Radiação luminosa no manejo de Urochloa brizantha com glyphosate

Abstract

Plants submitted to different light intensities modify their morphophysiology, which can directly influence the sensitivity to glyphosate. The objective was to evaluate the effect of light intensities in cultivation environments on the physiology and production of Urochloa brizantha and its relationship with sensitivity to the herbicide glyphosate. The experiment was conducted in the field in a randomized block design, with five replications, in a split plot scheme. In the plots three types of environments were allocated, being full sun, intermittent shading with three hours of direct light exposure and continuous shading, and in the subplots the doses 0, 480, 960 and 1440 g. i. a. ha-1 of glyphosate. At 14 days after application (DAA), glyphosate control, photosynthetic rate, stomatal conductance, transpiratory rate were evaluated and the water use efficiency was determined (USA). At the same time, the quantum efficiency of photosystem II, electron transport rate and the ratio between variable and maximum fluorescence (Fv/Fm) were analyzed. Samples of the plants were taken for morpho-anatomical analysis at the time of application. To determine the final control, evaluations were made at 35 DAA, assigning visual scores on a scale from 0 to 100%. Control at 14 DAA was above 80% in shaded environments from the 995 g dose. i. at ha-1 and in full sun above the dose 1,449 g. i. a. ha-1. U. brizantha had the lowest photosynthetic rate at a dose of 1,160 g ha 1 and there was an interaction effect between environments and doses. The plants had equal Fv/Fm between the environments, however in the shaded environments the values were below the ideal, which is between 0.75 and 0.85. At 35 DAA, control values of U. brizantha above 80% were observed from the 765 g dose. i. a. ha-1 of glyphosate for treatments with shading. Already in conditions of full sun, values of 65% of control were reached in the highest dose, which is considered unsatisfactory for the management of this species. The best control in the shade has a high correlation with the morpho-anatomical and physiological damage caused by shading. This makes it possible to reduce the dose of glyphosate and maintain an efficient control of U. brizantha grown under light restriction.