Efeitos da inibição da protoporfirinogênio IX oxidase sobre as trocas gasosas e fluorescência da clorofila a em plantas de soja (Glycine max L. Merrill)

• Autor(a) principal: Carretero, Diego Martins
• Data de Publicação: 2008
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/4295
• topic: PROTOX; Herbicida; Clorofila; PROTOX; Herbicide; Chlorophyll; CNPQ::CIENCIAS AGRARIAS::AGRONOMIA::FITOTECNIA::FISIOLOGIA DE PLANTAS CULTIVADAS

The photosynthetic fixation of CO2 depends on the capture of light energy by the harvest complex (chlorophyll b and carotenoids), which transfer the energy to the photosystems reaction centers (chlorophyll a). The electrons transfer chain established between the photosystems generates reducing power necessary for the CO2 fixation in the Calvin cycle. The integrity of the chlorophyll a molecules (Chl a) may be affected by several environmental stressors, such as water shortage, nutrient deficiency, excess of light, pollutants or synthetic inhibitors. The effects of the chlorophyll synthesis inhibition, by synthetic inhibitors, in the physiological parameters of leaves in different stages of maturation are poorly understood. This research aimed to analyze the effect of lactofen, whose mechanism of action is the inhibition of the enzyme protoporphyrinogen IX oxidase (PROTOX), by measurements on gas exchange, fluorescence of chlorophyll a, chlorophyll, fluorescence image, integrity of membranes and synthesis of chlorophyll. Lactofen was applied to soybean plants at V6 stage. Then, the plants were exposed to different periods of irradiance accumulation (3, 6, 12, 24 and 48 hours). At the end of each period new and mature leaves were evaluated. It was observed that the lactofen effects were related to the duration of light exposure. In the first hours after the application, we observed an increase in oxidative stress, which affected membrane permeability and stomatal conductance. The chlorophyll a content in the young leaves, of the control treatment plants (FNC), increased significantly after 48 hours of light exposure when compared to those at the begging of the light exposure. However the parameters of gas exchange and chlorophyll fluorescence leaves did not differ significantly from the mature control leaves (FMC). For the young treated leaves FNT), exposed to 6 hours of light, reductions in net photosynthetic rate of assimilation (A) in stomatal conductance (gs) and the rate transpiratory (E) were observed concomitantly with an increase in the electrolytes leakage. However no significant reduction in chlorophyll a content was detected, as compared to the control, over the same period of light exposure. In mature treated leaves (FMT) not significant reductions in gas exchange parameters were observed. To evaluate the effects of metabolic and CO2 diffusion limitations, the net assimilation rate was analyzed under different CO2 internal concentrations (A/Ci curve) and different light intensity (A/PAR curve), on plants exposed to lactofen, and after 12 hours of light exposure. Significant differences were only observed in the FNT, in relation to control, for the following variables: maximum rate of photosynthesis saturated by CO2 (AmaxCO2) and light (AmaxPAR), the maximum rate of carboxylation held by Rubisco (Vc, max), the transport electrons rate for its regeneration (Jmax) and the apparent quantum yield of photosynthetic assimilation (α), the concomitant increase in the compensation points of CO2 (ГCO2) and light (ГPAR) and limitations of stomatal (Ls) and mesophilic (Lm) components. In mature treated leaves, compared to the control leaves, the application of lactofen increased the photorespiratory rate (R1) and the rate of electrons transmission to photorespiration (Jo). For FNT it was observed that the parameters of the chlorophyll fluorescence, quantum yield of dark-adapted samples (Fv/Fm), effective PS II quantum yield of illuminated samples (ΔF/Fm') and the rate electron transports (ETR) reduced after 6 hours of light FNT, while the minimum fluorescence (Fo) increased in the same period. The values of Fv/Fm, ΔF/Fm' and ETR in the FNT, after 48 hours of light, increased significantly, while Fo decreased. With the obtained fluorescence images it was possible to detected signal differences between the affected and not affected regions, in the same FNT. Thus, affected regions after 48 accumulated hours of light, showed reductions in Fv/Fm, Fo, ΔF/Fm', ETR, quantum yield of regulated non- photochemical energy loss in PS II (Y(NPQ)) and photochemical quenching (qP), with concomitant increase in the quantum yield of non-regulated non-photochemical energy loss in PS II (Y(NO)). The regions of FNT not affected by lactofen did not show the effect of PROTOX inhibition, probably because their photosynthetic machinery remained intact. When the figures obtained on the fluorescence images were analyzed according to the percentage of surface leaf area, increases were observed in areas presenting low absorbance (> 0.11), as they increased the accumulated hours of light in the FNT. Similar behavior was observed for the parameters Fv/Fm, Fo, ΔF/Fm', and qP, increasing the percentage of leaf surface in FNT unable to perform photosynthesis. The use of lactofen, especially in young soybean leaves inhibits the synthesis of chlorophyll, induces oxidative stress, causes damage in the photosynthetic machinery, reduces A and gs.