Glyphosate-resistant hairy fleabane (Conyza bonariensis) exhibits a larger number of trichomes and altered stomatal density relative to the susceptible counterpart

Abstract

Following the adoption of Roundup Ready crops, glyphosate spraying frequency increased, while the use of other herbicide modes of action was neglected. Herbicide-resistant biotypes were reported in three major Conyza species in Brazil, including Conyza bonariensis, increasing growers’ bottom line. Considering that leaf surface structures affect proper herbicide deposition, uptake, and performance, this study aimed to characterize epicuticular surface components in glyphosate-resistant (R) and -susceptible (S) C. bonariensis. Conyza spp. seeds were collected in 36 locations in Brazil, and plants were subjected to resistance screening tests by spraying glyphosate at 720 and 1440 g ae ha-1 (0.5X and 1X the label recommended rate, respectively). For resistance level characterization, C. bonariensis biotypes with contrasting responses were selected for glyphosate dose-response assays. Leaf tissues for epicuticular surface analysis were harvested from newly-obtained R and S biotypes at two growth stages. Histological cuts were made on a leaf area of 25 mm² with a blade. Samples were fixed in Karnowsky solution, gradually changed to 100% ethanol, critical-point dried with CO2, and coated with gold, followed by stomatal and trichome density quantification using scanning electron microscopy. Results indicated a poor control with glyphosate in 33 of 36 Conyza spp. biotypes, and a high (31.5) resistance factor was calculated after dose-response trials. Leaf surface analysis indicated that C. bonariensis leaves are amphistomatic and exhibit tectorial trichomes. A higher number of trichomes and altered stomatal density (number.mm²) were quantified in R compared to the S counterpart, potentially reducing glyphosate uptake and effectiveness.

Mechanistic understanding and sustainable management of non-target site herbicide resistance in modern day agriculture

“…Stomatal regulation has been linked with the uptake and effectiveness of glyphosate and overall resistance dynamics (Baccin et al, 2023). The study showed that glyphosate resistant Conyza bonariensis exhibited altered adaxial (leaf upper side) morphology, characterized by increased number of trichomes and modified stomatal density compared to the susceptible biotypes (Baccin et al, 2023). Similar modifications in leaf morphology were documented in glyphosate-resistant L multiflorum and paraquat-resistant C. sumatrensis populations (Galvani et al, 2012;Pereira, 2019).…”

“…Additionally, high plant branching casts shadows on lower parts of the plant, thus preventing effective herbicide application (Mohr et al, 2007). Stomatal regulation has been linked with the uptake and effectiveness of glyphosate and overall resistance dynamics (Baccin et al, 2023). The study showed that glyphosate resistant Conyza bonariensis exhibited altered adaxial (leaf upper side) morphology, characterized by increased number of trichomes and modified stomatal density compared to the susceptible biotypes (Baccin et al, 2023).…”