Phytotoxicity of extracts from two Caatinga species on lettuce germination and anatomy

Phytotoxic metabolites released by plants can be beneficial or harmful to other plant species that share the same environment. Therefore, studying these metabolites is important to determine the dominance of plant diversity and the natural succession of agroecosystems. The objective of this study was to evaluate the phytotoxic activity of different concentrations of aqueous and ethanolic extracts from the leaves and bark of <i>Anadenanthera macrocarpa</i> (Benth.) Brenan and <i>Tabebuia caraiba</i> (Mart.) Bureau, on germination and anatomical characteristics of lettuce. The treatments were established from the dilution of aqueous (20, 10, 8, 6, 4, and 0%) and ethanolic (20, 10, 8, 6, 4, and 0 mg mL^-1) extracts. The percentage of germination, number of abnormal seedlings, germination speed index, average germination time, and tissue anatomy of the lettuce root were evaluated. The tested extracts reduced the germination percentage by up to 36% and the germination speed index by up to 83%; increased the number of abnormal seedlings by up to 81%, causing greater damage to exoderm and xylem tissues. In addition, the average germination time of lettuce increased by up to three days. Therefore, both species studied have allelopathic potential on the germinal metabolism of lettuce.