Structural and ultrastructural variations in roots of Calopogonium mucunoides Desv. treated with phenolic compounds from Urochloa humidicola (Rendle) Morrone & Zuloaga and phenolic commercial standards

Urochloa humidicola (Rendle) Morrone & Zuloaga is an exotic forage grass cultivated in Brazil. Previous studies have identified different phenolic compounds produced by this species, including flavonoids and p-coumaric acid, which have been well-known as allelochemical compounds. We evaluated the morphology, anatomy and ultrastructure of Calopogonium mucunoides roots exposed to phenolic compounds found in U. humidicola. Leaves of U. humidicola were dried, crushed and submitted to extraction in methanol solution, which was fractionated through partition using organic solvents. In the bioassays, the hydromethanolic leaf extract of U. humidicola, commercial standard solutions of phenols and a control were all tested against the germination and seedling growth of C. mucunoides. The seedlings of C. mucunoides were measured, morphologically described, and the roots were processed according to standard techniques for plant anatomy and electron microscopy. Urochloa humidicola leaf extract inhibited root development, although they stimulated shoot elongation. The p-coumaric acid strongly inhibited plant development, caused root tip necrosis and decreased production of lateral roots, as well as changes in anatomy. Seedlings treated with quercitrin, luteolin and fisetin showed greater lengths, whereas kaempferol negatively affected root growth. Also, U. humidicola extract, p-coumaric acid and kaempferol caused marked changes in root cell ultrastructure. These results suggest a potential phytotoxic effect caused by U. humidicola leaf compounds isolated against C. mucunoides seedlings, although the association of these compounds is suggested to be antagonistic.