The action of herbicides on weeds that develop under water deficit can be compromised, because the routes of penetration of hydrophilic herbicides are reduced due to the lower hydration of the cuticle in these plants. Moreover, hydrophobic compounds found in the epicuticular wax coating of plants under water stress, hinder the penetration of hydrophilic compounds. This study evaluated the control efficiency of glyphosate on guineagrass plants when submitted to different water deficits. The study was conducted in a greenhouse, and treatments were composed of guineagrass plants submitted to three soil water conditions [low water deficit (13%), intermediate water deficit (10%) and high water restriction (8%)], three doses of glyphosate (0.0, 270.0 and 540.0 g ha-1) and two phenological stages of plant development (4-6 leaves and 1-3 tillers). The water management started when plants presented two developed leaves. Visual evaluations were performed 7, 14, 21 and 35 days after the application of the herbicide; the morpho-physiological parameters of the specific leaf area, stomatal conductance and the difference between the environment temperature and the leaf temperature on the day of the herbicide application were also analyzed at the end of the study, as well as the dry matter of shoot and root. With the increase of the water restriction, there was a decrease in the analyzed morpho-physiological parameters, as well as in the dry matter accumulation of the shoot and roots of the studied plants. Plant control was more efficient when a 540 g ha-1 dose of glyphosate was applied, and when they were controlled at their vegetative stage of 1-3 tiller, and with a water management of 13%. It is possible to state that guineagrass under water restriction have less control efficacy when treated with glyphosate.

This research aimed at studying the action of the herbicides imazethapyr and lactofen on the root system and rhizobium/plant interaction, when they are sprayed at two phenological development stages of soybean cultivars under different soil water management. The soybean cultivars MG/BR 46 Conquista (conventional) and BRS Valiosa (RR) were submitted to three minimum soil water potential (Ψs): -0.03; -0.07 and -0.5 MPa at the V2 and V4 phenological stages. The herbicides imazethapyr at the rate (100 g a.i. ha-1) and lactofen at the rate (180 g a.i. ha-1) were sprayed 10 days after the start of water restrictions. Twenty-one days after the application of the herbicides, root dry matter and root nodulation were evaluated. The BRS Valiosa (RR) soybean cultivar presented better nodule formation and a more efficient and more adapted root system to support moderate drought stress conditions (-0.07 MPa). The MG/BR 46 Conquista (conventional) soybean cultivar presented lower the root system growth when submitted to the spraying of imazethapyr and lactofen. The activity of N2-fixating bacteria was affected by the spraying of the herbicides imazethapyr and lactofen.

Water deficit is a limiting factor for the soybean yield; it triggers different physiological and anatomical adaptations that have deleterious effects on the plants and can affect the selectivity of herbicides, causing production losses. In this context, the objective of this work was to evaluate the action of the chlorimuron-ethyl herbicide when applied at different stages of soybean plants, using conventional and transgenic cultivars, and different soil water potentials. A rate of 20 g ha-1 of the chlorimuron-ethyl herbicide was applied to two soybean cultivars (MG/BR46-Conquista - conventional, and BRS-Valiosa-RR - transgenic) at two phenological stages (V2 - first fully expanded trifoliate leaves, and V4 - third fully expanded trifoliate leaves), using three soil water potentials (-0.03 MPa, -0.07 MPa, and -0.5 MPa). Phytotoxicity, and plant height were evaluated at 3, 7, 14, and 21 days after the herbicide application. The shoot dry weight, root dry weight, and root system nodulation were evaluated. The soybean plants had lower phytotoxicity when subjected to application of chlorimuron-ethyl under water deficit conditions. The use of chlorimuron-ethyl reduced the growth and biomass of soybean plants and affected the plants' root system nodulation. The transgenic cultivar (BRS-Valiosa-RR) presented better performance when subjected to a moderate water deficit (-0.07 MPa), which contributes to biological nitrogen fixation.