Can occur hormesis in guinea grass using glyphosate?

Glyphosate is the most widely used herbicide in the world, due to several aspects related to the reduction of labor costs and its mode of action. Thus, as it has a broad spectrum of control and is highly effective, it is the main option for weed control in various agricultural production systems (Duke, Powles, 2008; Velini et al., 2009). However, recent studies have shown that low doses of this herbicide can act stimulating the growth of several plant species (Velini et al., 2008; Cedergreen, Olesen, 2010; Carvalho et al., 2013; Pereira et al., 2013; Carbonari et al., 2014; Nascentes et al., 2018), possibly due to changes in the shikimic acid pathway (Velini et al., 2008), which is responsible for approximately 20% of the carbon assimilated by plants (Haslam, 1993). Such biphasic dose-response effect, in which low doses cause a stimulatory effect, followed by high doses causing a toxic effect, is known as hormesis (Calabrese, Baldwin, 2002). Treating a crop with subdoses of a herbicide for a desirable phenotypic change can be valuable (Duke et al., 2006); for example, when glyphosate is applied in subdoses to stimulate the accumulation of sucrose and prevent flowering in sugar cane (Velini et al., 2009; Araldi, 2010). In a different way, it is also worth mentioning the work of Pereira et al. (2013), that when applying reduced doses (from 3.6 to 36.0 g a.e. ha-1) of glyphosate in eucalyptus seedlings, reported increases of up to 22% in the dry matter of the crop. Doses within this range were also responsible for the increase of up to 20% in the dry matter of non-resistant soybean, compared to the control (Velini et al., 2008). For forage species (Brachiaria brizantha), Nascentes et al. (2015) observed a growth 31.8% greater than plants without application, for the dose of 10.5 g a.e. ha-1. In spite of this, studies that aim to increase the vegetative mass of Panicum maximum for grazing, in response to a hormetic effect, are incipient in the literature. The guinea grass (P. maximum syn. Megathyrsus maximus) is considered an important grass in pasture areas in several regions of Brazil, being one of the first forage species introduced in the country (Souza, 1999). Thus, the knowledge of their physiological and morphological characteristics is essential for the establishment of adequate management (Rodrigues, Reis, 1995). Thus, the study of the hormetic effect caused by low doses of glyphosate can be a viable alternative for the management of this species, aiming to an increase in plant growth and consequently production of green mass. In addition to cultures varying in their responses due to the application of different glyphosate doses, as mentioned above, Belz and Duke (2014) pointed out Abstract: Background: Recent studies have demonstrated an increase in plant growth due to the application of low doses of glyphosate, which is part of the phenomenon known as hormesis. Objective: The present study aimed to evaluate the effects of glyphosate doses on the physiological and morphological parameters of Panicum maximum cv. Mombaça. Methods: The experiment was carried out in a greenhouse for 49 days after the application of glyphosate, in pots arranged in a completely randomized design, with four replications. The treatments consisted of ten doses of glyphosate (0; 3.78; 8.10; 16.64; 33.48; 67.7; 135; 270; 540 and 1,080 g a.e. ha-1). Evaluations of growth, gas exchange, leaf temperature and plants’ intoxication were carried out. The data were subjected to ANOVA and when significant, regression analysis was performed using the models for dose-response curves. Results: The physiological parameters, as well as the growth, were positively affected by the subdoses of glyphosate, with higher values than the control. The leaf temperature increased progressively according to the increase in doses. Conclusion: Subdoses of 3.78 to 270 g a.e. ha-1 of glyphosate promoted an increase in plant’s growth and physiological parameters.