The Effect of Sethoxydim Herbicide on the Physiological Parameters, Photosynthetic Enzymes and Antioxidant System in Foxtail Millet

Foxtail millet (<i>Setaria italica</i> L.) possesses characteristics such as strong stress tolerance and high yield. However, weeds compete with foxtail millet, leading to reduced crop yield, degraded quality, and even the promotion of pest and disease spread. Chemical weed control is currently the most practical and feasible method for preventing weed damage in foxtail millet production, but herbicides can harm the main crop, resulting in reduced yield. To investigate the effects of sethoxydim on the growth and development of foxtail millet, this experiment adopted a pot design, setting four concentration gradients for foliar spraying: 0.75, 1.5 (recommended dosage), 3 and 6 L of active ingredient per hectare (L ai ha^-1). Sethoxydim treatment hindered electron transport in photosynthesis, leading to a decrease in adenosine triphosphate synthesis and consequently a decline in the photosynthetic parameters of both photosystem I and photosystem II. Meanwhile, the activities and related gene expression of phosphoenolpyruvate carboxylase (PEPC), NADP-malate dehydrogenase (NADP-MDH) and pyruvate phosphate dikinase (PPDK) all showed a decreasing trend. In contrast, the activities and related gene expression of NADP-malic enzyme (NADP-ME) and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco); the contents of soluble protein and soluble sugar; and the activities of antioxidant enzymes including malondialdehyde (MDA), superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), along with their related gene expression, exhibited a trend of first increasing and then decreasing, reaching their peak at a dosage of 1.5 L ai ha^-1 (T2 treatment group). Meanwhile, the continuous rise in O₂^{·^-} and H₂O₂ contents indicated enhanced accumulation of reactive oxygen species (ROS) in plants under herbicide stress. These results show that at the recommended dosage, although sethoxydim causes certain damage to foxtail millet, the plant can maintain certain photosynthetic functions and physiological stability through self-regulation and gradually return to normal.