Effects of Sethoxydim on Root Morphology and Rhizosphere Microecology of Foxtail Millet Seedlings

Sethoxydim, a cyclohexanedione herbicide inhibiting acetyl-CoA carboxylase (ACCase), is widely used for post-emergence control of graminaceous weeds in foxtail millet (Setaria italica (L.) Beauv.) fields. This study investigated under field conditions the effects of sethoxydim on root morphology and rhizosphere microecology using two cultivars with contrasting tolerance: the relatively tolerant hybrid Zhangzagu 10 and the relatively sensitive conventional Zhonggu 19. Previous studies on sethoxydim in foxtail millet have focused primarily on shoot physiology, whereas integrated assessments of root morphology, soil enzyme activity, and rhizosphere microbiome remain scarce. Sethoxydim was applied at 112.5, 225.0, or 337.5 g a.i./hm2 (0.5×, 1×, and 1.5× the recommended dose), with water as control. Root morphology and rhizosphere enzyme activities were measured at 0, 7, 14, 21, and 28 days after application, and bacterial and fungal communities were analyzed by high-throughput sequencing. Sethoxydim inhibited root growth in both cultivars, but Zhonggu 19 was consistently more sensitive. At 337.5 g a.i./hm2, root length, surface area, diameter, and volume were markedly reduced in Zhonggu 19, whereas Zhangzagu 10 showed milder inhibition and even slight stimulation at lower doses. Rhizosphere urease, sucrase, and catalase activities initially decreased then recovered. Sethoxydim altered α-diversity and community structure of rhizosphere bacteria and fungi. These preliminary findings suggest that sethoxydim may be tentatively applied at or below the recommended dose (≤225.0 g a.i./hm2) under similar field conditions, particularly for tolerant cultivars such as Zhangzagu 10, though confirmation through multi-year trials is required.