Journal of Agricultural Science; Vol. 7, No. 12; 2015 ISSN 1916-9752 E-ISSN 1916-9760 Published by Canadian Center of Science and Education 151 Allelopathy of Miconia spp. (Melastomataceae) in Lactuca sativa L. (Asteraceae) Marcos Aurélio Figueiredo dos Santos 1 , Maria Arlene Pessoa da Silva 1 , Antônio Carlito Bezerra dos Santos 1 , Sarah Ribeiro Alencar 1 , Isabella Hevily Silva Torquato 2 , Amanda Oliveira Andrade 1 , Natalia Cavalcante da Costa 2 , Maria Elizete Machado Generino 1 , Hemerson Soares Landim 2 & Alison Honorio de Oliveira 1 1 Departamento de Ciencias Biologicas, Universidade Regional do Cariri-URCA, Brazil 2 Departamento de Química Biologica, Universidade Regional do Cariri-URCA, Brazil Correspondence: Marcos Aurélio Figueiredo dos Santos, Departamento de Ciencias Biologicas, Universidade Regional do Cariri-URCA, Brazil. E-mail: biologo_100mil@hotmail.com Received: August 27, 2015 Accepted: September 25, 2015 Online Published: November 15, 2015 doi:10.5539/jas.v7n12p151 URL: http://dx.doi.org/10.5539/jas.v7n12p151 Abstract The aim of this work is to study the effects of the brute aqueous extract (BAE) of seven species of the genus Miconia occurring in the Chapada do Araripe-CE on Lactuca sativa, in addition to identifying the classes of secondary metabolites present in the extracts. The treatments consisted of four concentrations of leaf extract (25, 50, 75 and 100%), in addition to a control (0%) of distilled water, with five replicates each in a totally randomized experiment design layout for seven days. The following were assessed: number of germinated seeds, germination speed index (GSI), caulicle and radicle root length of the seedlings, occurrence of radical root necrosis, changes in the mitotic index and occurrence of chromosomal aberrations. The classes of secondary metabolites present in the extracts were identified through color changes and/or formation of precipitation. The brute aqueous extract of the leaves of M. albicans, M. alborufescens and M. stenostachya inhibited the germination of lettuce. All the BAEs of Miconia spp. had an adverse effect on the growth of the caulicle and radicle root of L. sativa. The extracts of M. albicans, M. ibaguensis, M. lingustroides and M. stenostachya were cytotoxic for the lettuce seedlings. Chromosomal aberrations were observed in all tested extracts. The metabolites found were hydrolysable and condensed tannins, flavonoids, flavones, flavanonols, chalcones, aurones and alkaloids. The tested species of Miconia showed inhibitory allelopathic activity within the parameters tested. The classes of secondary metabolites found could be responsible for the allelopathic and cytotoxic effects observed. Keywords: allelopathic activity, chromosomal aberrations, cytotoxicity, secondary metabolites 1. Introduction Allelopathy, defined as the positive or negative interference of one plant on other plants, fungi, insects and algae (Aires, 2007), is provoked by chemical substances called allelochemicals. These allelochemicals are products of secondary metabolism which inhibit or promote the germination and/or growth of the receptor organism (Aires, 2002) and are released into the environment by leaching, volatilization, radicle exudation and decomposition of plant parts. Derived from different metabolic pathways, the allelochemicals are products which originate in glucose metabolism and whose intermediate products are acetate and shikimic acid. Some allelochemicals result from a unit of shikimate and one or more units of acetate, or from derivatives such as anthraquinones, flavonoids and condensed tannins (Brito, 2010). According to Pessotto and Pastorini (2007), the allelochemicals cause interference in seed conservation, dormancy and germination, seedling growth and adult plant vegetative vigor when they affect vital respiration functions, photosynthesis, cell division, nutrition and reproduction. The allelochemicals affect the seed germination or the seedling growth of the receptor plants, thus making impossible the development of species which are intolerant to these compounds (Azevedo Neto, 2010). However, these effects could be considered secondary manifestations caused by primary events which take place at cellular and molecular level (Aires, 2007). Nevertheless, the plants which produce allelochemicals are acting in self-defense and the release must be