Hormetic effects of glyphosate on niger (Guizotia abyssinica): field insights into growth, physiology, and yield responses.

A field experiment conducted in 2023 evaluated the physiological effects of varying glyphosate concentrations (0, 1.5, 3, 6, 12, 25, 50,100, 200 and 400 g a.i. ha^-1) on the growth and development of niger (Guizotia abyssinica Cass). A hormetic response was observed at 6 g a.i. ha⁻¹, where the grain yield increased by 33% compared to the control, along with enhanced branching and inflorescence production. This dosage also corresponded with elevated catalase (CAT) activity and reduced levels of hydrogen peroxide (H₂O₂) and proline. Maximal activities of ascorbate peroxidase (APX) and guaiacol peroxidase (GPX) were observed at 50 and 100 g a.i. ha⁻¹, respectively. Glyphosate concentrations of 6 g a.i. ha⁻¹ and below induced less than 10% chlorosis, from which the plants fully recovered. In contrast, applications of 6, 12, 25, and 50 g a.i. ha⁻¹ resulted in reductions in plant dry matter by 34%, 48%, 46%, and 50%, respectively. Higher dosages ranging from 100 to 400 g a.i. ha⁻¹ caused severe phytotoxicity (> 80%) and ultimately led to plant death. The results of this experiment suggest that a very low glyphosate dosage of 6 g a.i. ha⁻¹ may affect physiological processes in niger plants, potentially contributing to increased grain production. However, such responses to low-dose herbicide application are known to be highly variable and influenced by factors such as environmental conditions, plant development, and species-specific sensitivity. Further investigation is required to clarify the underlying mechanisms and to determine the consistency and practical significance of these effects under field conditions.