Barnyard Grass Stress Triggers Changes in Root Traits and Phytohormone Levels in Allelopathic and Non-Allelopathic Rice

SIMPLE SUMMARY: Barnyard grass (Echinochloa spp.) is one of the most dominant and noxious weed species in rice paddies. However, allelopathic rice is known to interfere with the growth of barnyard grass through allelochemical-mediated root interactions. Here, the allelopathic rice cultivar, “PI312777” (PI), and the non-allelopathic rice cultivar, “Lemont” (LE), were used as the materials for analysis and were subjected to the stresses of quinclorac-resistant (R) and -susceptible (S) barnyard grass, respectively. We first investigated changes in the morphological traits of the roots of two rice cultivars. Meanwhile, the levels of different plant hormones, including auxin, abscisic acid, jasmonic acid, and salicylic acid, known for their diverse adaptation strategies to biotic stress, were analyzed. The results indicated that PI demonstrated the greater competitive ability, compared to LE, under barnyard grass-induced stress, with respect to shoot and root biomass, the number of root tips, and root tip length in rice with root diameters of <0.5 mm. Salicylic acid (SA) and jasmonic acid (JA) were found to have the potential to suppress barnyard grass growth in allelopathic rice. The amounts of SA and JA demonstrated a significant correlation with the number of root tips and root tip length in rice with root diameters of <0.5 mm. Our research may aid in the development of strategies for reducing the environmental impact of herbicides through the prudent selection of non-chemical control tactics. ABSTRACT: Despite the growing knowledge concerning allelopathic interference with barnyard grass, little is understood regarding the competitive physiological mechanisms of the interaction between allelopathic rice and herbicide-resistant barnyard grass. A hydroponic system was employed to investigate the root morphological traits and different phytohormonal changes in allelopathic and non-allelopathic rice cultivars when co-planted with quinclorac-resistant and -susceptible barnyard grass, respectively. The results show that shoot and root biomass were greater in PI. Barnyard grass stress induced an increase in shoot and root biomass in PI at 7 and 14 days of co-culturing rice and barnyard grass. Especially under the stress of quinclorac-resistant barnyard grass, the shoot biomass of PI increased by 23% and 68%, respectively, and the root biomass increased by 37% and 34%, respectively. In terms of root morphology, PI exhibited a significantly higher fine-root length, in root diameters of <0.5 mm, a greater number of root tips, and longer root tips compared to LE. The response to quinclorac-resistant barnyard grass stress was consistent in terms of the SA and JA content. The obvious accumulation of SA and JA was observed in two rice cultivars under quinclorac-resistant barnyard grass stress, with greater amounts of SA and JA in PI. The significant decrease in auxin (IAA) and abscisic acid (ABA) content in rice was detected from 7 to 14 days under co-culture stress. Additionally, highly significant and positive correlations were found between SA and JA content, and the number of root tips and root tip length at root diameters of 0–0.5 mm in rice.