CR) and fresh matter (BF) was reduced as the concentration increased. This can be attributed to the reduction of the mitotic index (IM) of 29.02% in the control (treatment) to 1.5% in the concentration 500 µM of CuSO4. Although this concentration has been toxic, it was not clastogenic in the tested conditions. To the content of proline, concentration-dependent behavior was observed and the activity of superperoxide dismutase (SOD) was significant. It is noteworthy that the copper was cytotoxic, but the antioxidant system of the target plant prevented the lipid peroxidation of the membranes. To H2O2, it had a significant reduction of G% in the concentrations over 5mM. The CR and IM were negatively influenced and showed concentration-dependent behavior. The same behavior was observed to proline, that present higher accumulation in the concentration of 7.5 e 10 mM, differing of the others treatments. Furthermore, the activity of the enzymes (SOD) and ascorbato peroxidase (APX) was significant. It is noteworthy that the SOD had higher activity and thus, possibly, associated to the proline has attenuated the lipid peroxidation of the membranes. The phytotoxicity of H2O2 interfered negatively in the initial growth of L. sativa. The protector effect of the NO was verified by use of Sodium nitroprusside (SNP) as a donor of NO. Seeds of L. sativa were singly exposed to 250 and 500 µM of CuSO4 and to 100, 250 and 500 µM of the protector factor and to the combination of those two, being the water used as negative control. It has no significant difference for G%. CR treatments with 250 e 500 µM of SNP differed from each other and from the others treatments, reducing the growth in a concentration-dependent behavior. The combination SNP plus copper prevented the phytotoxic effect of this metal in the higher concentrations. The organism-test exposed to water showed IM 26.24%, while the treatments with singly copper the IM was 1.36% e 1.18% for the concentration of 250 and 500 µM, respectively. Thus, NO attenuated the cytotoxic effect of copper, however, singly in the higher concentrations showed itself cytotoxic. BF presented significant difference between treatments, being 250 and 500 µM of copper those that showed lower accumulation of biomass. When 250 and 500 µM of SNP were combined with 250 µM of copper the toxic effects of copper were attenuated, because there was not reduction of BF. The accumulation of proline supports to evidence the protector effect of NO. The cellular division, CR and BF were reduced when L. sativa was exposed to copper. The exogenous application of NO was capable of reducing the deleterious effects of copper.