Glyphosate Applied at the Early Reproductive Stage Impairs Seed Production of Glyphosate-Resistant Hairy Fleabane

Abstract

Glyphosate-resistant hairy fleabane [Conyza bonariensis (L.) Cronq.] is one of the most important weeds in the world. Among the factors that make this weed species widely distributed in the most diverse environments is the high seed production capacity and dispersal. Hairy fleabane plants not controlled by herbicide application regrowth and overcome crop canopy, use environmental resources, interfere with crops, and complete their life cycle by producing thousands of seeds and replenishing the seed bank. Management strategies that reduce production and viability of hairy fleabane seeds can be adopted within the integrated management to reduce the seed bank and prevent further infestations. In this way, experiments were carried out in a greenhouse and laboratory of seed analysis to evaluate the effect of glyphosate (1,480 g a.e. ha-1) on the production and viability of glyphosate-resistant hairy fleabane seeds when applied at the vegetative and reproductive stages. Seed production was reduced by 68.4 and 100% when glyphosate was applied on hairy fleabane plants at the vegetative and early reproductive stages, respectively, regarding to the control. The viability of hairy fleabane seeds was not influenced by treatments at the evaluated stages. However, glyphosate treatment reduced the hairy fleabane seed production when applied at the vegetative stage . Hairy fleabane seed production is not feasible when glyphosate is applied at the early reproductive stage.

Glyphosate Resistance Affect the Physiological Quality of Conyza bonariensis seeds

“…Biotypes of C. bonariensis showed results higher than 60% for total germination (Figure 1B) and vigor (Figure 1A). These results are considered high in relation to those found by Piasecki et al (2019a), who reported about 38% germination for glyphosate-resistant C. bonariensis. Germination of hairy fleabane seeds is influenced by the environment (light, temperature, salinity, and depth of sowing), with values ranging from 18 to 32% under ideal conditions (Nandula et al, 2006, Wu et al, 2007Davis and Johnson, 2008).…”

“…Molecular identification of species of the biotypes used in the present study indicated that B01R, B11R, B29R, B17S, and B20S are from the species C. bonariensis. B48S presented different molecular pattern in genotyping analyses, being necessary the complementation with morphological analysis, which was performed by an expert in the field, who concluded that the species in question is C. blakei (Cabrera) Cabrera (Table 1) (Piasecki et al, 2019a). According to Lorenzi (2000), the genus Conyza includes approximately 100 species, which are distributed throughout the world, although they are originated from the Americas.…”

“…They have high seed production, i.e., a single plant can produce more than 200,000 (Bhowmik and Bekech, 1993). Studies conducted in Brazil by Piasecki et al (2019a) with glyphosate-resistant C. bonariensis estimated production of about 120,000 seeds per plant. In addition to high seed production, they have morphological features that favor dispersal, which can exceed the distance of 100 m (99% of seeds up to 100 m and 1% up to 500 m) (Dauer et al, 2007).…”

Stressful Conditions Affect Seed Quality in Glyphosate Resistant Conyza bonariensis (L.)

“…The viability and germination potential of C. bonariensis seeds were similar between biotypes within each collection site. The viability of seeds was low (±50% or less) in relation to other studies that have reported viability rates between 65% and 80% in R and S Conyza populations [11,19]. However, there are also cases with lower germination rates [2,23].…”

“…Great efforts have been made to characterize the factors involved in the glyphosate resistance, dispersal, and management of C. bonariensis, including growth analysis and bioecological aspects [10]. In considering its seed biology, there is abundant information regarding Conyza's seed germination growing degree days requirements for different sowing dates (autumn, winter, and spring) and agricultural environments [10], as well as its seed viability [11,15], emergence times over a growing season [16], seed longevity [17], and some aspects related to seed quality, such as factors related to seed production, dead seeds, empty seeds, and dormant seeds, among others parameters [18,19]. However, studies on how stress conditions (cold or heat) affect the seed quality of glyphosate-resistant C. bonariensis-which may contribute to its management by reducing seed persistence, reducing the soil seed bank, and consequently preventing the emergence of weeds at critical times [20]-are scarce.…”