Physiological and Biochemical Mitigation of Tembotrione-Induced Phytotoxicity in Sorghum by Ascophyllum nodosum Extracts

Weed interference and herbicide-induced phytotoxicity, particularly from HPPD inhibitors such as tembotrione, represent significant limitations to yield stability in grain sorghum. Developing strategies to enhance crop tolerance without compromising weed control is of high practical interest. This study tested the hypothesis that a commercial Ascophyllum nodosum-based biostimulant can mitigate tembotrione-induced oxidative stress and phytotoxicity in sorghum without compromising the weed-control activity of the herbicide. Sorghum plants at the V4 phenological stage (four fully expanded leaves) were subjected to five treatments: (1) untreated control; (2) biostimulant application alone; (3) tembotrione application alone; (4) simultaneous application of tembotrione and biostimulant; and (5) tembotrione followed by biostimulant application after six days of application (6 DAT). After 10 days of treatment, photosynthetic pigment synthesis, primary photochemistry, gas exchange, antioxidant metabolism, phytotoxicity levels, growth parameters, and yield indices were evaluated. The results support the hypothesis that A. nodosum-based biostimulants can act as effective mitigating agents. The biostimulant sustained carotenoid levels and preserved the stability of the photosynthetic apparatus (PSII), counteracting HPPD enzyme inhibition caused by the herbicide. Isolated biostimulant application upregulated net photosynthesis by 60%, while simultaneous co-application with tembotrione preserved membrane integrity and the leaf area index. Furthermore, the efficacy of the mitigation strategy was highly time-dependent, as simultaneous co-application proved superior to the delayed (6 DAT) intervention. From an agronomic perspective, the biostimulant reduced visual injury and restored the grain number per plant to control levels under simultaneous co-application, although the final yield of combined treatments did not differ statistically from either the untreated control or the treatment of tembotrione alone. This study shows that the integration of A. nodosum extracts into the chemical management of sensitive crops represents a viable biotechnological strategy to enhance herbicide selectivity and yield stability.