The introduction of species is one of the major causes of biodiversity loss. Invasive exotic plants adapt, propagate and dominate, damaging both natural processes and native species. Thus, the objective was to evaluate the allelopathic effect of three exotic species in the caatinga, on the physiological quality of native forest seeds. Aqueous extracts of leaves of Azadirachta indica, Prosopis juliflora and Cryptostegia madagascariensis were tested to evaluate their effect on the seed germination and seedling vigor of Piptadenia moniliformis, Mimosa tenuiflora and Mimosa caesalpiniaefolia. Seed germination of M. tenuiflora was inhibited by the aqueous extracts of P. juliflora and A. indica in 100% and 42%, respectively, with a reduced GVI with the extracts of P. juliflora and C. madagascariensis. Regarding AEI, P. juliflora extract had positive effect on P. moniliformis seeds and negative on M. tenuiflora seeds, close to -1. A reduction in height of P. moniliformis was observed when the seedlings were in contact with the extracts of the invasive plants studied. For M. tenuiflora, the extract of P. juliflora prevented the formation of seedlings, and M. caesalpiniaefolia presented higher inhibitory effect with extracts of P. juliflora and A. indica. Therefore, M. tenuiflora showed to be sensitive to P. juliflora phytotoxicity.Resumo: A introdução de espécies é umas das maiores causas da perda da biodiversidade. As plantas exóticas invasoras se adaptam, propagam-se e exercem dominância, prejudicando tanto os processos naturais quanto as próprias espécies nativas. Assim, objetivou-se com este trabalho avaliar o potencial alelopático das espécies exóticas na caatinga, sobre a qualidade fisiológica de sementes florestais nativas. Extratos aquosos das folhas de Azadirachta indica, Prosopis juliflora e Cryptostegia madagascariensis foram testados para avaliar seu efeito sobre a germinação de sementes e vigor das plântulas de Piptadenia moniliformis, Mimosa tenuiflora e Mimosa caesalpiniaefolia. Na avaliação da germinação, as sementes de M. tenuiflora foram inibidas com extratos aquosos de P. juliflora e A. indica em 100% e 42%, respectivamente, com IVG reduzido com os extratos de P. juliflora e C. madagascariensis. Quanto ao RI, o extrato de P. juliflora teve efeito positivo nas sementes de P. moniliformis e negativo próximo a -1 nas sementes de M. tenuiflora. Nas plântulas de P. moniliformis, observou-se redução na altura, quando em contato com os extratos das plantas invasoras pesquisadas. Para a M. tenuiflora, o extrato de P. juliflora impediu a formação de plântulas, e as de M. caesalpiniaefolia apresentaram melhor efeito inibitório com extratos de P. juliflora e A. indica. Assim, a M. tenuiflora se mostrou sensível a fitotoxidez da P. juliflora. Palavras-chave:Germinação. Invasão biológica. Potencial alelopático.

Biological invasions of exotic plants can seriously contribute to ecological imbalances within areas to which they have been introduced, affecting, in addition to native plants, cultivated species. Consequently, the objective of the current study was to evaluate effects of allelopathic extracts of three exotic species on the physiological quality of crambe seeds. Aqueous extracts of Prosopis juliflora (Sw.) DC (algaroba) were formulated; Azadirachta indica A. Juss (Nim) and Cryptostegia madagascariensis Bojer ex Decne (viuvinha) at 20% (200 g of dried leaves per liter of distilled water) placed in B.O.D at 25 ° C for 24 h. Petri dishes, lined with germitest paper sheets, were moistened with aqueous extracts at 2,5 times de paper weight. In the experimental trial, we used the completely randomized, with four treatments (three extracts + control treatment) and four replicates of 25 seeds. The first and second germination counts were evaluated in the fourth and tenth days respectively; germination rate index (GRI), allelopathic effect index (AEI), seedling length and dry mass. P. juliflora and C. madagascariensis reduced germination by 84 and 91%, respectively, in the first count. The GRI was affected by the three species, while the AEI showed greater allelopathy for C. madagascariensis, reaching -0.90. Seedlings lengths were reduced in the presence of P. juliflora and C. madagascariensis. For crambe seeds, all extracts negatively affected their physiological potential.