Alterações fisiológicas e bioquímicas em plantas de soja à toxidez por ferro em condições de normóxia e hipóxia

Abstract

Iron is an essential micronutrient for the maintenance of plant’s life, it is involved in growth and developmental processes of plants.The aim of this study was to evaluate the effect of toxic levels of Fe on biochemical and photosynthetic characteristics, visual symptoms of toxicity and nutritional state of soybean plants at normoxia and hypoxia conditions. Plants were grown in a hydroponic system and the experiments were carried out with root systems of plants under normoxia (air flushing) e hypoxia (N2 flushing), in Hoagland solution containing different concentrations of iron (Ferric- EDTA). The research was carried out in two steps: In the first step, tests were done in order to select and test iron concentrations to analyse responses of plants under normoxic conditions (0,9; 13,5 e 18 mM), and posteriorly under hypoxia conditions (0,9; 4,5; 9,0; 13,5 e 18 mM) for a 144 h period. Gas exchange parameters (A, Ci, gs, E) and chlorophyll index were evaluated at 72 h and 144h of exposition to treatments and dry mass accumulation and mineral composition analysis in leaves and roots, at 144h of exposition. In the second step, plants were exposed to Fe concentrations (0,9; 9,0; 18; 27 e 36 mM) under normoxia and hypoxia, for a period of 72 hours. Parameters of gas exchange (A, Ci, gs, E e A/Ci) were evaluated after 24h of exposition to treatments. At 72h, biochemical analysis were evaluated: antioxidant enzymes activities (SOD, CAT e APX), levels of lipid peroxidation, chloroplastic pigments, proline, composition of mineral nutrients, toxicity symptons and dry mass accumulation. Excess of iron caused a biomass reduction of soybean plants in all experiments, and the effect was more severe in plants submitted to hypoxia. In the first step, under normoxia, Ci and chlorophyll index decreased, and transpiration increased in plants submitted to highest iron concentration in both periods. In the second step of this research, SOD, CAT and APX activities were higher according to increase of iron concentrations in both conditions. Fe increase in tissues, resulted in accumulation of proline in leaves, with higher intensity in hypoxia treatment. Excess of iron affected uptake and translocation of nutrients. Results suggest that soybean plants when exposed to high concentrations of iron, show symptoms of toxicity that weaks the functionality of photosynthetic apparatus and distribution of other nutrients in the whole plant, however in a opposite way, enzimatic and non-enzimatic antioxidant system increased in order to attenuate the stress caused by iron.