Photosynthesis, the fundamental physiological process of plant responsible for the growth and yield of crops, is strongly affected by environmental stresses. Several methods have been used to study changes in the physiological parameters of plants exposed to stresses. The work aimed to study physiological parameters related to photosynthesis in leaf discs of soybean plants exposed to a photosystem II-inhibiting herbicide. Soybean leaf discs obtained from mature leaves of plants in the vegetative stage immersed in bentazon herbicide solutions at concentrations of 0, 100, 250 or 500 µM were evaluated. In experiment I, the effect of the herbicide on chlorophyll a fluorescence transient was measured using a portable fluorometer. In the second experiment, the effect of the herbicide on modulated chlorophyll a fluorescence and gas exchange were evaluated, with the latter being measured with an infrared gas analyzer. The evaluations of transient and modulated fluorescence provided additional information on the photosynthetic activity of soybean leaf discs exposed to the action of bentazon. For the fluorescence transient analysis, performance indices were the parameters most sensitive to the action of bentazon, showing a decrease of approximately 70 % at a dose of 500 µM. For the modulated fluorescence analysis, the photochemical quenching coefficient, the electron transport rate, the photochemical efficiency of photosystem II and the net assimilation rate, decreased in response to herbicide application, with values that were almost equal to zero at a dose of 500 µM, which are the parameters that showed the greatest sensitivity to bentazon in soybean.
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